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Sarcosine and betaine crystals upon cooling: structural motifs unstable at high pressure become stable at low temperatures† E. A. Kapustin,*ab V. S. Minkov*ac and E. V. Boldyreva*ac The crystal structures of N-methyl derivatives of the simplest amino acid glycine, namely sarcosine (C3H7NO2) and betaine (C5H11NO2), were studied upon cooling by single-crystal X-ray diffraction and single-crystal polarized Raman spectroscopy. The effects of decreasing temperature and increasing hydrostatic pressure on the crystal structures were compared. In particular, we have studied the behavior upon cooling of those structural motifs in the crystals, which are involved in structural rearrangement during pressure-induced phase transitions. In contrast to their high sensitivity to hydrostatic compression, the crystals of both sarcosine and betaine are stable to cooling down to 5 K. Similarly to most a-amino acids, the crystal structures of the two compounds are most rigid upon cooling in the direction of the main structural motif, namely head-to-tail chains (linked via the strongest
Received 4th November 2014, Accepted 16th December 2014
N–H O hydrogen bonds and dipole–dipole interactions in the case of sarcosine, or exclusively by dipole– dipole interactions in the case of betaine). The anisotropy of linear strain in betaine does not differ much
DOI: 10.1039/c4cp05094k
upon cooling and on hydrostatic compression, whereas this is not the case for sarcosine. Although the interactions between certain structural motifs in sarcosine and betaine weaken as a result of phase transitions
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induced by pressure, the same interactions strengthen when volume reduction results from cooling.
Introduction Variations of temperature and pressure are widely used as very powerful tools to study interactions in a crystal structure. A comparison of the crystal structures upon cooling and upon increasing hydrostatic pressure provides valuable information about their properties. This holds for the data on bulk compressibility, strain anisotropy, phase transitions accompanied by ordering/disordering of molecular groups and fragments, the changes of molecular conformations, switching over of hydrogen bonds, proton migration within a hydrogen bond from a donor to an acceptor, etc.1 One can get a better insight into crystal structure instability and understand which changes in the intermolecular contacts are responsible for the structural rearrangement. Although pressure and temperature are both a
Novosibirsk State University, Pirogov street, 2, Novosibirsk 630090, Russian Federation. E-mail: [email protected], [email protected], [email protected] b Department of Chemistry, University of California, Berkeley, California 94720, USA c Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze street, 18, Novosibirsk 630128, Russian Federation † Electronic supplementary information (ESI) available. CCDC 1004185–1004202. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cp05094k
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scalars, the same value of volume decrease upon cooling and on hydrostatic compression can result from different structural changes at the microscopic level. A consequence of cooling is the decrease of thermal displacements, which causes the compression of the structure. In contrast to this, hydrostatic compression accounts for direct shifts of the atomic coordinates, so that the ‘‘free volume’’ is eliminated. For a simple cubic structure, the coefficients of thermal expansion and ¨neisen equation.2 isothermal compressibility are related by Gru However, this simple relation is not applicable to low-symmetry structures. The bulk compressibility is not sufficient, to describe their behavior upon compression. Linear strain tensor should be calculated, to characterize the anisotropy of lattice strain.2 According to the Neumann’s principle, the symmetry of structure response to variation of temperature or pressure depends on the symmetry of the crystal structure and must be the same;2,3 however, for any non-cubic structures this does not mean that also the anisotropy of strain upon cooling and upon hydrostatic compression must be identical. There are many different ways to compress a low-symmetry crystal structure without violating the Neumann’s principle. The directions of minimum and maximum strain may be radically different, the ratio between maximum and minimum strain is highly variable. In low-symmetry monoclinic and triclinic structures, also the directions of all or some of the principal axes of strain
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tensors corresponding to compression upon cooling and with increasing pressure may be different.4 For molecular crystals these differences account for specific structure-sensitive features. The crystals of organic compounds often combine molecular flexibility with the presence of several types of intermolecular interactions, some of which are specific and directional. The directions in which a crystal structure is most rigid usually correlate with the directions of selected structural motifs. Numerous examples are known, when cooling and increasing pressure have the same effect on a crystal structure (see examples in ref. 5,6), and when they do not.1,4,7,8 The aim of the present study was to compare the effect of cooling on the crystal structures of sarcosine and betaine with the data on hydrostatic compression of these crystals obtained previously.9 The choice of the system Sarcosine and betaine are the single and triple methylated derivatives of the simplest amino acid glycine. The double methylated derivative, N,N-dimethylglycine, has been studied in detail previously.10 The idea to choose the crystals of such compounds was very clear. The crystals of amino acids consist of zwitterions (with positively charged +NH3 group and negatively charged COO ), which are linked to each other by charge-assisted hydrogen bonds. These hydrogen bonds form infinite head-to-tail chains, which are the main structural motifs in the crystals of amino acids.11 In order to understand the role of such hydrogen bonds in the stability or instability of a crystal structure, the amino group was modified in such a way, so that to exclude the formation of selected hydrogen bonds. Sarcosine has two nonsubstituted hydrogen atoms in the amino group, and the molecules in its crystal are linked into a three-dimensional hydrogen bonded framework, resembling the crystal structure of b-glycine. In betaine all hydrogen atoms in the amino group are substituted for methyl groups, and thus cannot form any hydrogen bonds, but are involved exclusively in the electrostatic interactions between positively and negatively charged parts of a zwitterion. In a very recent work increasing hydrostatic pressure was shown to result in the phase transitions in sarcosine and in betaine.9 In sarcosine, a first order phase transition occurs at B0.8–1.3 GPa and is accompanied by changing molecular conformation, the formation of three-centered bifurcated hydrogen bonds and the fragmentation of a single crystal to fine powder. Betaine undergoes an extended phase transition at 1.4–2.9 GPa with disordering of the trimethylamino-groups and cracking of a crystal onto several large fragments. Both pressure-induced phase transitions are kinetically controlled and strongly depend on the rate of pressure variation. We wanted to see if the same structural units, which are responsible for the crystal structure instability at high pressure, will also account for an instability on cooling.
Materials and methods Samples Single crystals of sarcosine and betaine were obtained as described previously.9 Since betaine readily forms hydrates
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and perhydrates in the presence of even traces of water or hydrogen peroxide,12,13 plate-shaped single crystals of anhydrous betaine were obtained using anhydrous methanol. A commercial sample of betaine (Aldrich, CAS No. 107-43-7) was dissolved in methanol at B50 1C to reach saturation, at this point the heating was switched off and the solution was kept at room temperature to cool down, then put into the fridge at B2 1C. Prismatic colorless single crystals of sarcosine were crystallized from its saturated aqueous solution by slow evaporation at room temperature. During low temperature single crystal X-ray diffraction experiments the crystal of betaine was covered by a thin layer of low viscosity CryoOil to prevent hydration. In the case of Raman experiments, crystals were quickly removed from their mother liquor, wiped dry, loaded into the cryostat, and vacuumized. Using any moisture protectors for crystals was not appropriate because a thin film of any protective agent on a sample surface induces strong fluorescence in Raman spectra. X-Ray diffraction Data on crystal structures upon variation of temperature were collected using a Stoe IPDS-II single-crystal X-ray diffractometer (Mo Ka, l = 0.71073 Å). The 700-series Oxford Cryostreams cooling system was used to vary the sample temperature (precision of 0.1 K was maintained). For both sarcosine and betaine, single crystal data were collected at 295 K and then upon cooling from 275 to 100 K with a temperature step of 25 K. The data collection, indexing and integration of reflections, as well as data reduction were performed using X-Area software.14 X-SHAPE software15 was used to index crystal faces of sarcosine and betaine for further experiments on polarized Raman spectroscopy. Crystal structures of sarcosine and betaine at all temperatures were solved with direct methods using SHELXS and refined using SHELXL16 integrated in the X-Step32 shell.17 All H atoms in the structures of betaine at different temperatures, as well as the H atoms bonded to N atom in structures of sarcosine at different temperatures, were found in difference Fourier maps, and their positions were refined freely. H atoms bonded to C atoms in the structures of sarcosine were placed geometrically and refined using a riding model, with default distances of methyl C–H = 0.98 Å and secondary C–H = 0.97 Å. The thermal parameters of all H atoms were set as Uiso(H) = 1.5Ueq(C) for terminal methyl groups, or 1.2Ueq otherwise. The parameters characterizing data collection and refinement, as well as crystal data are summarized in Tables S1 and S2 of ESI.† Mercury18 and PLATON19 were used for visualization and analysis of the crystal structures. CCDC 1004185–1004193 for sarcosine and 1004194–1004202 for betaine (ESI†). Raman spectroscopy Polarized Raman spectra of oriented single crystals were collected using a triple-grating Horiba Jobin Yvon Lab-Ram HP spectrometer equipped with a N2-cooled detector coupled to an Olympus BX41 microscope. Excitation was supplied by an Ar+ laser (l = 488 nm) with a 2 cm 1 spectral resolution. The lowtemperature Raman spectra were recorded using a helium cryostat JANIS ST-500HT; oriented single crystals of sarcosine
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and betaine were wrapped in a thin indium foil to provide a better thermal contact, so the only upper face was accessible for a laser beam. Polarized and unpolarized Raman spectra of sarcosine were measured at 295 K, then in the range from 280 to 20 K with a step of 20 K, and at 5 K; for betaine – from 300 to 20 K with a step of 20 K, as well as at 10 and 5 K. The directions of the polarization vectors of the incident and scattered light coincided with each other and the crystallographic axes and were designated as aa, bb, and cc in the case of coincidence of the crystallographic axes a, b, and c. The Raman spectrum of a crystal of sarcosine with a deuterated amino group was also recorded.
