A study of van der Waals complexes of 1,2-dichloroethane in paraffin oil by FTIR spectroscopy and ab initio calculations.

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx

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A study of van der Waals complexes of 1,2-dichloroethane in paraffin oil by FTIR spectroscopy and ab initio calculations A.I. Fishman ⇑, A.I. Noskov, R.M. Aminova, R.A. Skochilov Kazan Federal University, Institute of Physics, Kremlevskaya str. 18, Kazan 420008, Russia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

Weak interactions of 1,2-

dichloroethane in paraffin oil were studied. Conformers of 1,2-dichloroethane are involved in the complexation, forming tg-dimer. Structure of tg-dimer were determined from the DFT calculations. The complexation enthalpy was determined from FTIR spectra.

a r t i c l e

i n f o

Article history: Available online xxxx Keywords: Van der Waals complex FTIR spectroscopy Glass transition Isosbestic point Factor analysis 1,2-Dichloroethane

a b s t r a c t Weak molecular interactions of 1,2-dichloroethane dissolved in paraffin oil were investigated by FTIR spectroscopy. Occurrence of isosbestic points in the spectra along with the factor analysis showed that DCE DCE dimers are formed in solutions at DCE concentrations between 7 and 15 vol.%. It was found that both trans and gauche conformers are involved in the complexation, forming a tg-dimer. From the spectra collected at 200–222 K, the complexation enthalpy was determined: 4.2 ± 0.4 kcal mol1. The equilibrium geometry of tg-dimer and the vibrational frequencies were determined from the density functional calculations performed at B3LYP/6-311++G(d,p) and 6-31G(d,p) levels. The C–C bonds of the two molecules involved in tg-dimers were found to be oriented nearly perpendicular to each other. The complexation energy calculated using 6-31G(d,p) and 6-311++G(d,p) basis sets was found to be 1.59 and 1.52 kcal mol1, respectively. Ó 2014 Published by Elsevier B.V.

Introduction Molecular complexes have been attracting considerable attention due to the fact that intermolecular interactions play a crucial role in reaction and dissolution kinetics, adsorption thermodynamics [1,2], self-organization of molecules, and formation of supramolecular systems with non-typical physical and chemical properties [3]. Also it is known that intermolecular, in particular dispersive, forces govern major physical phenomena in solids [4]. ⇑ Corresponding author. Tel.: +7 9172696039. E-mail address: [email protected] (A.I. Fishman).

It has been reported that non-classical hydrogen bonds such as C–H X (X = N, O, Cl, etc.) significantly influence stability of metal complexes and the extension of the complex networks [5]. Such bonds need to be accounted for is engineering of crystals [6,7] and biomolecular systems [8]. C–H X bonds stabilize molecular conformation [9] which affects biological activity [10]. Such bonds are observed in synthetic receptors where they play an important role binding halogenated guests [11]. Significant progress in theoretical analyses of weak molecular interactions along with development of new experimental approaches in this area is due to works of different research groups, including the one headed by prof van der Veken [12–17].

1386-1425/$ - see front matter Ó 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.saa.2014.01.010

Please cite this article in press as: A.I. Fishman et al., A study of van der Waals complexes of 1,2-dichloroethane in paraffin oil by FTIR spectroscopy and ab initio calculations, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.01.010

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A.I. Fishman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx

It is known that alkyl chlorides can function as hydrogen bond acceptors [18,19]. The simplest representative of this class is 1,2dichloroethane (DCE, CAS no. 107-06-2). This chemical has been a subject of a considerable number of studies due to its hindered internal rotation and the nature of the potential barrier associated with this motion. DCE is a small flexible molecule which in liquid and solutions occurs in two stable conformations: gauche (g) and trans (t) [20,21]. It is well known that conformational equilibria in small molecules can be strongly affected by the environment, like solvent of solid matrix. This allows using conformationally mobile molecules as probes that are sensitive to confinement, for instance in zeolites and in inorganic mesoporous materials [22,23]. The sensitivity of the conformational dynamics to local mobility of the environment is the basics of so called ‘‘method of conformational probes.’’ This method allows studying local mobility in polymers and low-molecular-weight glass-forming liquids [24–26]. The approach is based on imbedding a small amount of a probe compound into the studied polymer or a glass-forming liquid (called hereafter ‘‘matrix’’) and following the temperature dependences of the probe’s conformationally-sensitive bands via FTIR spectra. Lowering the temperature may cause freezing-in of the molecular mobility of the matrix and this in turn may freeze-in the conformational transitions in the probe. From the behavior of FTIR band intensities, the freezing-in temperature, Tf, can be determined. Based on the ‘‘activation volume’’ of the probe (i.e., the minimum volume required for the conformational transitions to occur), one can estimate the size of the mobile units of the matrix which also freeze-in at Tf. DCE is suitable for use as a conformational probe for two main reasons: (i) it exhibits several strong infrared absorption bands of trans and gauche conformations that do not overlap each other; (ii) the barrier to internal rotation in the gas phase is relatively low (3 kcal mol1) [20]. In Ref. [27] the conformational mobility of DCE embedded into a glass forming matrix (paraffin oil) was investigated. It has been shown that the conformational mobility in DCE occurs below the glass transition temperature of paraffin oil. It has been also found that at DCE concentration exceeding 7 vol.%, features of complexation develop in the FTIR spectra. Of our interest was a more thorough investigation of the possible complexation of DCE. In particular it was of interest to reveal possible hydrogen bonding between DCE molecules. Since the complex formation of DCE in paraffin oil may freeze-in at lower temperatures, this would provide us an additional way of characterizing molecular mobility in the glass-forming matrix. Of special interest was a possibility of separating the diffusion of DCE molecules during the complex formation from the internal rotation in the same molecules. In this paper, FTIR spectra of DCE solutions in a glass-forming liquid (paraffin oil) were studied in a wide range of concentrations and temperatures. In our focus were complexes formed between DCE molecules that were proven to occur at certain conditions. Conclusions of the experimental study were supported by factor analysis and quantum chemical calculations of the electronic and spatial structure of the complexes along with the energy of the complexation. Experimental DCE was purchased from Sigma–Aldrich (CAS 107-06-2, assay P99%). Paraffin oil was used as a glass-forming liquid. It was a product of Cumberland Swan (NDC 0869-0831-43, purity 99.9%, refractive index nD = 1.478). These compounds were used without further purification. All spectra were recorded with a Bruker Tenzor-27 Fouriertransform spectrometer. The spectral resolution was 1 cm1, each

spectrum was averaged over 32 consecutive scans. The thickness of the absorbing layer was 50 lm. The sample with the solution of a given concentration was thoroughly stirred for at least 5 min. A standard cryostat was used for sampling in the temperature range of 100–300 K. The cryostat was cooled by liquid nitrogen. The cooling rate used in our experiments was in the range 0.5–1 K min1. The sample temperature was measured by a platinum thermometer with an accuracy of ±1 K. The spectra of the pure glass-forming liquids were subtracted from the spectra of the probe solutions, being collected at the same temperature. The areas under the analytical bands (i.e., integrated intensities) were determined using OPUS software supplied with the spectrometer. Quantum chemical calculations were used for modeling the structure of molecular complexes. The complex geometries were optimized within the framework of the density functional theory (DFT) [28,29] using Beckes three-parameter hybrid method [30] employing the correlation functional of Lee, Yang and Parr (B3LYP) [31], which includes both local and non-local terms with the 6-31G(d,p) and 6-311++G(d,p) basis sets. For correct description of weak noncovalent interactions, the polarization and diffuse functions were taken into account. The standard enthalpies of the monomers and dimers and their Gibbs free energies in the gas phase (T = 298.15 K, P = 1 atm) were calculated taking into account zero-point vibrational energies and corresponding thermal corrections to the electronic energies. All the DFT calculations were performed with the Gaussian 98 program package [32]. Results and discussion Vibrational spectra and stable conformations of DCE were extensively studied previously [21,33]. We obtained the infrared spectra of DCE solutions in paraffin oil at different concentrations of DCE. It was found that at concentrations higher than 7 vol.%, some absorption bands of both t- and g-conformations of DCE (1230 (t), 685 (g), 661 (g) cm1) start exhibiting an asymmetry, which becomes more pronounced upon increasing the concentration of DCE. As an example, Fig. 1 shows infrared spectra of the region 1245–1220 cm1 for DCE solutions in paraffin oil at different concentrations and the temperature of 201 K. The absorption band at 1230 cm1 is assigned to vibrations b(CCH) + d(HCCl) of t-conformation of DCE. Upon increasing the DCE concentration, a new absorption peak develops at the high-frequency side of the monomer band. The intensity of this new peak increases significantly with the concentration of the solution. Similar changes were also observed in other spectral regions. Significant redistribution of the doublet’s components is observed when the solution is cooled down from 222 to 200 K (Fig. 2). Such temperature changes were found to be reversible. The isosbestic points at frequencies of 1233, 659, 665, 682 cm1 (Fig. 2) are observed in the infrared spectra of DCE. The presence of an isosbestic point indicates that only two species are involved in the equilibrium. It is worth noting that the described above complications of the FTIR spectra were observed at temperatures below 220 K. At these temperatures, a small fraction of paraffin oil can turn into crystalline state [27]. To prove that the doublets observed in FTIR spectra are not associated with paraffin oil crystallinity, the temperature and concentration dependences of infrared bands of DCE were studied for its solutions in n-hexane (CAS no. 110-543). This study was performed at DCE concentrations of 10 and 15 vol.% and at temperatures in the range of 220–170 K. Fig. 3 shows infrared spectra of DCE solution in n-hexane (C = 15 vol.%) collected at different temperatures.