Results and discussion Crystal structures under ambient conditions Interestingly, no polymorphs have been reported for sarcosine and betaine under ambient conditions, although glycine, which can be considered as a ‘‘precursor’’ for them, has three polymorphic modifications. Both N-derivatives of glycine are extensively used for crystallization of various salts, because of the possibility to use them as piezoelectric materials (Cambridge Structural Database, Version 5.34 May 2013,20). In these salts sarcosine crystallizes in different forms. For instance, in the case of strong inorganic acids, such as nitric, hydrochloric, hydrobromic, hydroiodic, perchloric, tetrafluoroboric, hexafluorosilicic, methanesulfonic, sarcosine crystallizes as a salt in the zwitterionic and protonated forms simultaneously.21 As for weak acids, sarcosine crystallizes either as a zwitterion (ellagic, telluric, L-ascorbic, pyromellitic acids22), or in the protonated form only (maleic and phosphoric acids23). Both anhydrous sarcosine and betaine crystallize in the orthorhombic space groups (non-centrosymmetric P212121 and centrosymmetric Pnma, respectively) with one zwitterion per asymmetric unit. A comparative study of crystal structures of both sarcosine and betaine under ambient conditions is described in detail in ref. 9. Having two H atoms in the amino group, each sarcosine zwitterion forms two N–H O hydrogen bonds. For both bonds, the same O1 atom of the carboxylate group acts as an acceptor. The shorter N1–H1n O1 hydrogen bond with d(N O) of 2.7598(14) Å links zwitterions into infinite zigzag head-to-tail chains of C(5) type along the crystallographic axis c [for graph set notations see ref. 24]. Infinite zigzag head-to-tail chains of the same type built with a longer N1–H2n O1 hydrogen bond with d(N O) of 2.7885(13) Å are directed along the crystallographic axis b (Fig. 1). Since in betaine all H-bonding donors are replaced by methyl groups, infinite head-to-tail chains are built with zwitterions linked exclusively via dipole–dipole interactions. In sarcosine, infinite head-to-tail chains form a threedimensional H-bonded framework, which resembles the crystal structure of b-glycine. In the case of betaine infinite chains are directed along the crystallographic axis a (Fig. 1). Several projections of sarcosine and betaine crystal structures are shown in Fig. S1 and S2 (ESI†).
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Fig. 1 Infinite head-to-tail chains in sarcosine linked via N1–H1n O1 (a) and N1–H2n O1 (b) hydrogen bonds; infinite head-to-tail chains in betaine held together via dipole–dipole interactions (c).
Effect of cooling on the crystal structures According to Raman spectra, no phase transitions were observed during cooling of sarcosine and betaine crystals from ambient temperature down to 5 K. Variable-temperature singlecrystal X-ray diffraction study showed that lattice strain was strongly anisotropic and different from that upon hydrostatic compression. Effect of cooling on sarcosine In the case of sarcosine the distortion of the crystal structure was strongly anisotropic, but continuous. The trends of relative changes in cell parameters appeared to be very similar to those in the b-polymorph of glycine. In addition, polarized Raman spectroscopy revealed the formation of a self-trapped state in sarcosine upon cooling, similar to what was previously reported for glycine.25 This self-trapped state was formed exclusively in the head-to-tail chains directed along the crystallographic axis c. Sarcosine appeared to be quite a rigid zwitterion towards the variations in temperature, similarly to the zwitterion of glycine. Thus, upon cooling down to 100 K, the zwitterion of sarcosine became less planar by 1.4(3)1 in terms of the torsion angle N–C–C–O of the main backbone. This value is comparable with the changes of torsion angles of a-, b-, g-glycine upon cooling down to 100 K (0.5(1), 0.6(1) and 0.4(1)1, respectively).26,27 At the same time the conformation of the methyl group also did not change significantly (less than 11). The C1–O1 bond elongated slightly more than the C1–O2 one, due to the strengthening of the N1–H1n O1 and N1–H2n O1 hydrogen bonds (see Fig. S3 in ESI†). This is similar to cooling of both polymorphs of N,N-dimethylglycine, where also only one of the two oxygen
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atoms per zwitterion involved in the formation of H-bonds.10 However, the delocalization of electron density within the carboxylate group did not change significantly upon cooling. The dependence of atomic displacement parameters of O1, O2 and N1 atoms upon cooling down to 100 K was smooth, revealing no phase transition as expected. In general, values of principal elements Uii are comparable with those of the three polymorphs of glycine in the same range of temperatures.27 The behavior of N1–H1n O1 and N1–H2n O1 hydrogen bonds upon cooling is shown in Fig. 2. As in the polymorphs of glycine,26,27 orthorhombic N,N-dimethylglycine,10 L-alanine,28 and DL-alanine,29 all the H-bonds in sarcosine become stronger upon cooling down to 100 K. So, the d(N1 O1) decreases by 0.60(9)% and 0.63(9)% for N1–H1n O1 and N1–H2n O1 hydrogen bonds, respectively, while the +(N–H O) does not change at all. The behavior of the H-bonds in sarcosine upon hydrostatic compression is different: the stronger N1–H1n O1 hydrogen bond compresses with increasing pressure, while the weaker N1–H2n O1 bond – expands.9 The elongation of the weaker H-bond up to a certain value results in the phase transition and the formation of a new, strong three-centered H-bond. Thus, the process of switching of the weak N1–H2n O1 hydrogen bond to the bifurcated one acts as a trigger for the reconstructive phase transition. Upon cooling, in contrast, this weak H-bond becomes stronger, and the ambient pressure phase is
Fig. 2 Changes in donor–acceptor distances of N1–H1n O1 (a) and N1–H2n O1 (b) hydrogen bonds. All curves are guides to the eye.
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becoming even more stable. This process also accounts for the different anisotropy of crystal structure compression upon cooling, as compared with that upon increasing pressure. Relative changes in the cell parameters of sarcosine upon cooling are plotted in Fig. 3. The structure is most rigid in the direction of zigzag head-to-tail chains formed by zwitterions along b and c crystallographic axes. In fact, the crystal structure of sarcosine even expanded slightly along the direction of the head-to-tail chains built by the shortest N1–H1n O1 hydrogen bond, i.e. along the crystallographic axis c (0.33(1)% upon cooling down to 100 K). In the case of cooling of the three polymorphs of glycine down to 100 K, the structure was the least compressible in the direction of straight head-to-tail chains, built by the strongest N–H O hydrogen bond in the structure ( 0.32(2)%, +0.08(1)% and 0.07(2)% for a-, b-, gglycine respectively).26,27 At the same time, the structure of sarcosine is compressed upon cooling down to 100 K by 0.24(2)% along the crystallographic axis b, which coincides with the direction of zigzag head-to-tail chains built by the longer N1–H2n O1 hydrogen bond. Taking into account that the changes of zwitterion conformation are non-significant, and that the changes in the N O distances are similar, one can assume that the only reason for different compression of zigzag head-to-tail chains along axes b and c is the different
Fig. 3 Relative changes in cell parameters of sarcosine (a) and betaine (b) with variations in temperature: black circles – crystallographic axis a; red rhombs – axis b; blue triangles – axis c. The standard deviations of values are smaller than the size of symbols. All curves are guides to the eye.
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geometry of these chains. The angles W and j between neighboring zwitterions in chains along axes c and b, respectively, are shown in Fig. 1. While the value of angle W increases from 98.59(2)1 at 295 K to 99.05(2)1 at 100 K, the value of angle j remains constant (89.85(2) at 295 K and 89.83(2) at 100 K). Thus, with the decrease of the intermolecular distance within the head-to-tail chains, the flattening of angle W between the zwitterions makes the zigzag chain along the crystallographic axis c more straight (and so the structure expands), while the constancy of the value of angle j causes the compression of the chain along axis b. A similar effect has been reported earlier for glycylglycine30a and paracetamol.30b The crystal structure of sarcosine is compressed mostly along the crystallographic axis a ( 2.25(1)% at 100 K). Such a large (B10 times) linear strain along a selected axis, as compared with all other directions, accounts for the collapsing voids between the zigzag head-to-tail chains. In general, the anisotropy of lattice strain of sarcosine upon cooling correlates well with that of a-, b-, and g-glycine,26,27 the monoclinic polymorph of N,N-dimethylglycine,10 L- and DL-alanine,28,29,31,32 L- and 33,34 DL-serine, where the least compression has been observed along the head-to-tail chains built by the shortest H-bonds. The largest compression is normal to these motifs. Additional and important information of intermolecular interactions in sarcosine was obtained from polarized Raman spectra. In contrast to the high pressure study,9 the analysis of Raman spectra upon cooling did not reveal any phase transitions upon cooling down to 5 K. The blue shift of the modes in the region of the lattice vibrations 80–140 cm 1 was very smooth and small (B1–2 cm 1 upon decreasing the temperature from
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ambient to 5 K). At the same time, in the region of stretching vibrations of C–H (2900–3100 cm 1), the modes shifted to the red region by B3 cm 1. As for skeleton vibrations [n(C–C), n(C–N)], their frequencies almost did not change upon cooling, since the zwitterion conformation was preserved. Even non-polarized Raman spectra of sarcosine measured upon cooling showed the non-linear increase of intensity for the bands near 2400 cm 1 (Fig. S4 in ESI†). In order to make a better assignment of the Raman bands, the spectrum of the deuterated sarcosine at ambient temperature was also recorded. In Fig. 4 one can see the shift of n(N–H) from 3252 cm 1 to 2217 cm 1, what corresponds to n(N–D) vibrations. At the same time, the band at 2432 cm 1 disappeared after deuteration, which enables us to claim that it includes the vibrations of the N–H group. In the polarized spectra bands at B2400–2450 cm 1 appear only in aa- and cc-polarization and should correspond to the strongest N1–H1n O1 hydrogen bond (Fig. 5). One can notice a strong temperature dependence of the intensities of bands in this region (Fig. S5 in ESI†). Previously a similar effect of non-linear increase of intensity for several modes at a spectral region of about 2500–2800 cm 1 was observed in polarized Raman spectra of L-, DL-alanine and all the three polymorphs of glycine upon cooling.25 In the case of L-alanine it was possible to assign these modes to different phonon bands. The observed phenomenon was interpreted as a manifestation of the formation of ‘self-trapped states’ of N–H O vibration, i.e. the coupling between the internal N–H vibrations and the external N O vibrations (or lattice vibrations). As proposed by Davydov,35 such a coupled state may appear and propagate along an infinite onedimensional chain, in which the molecules are inked via H-bonds.
Fig. 4 Raman spectra of sarcosine (bottom) and its deuterated analogue d2-sarcosine (top). The shift to the low-wavenumber region of some bands caused by deuteration is highlighted by dotted pink lines.
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Fig. 6 Temperature dependence of COO stretching asymmetrical vibrations (black rhombs) and NH bending vibrations (red circles) in sarcosine at bb polarization.
Fig. 5 Polarized Raman spectra of sarcosine at 295 K and 5 K in the range of coupled N–H stretching vibrations and lattice vibrations (a). The cc-Raman spectra of sarcosine at different temperatures (b). The definitions aa, bb and cc imply the directions of polarization vector of incident (first symbol) and scattered (last symbol) light with respect to crystal axes.
In the case of polymorphs of glycine, L-, and DL-alanine, this selftrapped state arises along the infinite head-to-tail chains linked via the shortest N–H O hydrogen bonds. Hitherto, self-trapped states were observed in polarized Raman spectra of amino acid crystals in a specific direction, along the infinite straight head-to-tail chains.25 In sarcosine the head-to-tail chains are not straight, but of zigzag type, and the directions of the corresponding H-bonds linking the molecules in these chains do not coincide with the direction of the chain as a whole. This may be the reason why the band at 2432 cm 1 is also present in the Raman spectrum of aa-polarization. For a comparison, the analysis of polarized Raman spectra of the two polymoprhs of N,N-dimethylglycine did not reveal any formation of self-trapped states. In DMG-I there are no molecular chains, but only isolated H-bonded cycles, whereas in DMG-II the H-bonds in the chains are not equivalent.10 Based on Gilli correlation for N–H O hydrogen bonds [n(N–H) vs. d(N O)],36 the mode at 3252 cm 1 agrees well with the geometric parameters of any of the two N–H O hydrogen bonds in sarcosine. Unfortunately, this mode appeared to be very broad and weak, and could not be assigned reliably to a certain H-bond in the structure, even if polarized spectra were analyzed. In contrast to Raman spectra at high pressure, the shift of this band upon cooling was poorly measurable, so that the nas(COO ) and d(N–H) vibrations were analyzed instead, to monitor the changes in the H-bonds. The bending N–H vibrations mode shifted smoothly to the higher wavenumber region upon cooling (1635 cm 1 at 295 K to 1645 cm 1 at 5 K), while
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the asymmetrical stretching COO vibrations shifted to the low wavenumber region (1605 cm 1 at 295 K to 1602 cm 1 at 5 K) (Fig. 6). These simultaneous shifts of the two modes can be explained by a decrease in the donor–acceptor distance and by the strengthening of the N–H O hydrogen bonds upon cooling. Effect of cooling on betaine Relative changes in the cell parameters of betaine upon cooling down to 100 K are plotted in Fig. 3. The compression of the structure is strongly anisotropic, the structure being the least compressible in the direction of head-to-tail chains along the crystallographic axis a ( 0.23(2)% at 100 K). The directions of the largest compression coincide with crystallographic axes b ( 1.59(1)%) and c ( 1.34(1)%), and are related to collapsing of voids between bulky trimethylamino fragments of the neighboring infinite chains. Similar to sarcosine, the molecular geometry of betaine does not change much upon cooling. While the main backbone of the sarcosine zwitterion becomes slightly more twisted, in betaine the mirror plane, comprising atoms of the main backbone, is preserved during cooling down to 100 K (and even down to 5 K what can be supposed on Raman spectra since there is no significant changes in stretching and bending vibrations of C–C, C–N and C–H). As for the carboxylate group of betaine, the C–O distances continuously expand upon cooling (0.8(1)% for C1–O1 and 0.7(1)% for C1–O2). Taking into account both the elongation of C–O distances and the compression of the crystal structure along the crystallographic axis a i.e. along the chain, one can conclude that the dipole–dipole interactions between the zwitterions within the chain get stronger upon cooling. The distance between the N atom of the +N–(CH3)3 fragment and the O1 atom of the COO group of the neighboring zwitterion within the chain decreases from 3.667(2) Å at 295 K to 3.635(2) Å at 100 K (N1 O1–C1 angle of B1631 does not change). The changes in the polarized Raman spectra of betaine were also continuous in the range from 300 K down to 5 K and
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did not reveal any phase transitions. The blue shift of the stretching C–H vibrations upon cooling down to 5 K did not exceed 1 cm 1. Moreover, the modes at 3020 cm 1, as well as at 3050 cm 1, became narrower upon cooling, and no additional modes appeared. This excludes the possibility of disordering of the bulky +N–(CH3)3 fragment around the N1–C2 bond. The asymmetric n(COO ) shifted from 1616 cm 1 at 300 K to 1612 cm 1 at 5 K, also proving the strengthening of interactions between the zwitterions through the whole experimental temperature range. These results differ radically from those observed previously with increasing pressure.9 The pressureinduced disorder of the bulky –N–(CH3)3 fragments in betaine rotating around the N1–C2 bond manifested itself by nonmonotonic behavior of the stretching C–H vibrations in the region of 2900–3100 cm 1.11 Above 1.5 GPa the band of the asymmetric n(COO ) vibrations shifted to higher wavenumbers, indicating that the repulsion contribution between the electron shells of the O-atom and methyl groups of the neighboring zwitterion increased. The analysis of the short contacts in the crystal structure, the shifts of the n(C–H) upon cooling, as well as the two-dimensional Hirshfeld fingerprints showed that there are no C–H O hydrogen bonds in the structure. As could be expected, since there are no N–H O hydrogen bonds in the crystal structure of betaine, no self-trapped states have been observed. Comparison of the effects of cooling and increasing pressure on the crystals of sarcosine and betaine The values of bulk compressibility of sarcosine and betaine upon cooling differed significantly (Fig. 7), as well as the values of isothermal compressibility upon hydrostatic compression. The thermal expansion of betaine is larger than that of sarcosine (3.14(3)% and 2.17(3)% for a temperature change between 100 K and 298 K, respectively). For a comparison, thermal expansion of the three polymorphs of glycine in the same temperature range is even less, than in sarcosine (1.78(3)%, 1.94(4)% and 1.38(3)% for a-, b-, g-glycines, respectively26,27). The thermal expansion
Fig. 7 Relative changes in cell volumes of sarcosine (black circles) and betaine (red rhombs) with variations in temperature. All curves are guides to the eye.