Fig. 1. Infrared spectra in the region 1245–1220 cm1 for DCE solutions in paraffin oil at different DCE concentrations. The DCE concentration, vol.%, is shown next to the curves. The spectra are normalized by the maximum absorbance of 1230 cm1 peak. All spectra were collected at T = 201 K.

It can be seen from Fig. 3 that in hexane solution, a shoulder is developed at the high-frequency side of the peak at 1230 cm1 (i.e., the behavior is the same as observed for solutions of DCE in paraffin oil). The intensity of this shoulder (observed at 1235 cm1) increases upon cooling the sample. At 170 K, crystallization of the solution was observed, so 170 K was the lowest temperature for this particular study (the melting point of pure n-hexane is 178 K). Comparing the spectral shape of DCE bands in n-hexane and paraffin oil solutions, it can be concluded that the doublet structure of the bands is mainly due to molecular complexes and not related to partial crystallization of paraffin oil. Furthermore, it follows that complexes of DCE are developed at temperatures 220–170 K in both solutions. For different kinds of complexes, their number of components and individual FTIR spectra can be obtained using factor analysis. This approach is based on analyzing a set of spectra collected at the same temperature but at different concentrations of the solute [34,35]. No a priory hypotheses regarding the band shapes or other spectral parameters are required. This procedure reduces to a

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Fig. 3. Details of the infrared spectra of DCE solution in n-hexane (C = 15 vol.%) at different temperatures. From top to bottom the temperature of the solution decreases from 220 to 170 K. – The absorbance band of n-hexane.

minimum the subjective factor, related to the choice of analytical wavelengths and separation of overlapping bands. The set of infrared spectra obtained at the same temperature and DCE concentrations of 8, 10, 13 and 15 vol.% was simultaneously analyzed. Such analyses were performed for the spectra collected at temperatures 201, 209, 212, 217, and 223 K. To realize this approach, a computer program has been compiled in Mathcad 15.0 [36]. The spectral ranges of 1220–1330 cm1 (peak of t-conformer at 1230 cm1, b(CCH) + d(HCCl)) and 630–690 cm-1 (peaks of g-conformer at 675 b 655 cm1, Q(C–Cl) [21,33]) were analyzed. The sets of spectra were formed separately for each of these spectral regions. Each column of the absorption matrix contained the values of the absorbance of the corresponding spectrum from the set. Thus the matrix A had the dimension of (m; n), where m is the number of spectral points (m = 493 for the range 630–690 cm1, m = 916 for the range 1220–1330 cm1), and n is the number of spectra in the set (n = 4). Let us consider the results obtained by analyzing the set of spectra at temperature of 201 K and the spectral range

Fig. 2. Details of infrared spectra of DCE solution in paraffin oil (C = 8 vol.%) at different temperatures. From top to bottom the temperature of the solution decreases from 222 to 200 K.