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of the orthorhombic polymorph of N,N-dimethylglycine is 2.27(1)%, which is smaller than in betaine, but larger, than in sarcosine. The values of thermal expansion of methylated derivatives of glycine are larger than that of glycine itself, and this trend also holds for isothermal hydrostatic compression. In other words ‘‘the introduction of the bulky –CH3–groups to the amino-group of the ‘‘main’’ head-to-tail chains makes the crystal structures more compressible compared with the polymorphs of glycine’’.9 The most interesting observation of this study is that the anisotropy of linear strain for crystal structures of betaine and sarcosine differed significantly upon cooling and upon increasing pressure, even when scaled to the same value of relative volume change (Fig. 8). The scaling factors relating isothermal compressibility and thermal expansion for sarcosine and betaine are different. Thus, in sarcosine, the change in cell volume at 100 K corresponds to B0.22 GPa, while in betaine the volume change at the same temperature corresponds to B0.4 GPa. Bulk coefficients of thermal expansion of sarcosine and betaine are 115(2) MK 1 and 165(5) MK 1, respectively. At the same time, bulk coefficients of isothermal compressibility are 116(1) TPa 1 and 42(5) TPa 1, respectively. In betaine, the linear strain along crystallographic axis a (direction of infinite head-to-tail chains) is the least one both upon cooling and upon increasing pressure, and this correlates well with the strengthening of dipole–dipole interactions within the chain. However, the relative compressibility along b and c was opposite upon cooling and with increasing pressure. The difference between relative changes of b and c cell parameters increases upon cooling. The compression along these directions is determined by collapsing of voids between + N–(CH3)3 fragments of the neighboring chains. One can suppose that this collapse occurs differently upon cooling and upon hydrostatic compression. In sarcosine the direction of the largest compression along the crystallographic axis a upon increasing pressure and decreasing temperature correlated quite well. Similarly, minimum strain was observed along the crystallographic axis c (the head-to-tail chains built by the shortest N1–H1n O1 hydrogen bonds) both upon variations of temperature and pressure. Either upon cooling down to 125 K, or upon increasing the hydrostatic pressure up to 0.19 GPa, the changes of the W angle in the chain were also similar. However, the compression along the head-to-tail chains built by the longer N1–H2n O1 hydrogen bonds (along the crystallographic axis b) differed radically upon cooling and with increasing pressure. While the orientation of the molecules within the chain remained the same upon cooling, as well as upon increasing pressure (angle j did not change much), the N1–H2n O1 hydrogen bonds shortened upon cooling, but elongated upon increasing pressure. The remarkable difference in the behavior of N1–H1n O1 and N1–H2n O1 hydrogen bonds upon variations of temperature and pressure accounts for the different origin of these bonds. While N1–H1n O1 is the stronger (or the ‘main’ [definition proposed in ref. 10]) H-bond, the N1–H2n O1 bond is the weaker (or the ‘additional’) one. The compression of each kind of bonds upon increasing pressure and upon cooling differs, and
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Fig. 8 Comparison of the anisotropy of lattice strain of sarcosine (left column) and betaine (right column) upon cooling and with increasing pressure. Relative changes in cell parameters versus temperature (open circles) and versus pressure (filled circles) are scaled in such a way that relative changes in cell volume coincide upon cooling and increasing of pressure. The data on relative changes of volume versus pressure have been taken from the previous work.9
a similar effect has been observed previously for the three polymorphs of glycine,26 L-,28,37 and DL- (ref. 29 and 38) alanine by X-ray diffraction, as well as by polarized Raman spectroscopy.25,39 In all of these structures the ‘main’ H-bonds behaved similarly upon either increasing pressure, or upon cooling, while the response of the ‘additional’ H-bonds to the two actions was different, and could also trigger the phase transitions.40
Phys. Chem. Chem. Phys.
Conclusions The strain anisotropy of the crystal structures of sarcosine and betaine upon cooling was different in several respects from that upon increasing pressure, similarly to what has been observed for the polymorphs of glycine itself. A common feature for betaine, sarcosine, and for most other a-amino acids was the
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minimum strain along infinite head-to-tail chains. The differences in the anisotropy have been observed in the plane normal to the direction of minimum strain and were related to the behavior of the weakest hydrogen bonds and to the different mechanisms of collapsing the voids in the structure. In sarcosine, both upon cooling, and with increasing pressure, the compression along the crystallographic axis a (collapsing of voids between the chains) was 10 times larger than that along other axes. At the same time, the chains of different type in sarcosine compressed differently upon decreasing the temperature and with increasing pressure: while the chains built by stronger H-bonds compressed similarly, the distortion of the chains formed by weaker H-bonds differed radically upon cooling and increasing pressure. In betaine, having no H-bonds in the structure, collapsing of voids between the trimethylamino fragments upon cooling and with increasing hydrostatic pressure also differed. Another important difference in the structural response of sarcosine and betaine to temperature/pressure variations is that the structural strain upon cooling was continuous, in contrast to the pressure-induced phase transitions. In this respect, the behavior of the two methylated derivatives of glycine was also similar to that of the three polymorphs of glycine itself.26 One could suppose that the distortion of the weaker H-bond in sarcosine plays the key role in the pressure-induced phase transition, and that the same H-bond becomes stronger and stabilizes the crystal structure upon cooling. Similarly, in the case of betaine the ‘‘weakest element’’ in the structure is the rotation of the trimethylamino fragment around the C2–N1 bond: upon increasing pressure these fragments become disordered and dipole–dipole interactions within the head-to-tail chain get weaker (as could be seen from shifting of n(COO ) to higher wavenumbers due to repulsion of electron shells of O-atom and methyl groups), whereas with decreasing temperature the rotation of +N–(CH3)3 fragments is reduced and electrostatic interactions within the chain strengthen (asymmetric n(COO ) vibrations shift to lower wavenumbers).
Acknowledgements We acknowledge financial support from RFBR (grant No. 12-0331145), from the Ministry of Education and Science of Russian Federation (project No. 1828), and from Russian Academy of Sciences.
Notes and references 1 (a) E. V. Boldyreva, High Pressure Crystallography. From Fundamental Phenomena to Cutting-Edge Technological Applications, NATO Science for Peace and Security Series B: Physics and Biophysics, ed. E. V. Boldyreva and P. Dera, Kluwer, Dordrecht, 2010, pp. 147–160; (b) E. V. Boldyreva, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 218. 2 International Tables for Crystallography, Physical Properties of Crystals, 1st edn, 2003, vol. D, p. 522.
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3 J. F. Nye, Physical Properties of Crystals: Their Representation by Tensors and Matrices, Oxford Science Publications, 1st edn, 1957, reprinted with corrections many times after that. 4 (a) E. Boldyreva, J. Kivikoski and J. A. K. Howard, Acta Crystallogr., Sect. B: Struct. Sci., 1997, 53, 394; (b) E. Boldyreva, J. Kivikoski and J. A. K. Howard, Acta Crystallogr., Sect. B: Struct. Sci., 1997, 53, 405; (c) E. V. Boldyreva, D. Yu. Naumov and H. Ahsbahs, Acta Crystallogr., Sect. B: Struct. Sci., 1998, 54, 798; (d) E. V. Boldyreva, Cryst. Eng., 2003, 6, 235; (e) E. V. Boldyreva, J. Mol. Struct., 2003, 647, 159; ( f ) E. V. Boldyreva, T. N. Drebushchak, T. P. Shakhtshneider, H. Sowa, H. Ahsbahs, S. V. Goryainov, S. N. Ivashevskaya, E. N. Kolesnik, V. A. Drebushchak and E. B. Burgina, ARKIVOC, 2004, XII, 128; (g) R. Lee, J. A. K. Howard, M. R. Probert and J. W. Steed, Chem. Soc. Rev., 2014, 43, 4300. 5 V. S. Minkov, N. A. Tumanov, E. V. Boldyreva and R. Q. Cabrera, CrystEngComm, 2010, 12, 2551. 6 B. A. Zakharov, E. A. Losev and E. V. Boldyreva, CrystEngComm, 2013, 15, 1693. 7 B. A. Zakharov and E. V. Boldyreva, Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater., 2013, 69, 271. 8 E. V. Boldyreva, H. Ahsbahs and H.-P. Weber, Z. Kristallogr., 2003, 218, 231–236. 9 E. A. Kapustin, V. S. Minkov and E. V. Boldyreva, Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater., 2014, 70, 517. 10 E. A. Kapustin, V. S. Minkov, J. Stare and E. V. Boldyreva, Cryst. Growth Des., 2014, 14, 1851. 11 (a) S. N. Vinogradov, Int. J. Pept. Protein Res., 1979, 14, 281; (b) C. G. Suresh and M. Vijayan, Int. J. Pept. Protein Res., 1983, 22, 129; (c) E. V. Boldyreva, in Models, mysteries, and magic of molecules, ed. J. C. A. Boeyens and J. F. Ogilvie, Springer, Netherlands, 2007, vol. 169, pp. 13–14; (d) C. H. ¨rbitz, Acta Crystallogr., Sect. B: Struct. Sci., 2010, 66, 84; Go (e) V. S. Minkov, PhD thesis, Novosibirsk State University, ¨rbitz, A. B. Leirvåg and Ø. Jacobsen, 2011; ( f ) C. H. Go CrystEngComm, 2014, 16, 9631. 12 T. C. W. Mak, J. Mol. Struct., 1990, 220, 13. 13 V. S. Minkov, E. A. Kapustin and E. V. Boldyreva, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2013, 69, 416. 14 Stoe & Cie, X-AREA and X-RED, Darmstadt, Germany, 2006. 15 Stoe & Cie, X-SHAPE, Darmstadt, Germany, 2006. 16 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112. 17 Stoe & Cie, X-STEP32, Darmstadt, Germany, 2000. 18 C. F. Macrae, P. R. Edgington, P. McCabe, E. Pidcock, G. P. Shields, R. Taylor, M. Towler and J. van de Streek, J. Appl. Crystallogr., 2006, 39, 453. 19 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7. 20 F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci., 2002, 58, 380. 21 (a) M. Fleck, V. Ghazaryan and A. P. Petrosyan, Z. Kristallogr. – Cryst. Mater., 2012, 227, 819; (b) M. Fleck, V. Ghazaryan and A. P. Petrosyan, J. Mol. Struct., 2012, 1020, 160; (c) M. Fleck, V. Ghazaryan and A. P. Petrosyan, J. Mol. Struct., 2012, 1021, 130; (d) M. Fleck, V. Ghazaryan and A. P. Petrosyan,
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Cite this: DOI: 10.1039/c4cp05094k
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Sarcosine and betaine crystals upon cooling: structural motifs unstable at high pressure become stable at low temperatures† E. A. Kapustin,*ab V. S. Minkov*ac and E. V. Boldyreva*ac The crystal structures of N-methyl derivatives of the simplest amino acid glycine, namely sarcosine (C3H7NO2) and betaine (C5H11NO2), were studied upon cooling by single-crystal X-ray diffraction and single-crystal polarized Raman spectroscopy. The effects of decreasing temperature and increasing hydrostatic pressure on the crystal structures were compared. In particular, we have studied the behavior upon cooling of those structural motifs in the crystals, which are involved in structural rearrangement during pressure-induced phase transitions. In contrast to their high sensitivity to hydrostatic compression, the crystals of both sarcosine and betaine are stable to cooling down to 5 K. Similarly to most a-amino acids, the crystal structures of the two compounds are most rigid upon cooling in the direction of the main structural motif, namely head-to-tail chains (linked via the strongest
Received 4th November 2014, Accepted 16th December 2014
N–H O hydrogen bonds and dipole–dipole interactions in the case of sarcosine, or exclusively by dipole– dipole interactions in the case of betaine). The anisotropy of linear strain in betaine does not differ much
DOI: 10.1039/c4cp05094k
upon cooling and on hydrostatic compression, whereas this is not the case for sarcosine. Although the interactions between certain structural motifs in sarcosine and betaine weaken as a result of phase transitions
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induced by pressure, the same interactions strengthen when volume reduction results from cooling.
Introduction Variations of temperature and pressure are widely used as very powerful tools to study interactions in a crystal structure. A comparison of the crystal structures upon cooling and upon increasing hydrostatic pressure provides valuable information about their properties. This holds for the data on bulk compressibility, strain anisotropy, phase transitions accompanied by ordering/disordering of molecular groups and fragments, the changes of molecular conformations, switching over of hydrogen bonds, proton migration within a hydrogen bond from a donor to an acceptor, etc.1 One can get a better insight into crystal structure instability and understand which changes in the intermolecular contacts are responsible for the structural rearrangement. Although pressure and temperature are both a
Novosibirsk State University, Pirogov street, 2, Novosibirsk 630090, Russian Federation. E-mail: [email protected], [email protected], [email protected] b Department of Chemistry, University of California, Berkeley, California 94720, USA c Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze street, 18, Novosibirsk 630128, Russian Federation † Electronic supplementary information (ESI) available. CCDC 1004185–1004202. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cp05094k
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scalars, the same value of volume decrease upon cooling and on hydrostatic compression can result from different structural changes at the microscopic level. A consequence of cooling is the decrease of thermal displacements, which causes the compression of the structure. In contrast to this, hydrostatic compression accounts for direct shifts of the atomic coordinates, so that the ‘‘free volume’’ is eliminated. For a simple cubic structure, the coefficients of thermal expansion and ¨neisen equation.2 isothermal compressibility are related by Gru However, this simple relation is not applicable to low-symmetry structures. The bulk compressibility is not sufficient, to describe their behavior upon compression. Linear strain tensor should be calculated, to characterize the anisotropy of lattice strain.2 According to the Neumann’s principle, the symmetry of structure response to variation of temperature or pressure depends on the symmetry of the crystal structure and must be the same;2,3 however, for any non-cubic structures this does not mean that also the anisotropy of strain upon cooling and upon hydrostatic compression must be identical. There are many different ways to compress a low-symmetry crystal structure without violating the Neumann’s principle. The directions of minimum and maximum strain may be radically different, the ratio between maximum and minimum strain is highly variable. In low-symmetry monoclinic and triclinic structures, also the directions of all or some of the principal axes of strain
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tensors corresponding to compression upon cooling and with increasing pressure may be different.4 For molecular crystals these differences account for specific structure-sensitive features. The crystals of organic compounds often combine molecular flexibility with the presence of several types of intermolecular interactions, some of which are specific and directional. The directions in which a crystal structure is most rigid usually correlate with the directions of selected structural motifs. Numerous examples are known, when cooling and increasing pressure have the same effect on a crystal structure (see examples in ref. 5,6), and when they do not.1,4,7,8 The aim of the present study was to compare the effect of cooling on the crystal structures of sarcosine and betaine with the data on hydrostatic compression of these crystals obtained previously.9 The choice of the system Sarcosine and betaine are the single and triple methylated derivatives of the simplest amino acid glycine. The double methylated derivative, N,N-dimethylglycine, has been studied in detail previously.10 The idea to choose the crystals of such compounds was very clear. The crystals of amino acids consist of zwitterions (with positively charged +NH3 group and negatively charged COO ), which are linked to each other by charge-assisted hydrogen bonds. These hydrogen bonds form infinite head-to-tail chains, which are the main structural motifs in the crystals of amino acids.11 In order to understand the role of such hydrogen bonds in the stability or instability of a crystal structure, the amino group was modified in such a way, so that to exclude the formation of selected hydrogen bonds. Sarcosine has two nonsubstituted hydrogen atoms in the amino group, and the molecules in its crystal are linked into a three-dimensional hydrogen bonded framework, resembling the crystal structure of b-glycine. In betaine all hydrogen atoms in the amino group are substituted for methyl groups, and thus cannot form any hydrogen bonds, but are involved exclusively in the electrostatic interactions between positively and negatively charged parts of a zwitterion. In a very recent work increasing hydrostatic pressure was shown to result in the phase transitions in sarcosine and in betaine.9 In sarcosine, a first order phase transition occurs at B0.8–1.3 GPa and is accompanied by changing molecular conformation, the formation of three-centered bifurcated hydrogen bonds and the fragmentation of a single crystal to fine powder. Betaine undergoes an extended phase transition at 1.4–2.9 GPa with disordering of the trimethylamino-groups and cracking of a crystal onto several large fragments. Both pressure-induced phase transitions are kinetically controlled and strongly depend on the rate of pressure variation. We wanted to see if the same structural units, which are responsible for the crystal structure instability at high pressure, will also account for an instability on cooling.