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1220–1330 cm1. The eigenvalues of the covariance matrix (N = ATA) were as follows: 33.23; 0.48; 2.6 103; 1.2 104. The last two eigenvalues had changed more than twice while the first two remained almost unchanged when a random noise was imposed on the spectrum. It can be concluded that the solution contains two components (factors). This conclusion is also confirmed by the method of reproducing the experimental spectra with a limited number of factors [34]. It can be seen from Fig. 4 that only two factors are necessary for a satisfactory recovery of the experimental spectrum. Using the approach described in [37], the individual spectra of the components and their concentrations at each temperature were identified. The experimental spectra can be well described by a superposing these data. Similar results were observed for the spectral range 640–690 cm1. Thus, occurrence of the isosbestic points along with the results of factor analysis show that in the concentration range between 8 and 15 vol.% there are two components in solutions: the monomers and molecular complexes of DCE. It was mentioned above that the absorption bands of both t- and g-conformations exhibit features of complexation. This means that the complex involves molecules in both conformations. Following Refs. [15,38,39], the stoichiometry of the complexes was derived by studying a series of isothermal spectra of solutions with different concentrations, varying between 7 and 15 vol.%. The plot of the intensity of infrared band of the complex (Dcom) versus the products ðDt Þm ðDg Þn , where Dt and Dg are the integrated intensities of infrared bands of the t- and g-conformations, respectively, was built. The best linear correlation was obtained for the 1:1 stoichiometry. Based on the observed spectral transformations, the dimers include both t- and g-conformers, which means that tgdimers are formed. It should be noted that in the spectra of solutions with concentration above 15 vol.% the structure of the absorption bands becomes more complicated. Factor analysis shows that a third component develops in those solutions. Thus at DCE concentrations above 15 vol.%, some new types of complex species have to be taken into account. When the temperature of a solution (C = 7 vol.%) lowers down from 222 to 198 K the monomer’s bands intensities decrease while the tg-dimer’s band intensities increase (Fig. 2). This is due to the fact that the monomer–dimer equilibrium shifts towards dimers when the temperature goes down. Two factors should be taken into account when interpreting the temperature changes.

(i) Molecules of DCE may be involved in two parallel reversible reactions: conformational transition t M g and dimerization reaction t + g M tg-dimer. (ii) The solution can transform into glassy state [27]. The intensity dependence of the absorption band of the dimer D1234 upon the temperature is shown in Fig. 5. Three temperature ranges can be distinguished in the plot: I. 300 K > T > 240 K. The concentration of the complexes is negligibly small. II. 240 K > T > 200 K. In this temperature range, the band intensity gradually increases upon cooling. This indicates that tgdimers develop and their fraction increases in solution. These changes occur down to temperature 202 K, which is close to the glass transition temperature Tg of paraffin oil (Tg = 200 K [27]). III. T < 200 K. Below Tg the intensity slowly changes with the temperature. The transition from range II to range III indicates that the dynamics of complex formation is ‘‘frozen’’, and the ratio of the concentrations of the monomers and dimers becomes constant. This is due to the fact that below the glass transition temperature, the solution viscosity is so high that DCE molecules lose their translational mobility. At the same time, the conformational flexibility of DCE is known to occur down to temperature Tf = 172 K [27]. The standard complexation enthalpy DHocom was determined via Van’t Hoff equation [40]. For this, the equilibrium constant must be expressed in terms of the infrared bands intensities and plotted against reciprocal temperature:

Dcom DHocom ¼ Ln þ const: Dt Dg RT

ð1Þ

Spectra of solution with the concentration C = 8vol.% were collected at temperatures between 223 and 200 K and used for the analysis. The absorption bands of g-monomer noticeably overlap with bands of tg-dimer, which complicates the determination of their intensities. To avoid this problem, we used the equation, connecting the intensities of the absorption bands of t- and gconformations:

Dg DH 0 ¼ Ae RT ; Dt

ð2Þ

Fig. 4. Reproduced (dots) and experimental spectra (solid lines) using one (a) and two (b) factors. The set was formed from the spectra of solutions with concentrations equal to 8, 10, 13 and 15 vol.%. T = 201 K.