Materials and methods Samples Single crystals of sarcosine and betaine were obtained as described previously.9 Since betaine readily forms hydrates
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and perhydrates in the presence of even traces of water or hydrogen peroxide,12,13 plate-shaped single crystals of anhydrous betaine were obtained using anhydrous methanol. A commercial sample of betaine (Aldrich, CAS No. 107-43-7) was dissolved in methanol at B50 1C to reach saturation, at this point the heating was switched off and the solution was kept at room temperature to cool down, then put into the fridge at B2 1C. Prismatic colorless single crystals of sarcosine were crystallized from its saturated aqueous solution by slow evaporation at room temperature. During low temperature single crystal X-ray diffraction experiments the crystal of betaine was covered by a thin layer of low viscosity CryoOil to prevent hydration. In the case of Raman experiments, crystals were quickly removed from their mother liquor, wiped dry, loaded into the cryostat, and vacuumized. Using any moisture protectors for crystals was not appropriate because a thin film of any protective agent on a sample surface induces strong fluorescence in Raman spectra. X-Ray diffraction Data on crystal structures upon variation of temperature were collected using a Stoe IPDS-II single-crystal X-ray diffractometer (Mo Ka, l = 0.71073 Å). The 700-series Oxford Cryostreams cooling system was used to vary the sample temperature (precision of 0.1 K was maintained). For both sarcosine and betaine, single crystal data were collected at 295 K and then upon cooling from 275 to 100 K with a temperature step of 25 K. The data collection, indexing and integration of reflections, as well as data reduction were performed using X-Area software.14 X-SHAPE software15 was used to index crystal faces of sarcosine and betaine for further experiments on polarized Raman spectroscopy. Crystal structures of sarcosine and betaine at all temperatures were solved with direct methods using SHELXS and refined using SHELXL16 integrated in the X-Step32 shell.17 All H atoms in the structures of betaine at different temperatures, as well as the H atoms bonded to N atom in structures of sarcosine at different temperatures, were found in difference Fourier maps, and their positions were refined freely. H atoms bonded to C atoms in the structures of sarcosine were placed geometrically and refined using a riding model, with default distances of methyl C–H = 0.98 Å and secondary C–H = 0.97 Å. The thermal parameters of all H atoms were set as Uiso(H) = 1.5Ueq(C) for terminal methyl groups, or 1.2Ueq otherwise. The parameters characterizing data collection and refinement, as well as crystal data are summarized in Tables S1 and S2 of ESI.† Mercury18 and PLATON19 were used for visualization and analysis of the crystal structures. CCDC 1004185–1004193 for sarcosine and 1004194–1004202 for betaine (ESI†). Raman spectroscopy Polarized Raman spectra of oriented single crystals were collected using a triple-grating Horiba Jobin Yvon Lab-Ram HP spectrometer equipped with a N2-cooled detector coupled to an Olympus BX41 microscope. Excitation was supplied by an Ar+ laser (l = 488 nm) with a 2 cm 1 spectral resolution. The lowtemperature Raman spectra were recorded using a helium cryostat JANIS ST-500HT; oriented single crystals of sarcosine
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and betaine were wrapped in a thin indium foil to provide a better thermal contact, so the only upper face was accessible for a laser beam. Polarized and unpolarized Raman spectra of sarcosine were measured at 295 K, then in the range from 280 to 20 K with a step of 20 K, and at 5 K; for betaine – from 300 to 20 K with a step of 20 K, as well as at 10 and 5 K. The directions of the polarization vectors of the incident and scattered light coincided with each other and the crystallographic axes and were designated as aa, bb, and cc in the case of coincidence of the crystallographic axes a, b, and c. The Raman spectrum of a crystal of sarcosine with a deuterated amino group was also recorded.
Results and discussion Crystal structures under ambient conditions Interestingly, no polymorphs have been reported for sarcosine and betaine under ambient conditions, although glycine, which can be considered as a ‘‘precursor’’ for them, has three polymorphic modifications. Both N-derivatives of glycine are extensively used for crystallization of various salts, because of the possibility to use them as piezoelectric materials (Cambridge Structural Database, Version 5.34 May 2013,20). In these salts sarcosine crystallizes in different forms. For instance, in the case of strong inorganic acids, such as nitric, hydrochloric, hydrobromic, hydroiodic, perchloric, tetrafluoroboric, hexafluorosilicic, methanesulfonic, sarcosine crystallizes as a salt in the zwitterionic and protonated forms simultaneously.21 As for weak acids, sarcosine crystallizes either as a zwitterion (ellagic, telluric, L-ascorbic, pyromellitic acids22), or in the protonated form only (maleic and phosphoric acids23). Both anhydrous sarcosine and betaine crystallize in the orthorhombic space groups (non-centrosymmetric P212121 and centrosymmetric Pnma, respectively) with one zwitterion per asymmetric unit. A comparative study of crystal structures of both sarcosine and betaine under ambient conditions is described in detail in ref. 9. Having two H atoms in the amino group, each sarcosine zwitterion forms two N–H O hydrogen bonds. For both bonds, the same O1 atom of the carboxylate group acts as an acceptor. The shorter N1–H1n O1 hydrogen bond with d(N O) of 2.7598(14) Å links zwitterions into infinite zigzag head-to-tail chains of C(5) type along the crystallographic axis c [for graph set notations see ref. 24]. Infinite zigzag head-to-tail chains of the same type built with a longer N1–H2n O1 hydrogen bond with d(N O) of 2.7885(13) Å are directed along the crystallographic axis b (Fig. 1). Since in betaine all H-bonding donors are replaced by methyl groups, infinite head-to-tail chains are built with zwitterions linked exclusively via dipole–dipole interactions. In sarcosine, infinite head-to-tail chains form a threedimensional H-bonded framework, which resembles the crystal structure of b-glycine. In the case of betaine infinite chains are directed along the crystallographic axis a (Fig. 1). Several projections of sarcosine and betaine crystal structures are shown in Fig. S1 and S2 (ESI†).
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Fig. 1 Infinite head-to-tail chains in sarcosine linked via N1–H1n O1 (a) and N1–H2n O1 (b) hydrogen bonds; infinite head-to-tail chains in betaine held together via dipole–dipole interactions (c).
Effect of cooling on the crystal structures According to Raman spectra, no phase transitions were observed during cooling of sarcosine and betaine crystals from ambient temperature down to 5 K. Variable-temperature singlecrystal X-ray diffraction study showed that lattice strain was strongly anisotropic and different from that upon hydrostatic compression. Effect of cooling on sarcosine In the case of sarcosine the distortion of the crystal structure was strongly anisotropic, but continuous. The trends of relative changes in cell parameters appeared to be very similar to those in the b-polymorph of glycine. In addition, polarized Raman spectroscopy revealed the formation of a self-trapped state in sarcosine upon cooling, similar to what was previously reported for glycine.25 This self-trapped state was formed exclusively in the head-to-tail chains directed along the crystallographic axis c. Sarcosine appeared to be quite a rigid zwitterion towards the variations in temperature, similarly to the zwitterion of glycine. Thus, upon cooling down to 100 K, the zwitterion of sarcosine became less planar by 1.4(3)1 in terms of the torsion angle N–C–C–O of the main backbone. This value is comparable with the changes of torsion angles of a-, b-, g-glycine upon cooling down to 100 K (0.5(1), 0.6(1) and 0.4(1)1, respectively).26,27 At the same time the conformation of the methyl group also did not change significantly (less than 11). The C1–O1 bond elongated slightly more than the C1–O2 one, due to the strengthening of the N1–H1n O1 and N1–H2n O1 hydrogen bonds (see Fig. S3 in ESI†). This is similar to cooling of both polymorphs of N,N-dimethylglycine, where also only one of the two oxygen
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atoms per zwitterion involved in the formation of H-bonds.10 However, the delocalization of electron density within the carboxylate group did not change significantly upon cooling. The dependence of atomic displacement parameters of O1, O2 and N1 atoms upon cooling down to 100 K was smooth, revealing no phase transition as expected. In general, values of principal elements Uii are comparable with those of the three polymorphs of glycine in the same range of temperatures.27 The behavior of N1–H1n O1 and N1–H2n O1 hydrogen bonds upon cooling is shown in Fig. 2. As in the polymorphs of glycine,26,27 orthorhombic N,N-dimethylglycine,10 L-alanine,28 and DL-alanine,29 all the H-bonds in sarcosine become stronger upon cooling down to 100 K. So, the d(N1 O1) decreases by 0.60(9)% and 0.63(9)% for N1–H1n O1 and N1–H2n O1 hydrogen bonds, respectively, while the +(N–H O) does not change at all. The behavior of the H-bonds in sarcosine upon hydrostatic compression is different: the stronger N1–H1n O1 hydrogen bond compresses with increasing pressure, while the weaker N1–H2n O1 bond – expands.9 The elongation of the weaker H-bond up to a certain value results in the phase transition and the formation of a new, strong three-centered H-bond. Thus, the process of switching of the weak N1–H2n O1 hydrogen bond to the bifurcated one acts as a trigger for the reconstructive phase transition. Upon cooling, in contrast, this weak H-bond becomes stronger, and the ambient pressure phase is
Fig. 2 Changes in donor–acceptor distances of N1–H1n O1 (a) and N1–H2n O1 (b) hydrogen bonds. All curves are guides to the eye.
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becoming even more stable. This process also accounts for the different anisotropy of crystal structure compression upon cooling, as compared with that upon increasing pressure. Relative changes in the cell parameters of sarcosine upon cooling are plotted in Fig. 3. The structure is most rigid in the direction of zigzag head-to-tail chains formed by zwitterions along b and c crystallographic axes. In fact, the crystal structure of sarcosine even expanded slightly along the direction of the head-to-tail chains built by the shortest N1–H1n O1 hydrogen bond, i.e. along the crystallographic axis c (0.33(1)% upon cooling down to 100 K). In the case of cooling of the three polymorphs of glycine down to 100 K, the structure was the least compressible in the direction of straight head-to-tail chains, built by the strongest N–H O hydrogen bond in the structure ( 0.32(2)%, +0.08(1)% and 0.07(2)% for a-, b-, gglycine respectively).26,27 At the same time, the structure of sarcosine is compressed upon cooling down to 100 K by 0.24(2)% along the crystallographic axis b, which coincides with the direction of zigzag head-to-tail chains built by the longer N1–H2n O1 hydrogen bond. Taking into account that the changes of zwitterion conformation are non-significant, and that the changes in the N O distances are similar, one can assume that the only reason for different compression of zigzag head-to-tail chains along axes b and c is the different
Fig. 3 Relative changes in cell parameters of sarcosine (a) and betaine (b) with variations in temperature: black circles – crystallographic axis a; red rhombs – axis b; blue triangles – axis c. The standard deviations of values are smaller than the size of symbols. All curves are guides to the eye.
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geometry of these chains. The angles W and j between neighboring zwitterions in chains along axes c and b, respectively, are shown in Fig. 1. While the value of angle W increases from 98.59(2)1 at 295 K to 99.05(2)1 at 100 K, the value of angle j remains constant (89.85(2) at 295 K and 89.83(2) at 100 K). Thus, with the decrease of the intermolecular distance within the head-to-tail chains, the flattening of angle W between the zwitterions makes the zigzag chain along the crystallographic axis c more straight (and so the structure expands), while the constancy of the value of angle j causes the compression of the chain along axis b. A similar effect has been reported earlier for glycylglycine30a and paracetamol.30b The crystal structure of sarcosine is compressed mostly along the crystallographic axis a ( 2.25(1)% at 100 K). Such a large (B10 times) linear strain along a selected axis, as compared with all other directions, accounts for the collapsing voids between the zigzag head-to-tail chains. In general, the anisotropy of lattice strain of sarcosine upon cooling correlates well with that of a-, b-, and g-glycine,26,27 the monoclinic polymorph of N,N-dimethylglycine,10 L- and DL-alanine,28,29,31,32 L- and 33,34 DL-serine, where the least compression has been observed along the head-to-tail chains built by the shortest H-bonds. The largest compression is normal to these motifs. Additional and important information of intermolecular interactions in sarcosine was obtained from polarized Raman spectra. In contrast to the high pressure study,9 the analysis of Raman spectra upon cooling did not reveal any phase transitions upon cooling down to 5 K. The blue shift of the modes in the region of the lattice vibrations 80–140 cm 1 was very smooth and small (B1–2 cm 1 upon decreasing the temperature from
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ambient to 5 K). At the same time, in the region of stretching vibrations of C–H (2900–3100 cm 1), the modes shifted to the red region by B3 cm 1. As for skeleton vibrations [n(C–C), n(C–N)], their frequencies almost did not change upon cooling, since the zwitterion conformation was preserved. Even non-polarized Raman spectra of sarcosine measured upon cooling showed the non-linear increase of intensity for the bands near 2400 cm 1 (Fig. S4 in ESI†). In order to make a better assignment of the Raman bands, the spectrum of the deuterated sarcosine at ambient temperature was also recorded. In Fig. 4 one can see the shift of n(N–H) from 3252 cm 1 to 2217 cm 1, what corresponds to n(N–D) vibrations. At the same time, the band at 2432 cm 1 disappeared after deuteration, which enables us to claim that it includes the vibrations of the N–H group. In the polarized spectra bands at B2400–2450 cm 1 appear only in aa- and cc-polarization and should correspond to the strongest N1–H1n O1 hydrogen bond (Fig. 5). One can notice a strong temperature dependence of the intensities of bands in this region (Fig. S5 in ESI†). Previously a similar effect of non-linear increase of intensity for several modes at a spectral region of about 2500–2800 cm 1 was observed in polarized Raman spectra of L-, DL-alanine and all the three polymorphs of glycine upon cooling.25 In the case of L-alanine it was possible to assign these modes to different phonon bands. The observed phenomenon was interpreted as a manifestation of the formation of ‘self-trapped states’ of N–H O vibration, i.e. the coupling between the internal N–H vibrations and the external N O vibrations (or lattice vibrations). As proposed by Davydov,35 such a coupled state may appear and propagate along an infinite onedimensional chain, in which the molecules are inked via H-bonds.
Fig. 4 Raman spectra of sarcosine (bottom) and its deuterated analogue d2-sarcosine (top). The shift to the low-wavenumber region of some bands caused by deuteration is highlighted by dotted pink lines.
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Fig. 6 Temperature dependence of COO stretching asymmetrical vibrations (black rhombs) and NH bending vibrations (red circles) in sarcosine at bb polarization.
Fig. 5 Polarized Raman spectra of sarcosine at 295 K and 5 K in the range of coupled N–H stretching vibrations and lattice vibrations (a). The cc-Raman spectra of sarcosine at different temperatures (b). The definitions aa, bb and cc imply the directions of polarization vector of incident (first symbol) and scattered (last symbol) light with respect to crystal axes.
In the case of polymorphs of glycine, L-, and DL-alanine, this selftrapped state arises along the infinite head-to-tail chains linked via the shortest N–H O hydrogen bonds. Hitherto, self-trapped states were observed in polarized Raman spectra of amino acid crystals in a specific direction, along the infinite straight head-to-tail chains.25 In sarcosine the head-to-tail chains are not straight, but of zigzag type, and the directions of the corresponding H-bonds linking the molecules in these chains do not coincide with the direction of the chain as a whole. This may be the reason why the band at 2432 cm 1 is also present in the Raman spectrum of aa-polarization. For a comparison, the analysis of polarized Raman spectra of the two polymoprhs of N,N-dimethylglycine did not reveal any formation of self-trapped states. In DMG-I there are no molecular chains, but only isolated H-bonded cycles, whereas in DMG-II the H-bonds in the chains are not equivalent.10 Based on Gilli correlation for N–H O hydrogen bonds [n(N–H) vs. d(N O)],36 the mode at 3252 cm 1 agrees well with the geometric parameters of any of the two N–H O hydrogen bonds in sarcosine. Unfortunately, this mode appeared to be very broad and weak, and could not be assigned reliably to a certain H-bond in the structure, even if polarized spectra were analyzed. In contrast to Raman spectra at high pressure, the shift of this band upon cooling was poorly measurable, so that the nas(COO ) and d(N–H) vibrations were analyzed instead, to monitor the changes in the H-bonds. The bending N–H vibrations mode shifted smoothly to the higher wavenumber region upon cooling (1635 cm 1 at 295 K to 1645 cm 1 at 5 K), while
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the asymmetrical stretching COO vibrations shifted to the low wavenumber region (1605 cm 1 at 295 K to 1602 cm 1 at 5 K) (Fig. 6). These simultaneous shifts of the two modes can be explained by a decrease in the donor–acceptor distance and by the strengthening of the N–H O hydrogen bonds upon cooling. Effect of cooling on betaine Relative changes in the cell parameters of betaine upon cooling down to 100 K are plotted in Fig. 3. The compression of the structure is strongly anisotropic, the structure being the least compressible in the direction of head-to-tail chains along the crystallographic axis a ( 0.23(2)% at 100 K). The directions of the largest compression coincide with crystallographic axes b ( 1.59(1)%) and c ( 1.34(1)%), and are related to collapsing of voids between bulky trimethylamino fragments of the neighboring infinite chains. Similar to sarcosine, the molecular geometry of betaine does not change much upon cooling. While the main backbone of the sarcosine zwitterion becomes slightly more twisted, in betaine the mirror plane, comprising atoms of the main backbone, is preserved during cooling down to 100 K (and even down to 5 K what can be supposed on Raman spectra since there is no significant changes in stretching and bending vibrations of C–C, C–N and C–H). As for the carboxylate group of betaine, the C–O distances continuously expand upon cooling (0.8(1)% for C1–O1 and 0.7(1)% for C1–O2). Taking into account both the elongation of C–O distances and the compression of the crystal structure along the crystallographic axis a i.e. along the chain, one can conclude that the dipole–dipole interactions between the zwitterions within the chain get stronger upon cooling. The distance between the N atom of the +N–(CH3)3 fragment and the O1 atom of the COO group of the neighboring zwitterion within the chain decreases from 3.667(2) Å at 295 K to 3.635(2) Å at 100 K (N1 O1–C1 angle of B1631 does not change). The changes in the polarized Raman spectra of betaine were also continuous in the range from 300 K down to 5 K and
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did not reveal any phase transitions. The blue shift of the stretching C–H vibrations upon cooling down to 5 K did not exceed 1 cm 1. Moreover, the modes at 3020 cm 1, as well as at 3050 cm 1, became narrower upon cooling, and no additional modes appeared. This excludes the possibility of disordering of the bulky +N–(CH3)3 fragment around the N1–C2 bond. The asymmetric n(COO ) shifted from 1616 cm 1 at 300 K to 1612 cm 1 at 5 K, also proving the strengthening of interactions between the zwitterions through the whole experimental temperature range. These results differ radically from those observed previously with increasing pressure.9 The pressureinduced disorder of the bulky –N–(CH3)3 fragments in betaine rotating around the N1–C2 bond manifested itself by nonmonotonic behavior of the stretching C–H vibrations in the region of 2900–3100 cm 1.11 Above 1.5 GPa the band of the asymmetric n(COO ) vibrations shifted to higher wavenumbers, indicating that the repulsion contribution between the electron shells of the O-atom and methyl groups of the neighboring zwitterion increased. The analysis of the short contacts in the crystal structure, the shifts of the n(C–H) upon cooling, as well as the two-dimensional Hirshfeld fingerprints showed that there are no C–H O hydrogen bonds in the structure. As could be expected, since there are no N–H O hydrogen bonds in the crystal structure of betaine, no self-trapped states have been observed. Comparison of the effects of cooling and increasing pressure on the crystals of sarcosine and betaine The values of bulk compressibility of sarcosine and betaine upon cooling differed significantly (Fig. 7), as well as the values of isothermal compressibility upon hydrostatic compression. The thermal expansion of betaine is larger than that of sarcosine (3.14(3)% and 2.17(3)% for a temperature change between 100 K and 298 K, respectively). For a comparison, thermal expansion of the three polymorphs of glycine in the same temperature range is even less, than in sarcosine (1.78(3)%, 1.94(4)% and 1.38(3)% for a-, b-, g-glycines, respectively26,27). The thermal expansion
Fig. 7 Relative changes in cell volumes of sarcosine (black circles) and betaine (red rhombs) with variations in temperature. All curves are guides to the eye.
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of the orthorhombic polymorph of N,N-dimethylglycine is 2.27(1)%, which is smaller than in betaine, but larger, than in sarcosine. The values of thermal expansion of methylated derivatives of glycine are larger than that of glycine itself, and this trend also holds for isothermal hydrostatic compression. In other words ‘‘the introduction of the bulky –CH3–groups to the amino-group of the ‘‘main’’ head-to-tail chains makes the crystal structures more compressible compared with the polymorphs of glycine’’.9 The most interesting observation of this study is that the anisotropy of linear strain for crystal structures of betaine and sarcosine differed significantly upon cooling and upon increasing pressure, even when scaled to the same value of relative volume change (Fig. 8). The scaling factors relating isothermal compressibility and thermal expansion for sarcosine and betaine are different. Thus, in sarcosine, the change in cell volume at 100 K corresponds to B0.22 GPa, while in betaine the volume change at the same temperature corresponds to B0.4 GPa. Bulk coefficients of thermal expansion of sarcosine and betaine are 115(2) MK 1 and 165(5) MK 1, respectively. At the same time, bulk coefficients of isothermal compressibility are 116(1) TPa 1 and 42(5) TPa 1, respectively. In betaine, the linear strain along crystallographic axis a (direction of infinite head-to-tail chains) is the least one both upon cooling and upon increasing pressure, and this correlates well with the strengthening of dipole–dipole interactions within the chain. However, the relative compressibility along b and c was opposite upon cooling and with increasing pressure. The difference between relative changes of b and c cell parameters increases upon cooling. The compression along these directions is determined by collapsing of voids between + N–(CH3)3 fragments of the neighboring chains. One can suppose that this collapse occurs differently upon cooling and upon hydrostatic compression. In sarcosine the direction of the largest compression along the crystallographic axis a upon increasing pressure and decreasing temperature correlated quite well. Similarly, minimum strain was observed along the crystallographic axis c (the head-to-tail chains built by the shortest N1–H1n O1 hydrogen bonds) both upon variations of temperature and pressure. Either upon cooling down to 125 K, or upon increasing the hydrostatic pressure up to 0.19 GPa, the changes of the W angle in the chain were also similar. However, the compression along the head-to-tail chains built by the longer N1–H2n O1 hydrogen bonds (along the crystallographic axis b) differed radically upon cooling and with increasing pressure. While the orientation of the molecules within the chain remained the same upon cooling, as well as upon increasing pressure (angle j did not change much), the N1–H2n O1 hydrogen bonds shortened upon cooling, but elongated upon increasing pressure. The remarkable difference in the behavior of N1–H1n O1 and N1–H2n O1 hydrogen bonds upon variations of temperature and pressure accounts for the different origin of these bonds. While N1–H1n O1 is the stronger (or the ‘main’ [definition proposed in ref. 10]) H-bond, the N1–H2n O1 bond is the weaker (or the ‘additional’) one. The compression of each kind of bonds upon increasing pressure and upon cooling differs, and
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Fig. 8 Comparison of the anisotropy of lattice strain of sarcosine (left column) and betaine (right column) upon cooling and with increasing pressure. Relative changes in cell parameters versus temperature (open circles) and versus pressure (filled circles) are scaled in such a way that relative changes in cell volume coincide upon cooling and increasing of pressure. The data on relative changes of volume versus pressure have been taken from the previous work.9
a similar effect has been observed previously for the three polymorphs of glycine,26 L-,28,37 and DL- (ref. 29 and 38) alanine by X-ray diffraction, as well as by polarized Raman spectroscopy.25,39 In all of these structures the ‘main’ H-bonds behaved similarly upon either increasing pressure, or upon cooling, while the response of the ‘additional’ H-bonds to the two actions was different, and could also trigger the phase transitions.40
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Conclusions The strain anisotropy of the crystal structures of sarcosine and betaine upon cooling was different in several respects from that upon increasing pressure, similarly to what has been observed for the polymorphs of glycine itself. A common feature for betaine, sarcosine, and for most other a-amino acids was the
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minimum strain along infinite head-to-tail chains. The differences in the anisotropy have been observed in the plane normal to the direction of minimum strain and were related to the behavior of the weakest hydrogen bonds and to the different mechanisms of collapsing the voids in the structure. In sarcosine, both upon cooling, and with increasing pressure, the compression along the crystallographic axis a (collapsing of voids between the chains) was 10 times larger than that along other axes. At the same time, the chains of different type in sarcosine compressed differently upon decreasing the temperature and with increasing pressure: while the chains built by stronger H-bonds compressed similarly, the distortion of the chains formed by weaker H-bonds differed radically upon cooling and increasing pressure. In betaine, having no H-bonds in the structure, collapsing of voids between the trimethylamino fragments upon cooling and with increasing hydrostatic pressure also differed. Another important difference in the structural response of sarcosine and betaine to temperature/pressure variations is that the structural strain upon cooling was continuous, in contrast to the pressure-induced phase transitions. In this respect, the behavior of the two methylated derivatives of glycine was also similar to that of the three polymorphs of glycine itself.26 One could suppose that the distortion of the weaker H-bond in sarcosine plays the key role in the pressure-induced phase transition, and that the same H-bond becomes stronger and stabilizes the crystal structure upon cooling. Similarly, in the case of betaine the ‘‘weakest element’’ in the structure is the rotation of the trimethylamino fragment around the C2–N1 bond: upon increasing pressure these fragments become disordered and dipole–dipole interactions within the head-to-tail chain get weaker (as could be seen from shifting of n(COO ) to higher wavenumbers due to repulsion of electron shells of O-atom and methyl groups), whereas with decreasing temperature the rotation of +N–(CH3)3 fragments is reduced and electrostatic interactions within the chain strengthen (asymmetric n(COO ) vibrations shift to lower wavenumbers).
Acknowledgements We acknowledge financial support from RFBR (grant No. 12-0331145), from the Ministry of Education and Science of Russian Federation (project No. 1828), and from Russian Academy of Sciences.
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