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The calculations showed that in the tg-dimer, the C–C bonds of the two involved monomer molecules are nearly perpendicular to each other. The distances between the two carbon atoms to the unbound chlorine atoms are 2.77 Å and 2.72 Å for g-and t-conformers, respectively. The distances between the atoms of different conformers, calculated at B3LYP/6–311++G(d,p) level, are given in Table 1. The numbering of atoms is explained in Fig. 7. The equilibrium structure of the tg-dimer is shown in Fig. 7. The bond lengths and bond angles in the monomers and the dimer are given in Tables 2 and 3. As can be seen, the distance between the atoms Cl and H, belonging to different conformers, in most cases is close to the sum of their van der Waals radii (1.8 and 1.2 Å for Cl and H atoms, respectively [41]). This indicates that the stability of the tg-dimer is conditioned by cooperative effect of a number of weak non-covalent interactions. The sum of electronic and zero-point energies (a.e.) observed with 6-31G(d,p) and 6-311++G(d,p) basis sets and Fig. 5. The dependence of the absorption band intensity of the dimer (D1234) upon the temperature. C = 13 vol.%.

where DH0 = Hg Ht is enthalpy difference of the conformers and A is a constant. Eq. (1) can be rewritten as:

0

1

Dcom DHocom A ¼ Ln@ þ const: 0 H RT D2t exp DRT

ð3Þ

The magnitude of DH0 was determined from the analysis of the temperature dependence of the conformational equilibrium constant for solutions of DCE in paraffin oil at lower DCE concentrations (less than 5 vol.%), at which there are no signs of complex 0 1 formation. The obtained DH value was 0.52 ± 0.05 kcal mol . The dependences ln

D com

0

H D2t exp DRT

Table 1 The distances between atoms (r/Å), in the tg-dimer (numbering of atoms is shown in Fig. 7), calculated at B3LYP/6-311++G(d,p) level. Atoms

r

Cl3 Cl4 Cl4 C1 C1 H5 H5 H5

H13 H15 H13 H13 Cl12 Cl12 H13 C10

3.07 3.17 3.87 3.75 3.98 2.94 3.31 3.69

versus the reciprocal tempera-

ture obtained for the solution with concentrations C = 8 and 10 vol.% are shown in Fig. 6. Intensities of the peaks at 1234 cm1 (tg-complex) and 1230 cm1 (t-monomer) were used in the calculations. From the slope of the linear regression, the complexation enthalpy was determined: DHocom ¼ 4:2 0:4 kcal mol1. For more complete interpretation of the experimental data, ab initio calculations of the geometry and harmonic frequencies of the monomers (t- and g-conformations) and the tg-dimer were carried out. Fig. 7. The equilibrium structure of tg-dimer of DCE.

Table 2 Bond lengths, r/Å, for DCE monomers (t- and g-conformations) and tg-dimer. Bond

Monomer

tg-dimer

r

Fig. 6. The dependences ln 2 DcomDH0 upon reciprocal temperature. C = 8 and Dt exp RT 10 vol.%.

g

Cl4–C2 H8–C2 H7–C2 C1–C2 H5–C1 H6–C1 Cl3–C1

1.810 1.091 1.088 1.512 1.088 1.091 1.810

1.813 1.091 1.088 1.511 1.088 1.091 1.813

t

Cl12–C10 H15–C10 H16–C10 C9–C10 H13–C9 H14–C9 Cl11–C9

1.817 1.087 1.087 1.514 1.087 1.087 1.817

1.823 1.087 1.087 1.514 1.087 1.088 1.817


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Table 3 Bond angles, in degrees, for the DCE monomers (t- and g-conformations) and for the tg-dimer.

t

g

Table 5 Calculated and observed vibrational frequencies, in cm1, for t- and g-conformations (monomers) and tg-dimer of DCE.

Angle

Monomer

tg-dimer

Calculated frequencies

C10–C9–H13 C10–C9–H14 C9–C10–H15 C9–C10–H16 H13–C9–Cl11 H14–C9–Cl11 H15–C10–Cl12 H16–C10–Cl12

111.745 111.745 111.745 111.745 106.986 106.986 106.986 106.986

111.874 111.783 111.798 111.958 107.025 106.905 106.697 106.450

Monomer

C1–C2–H7 C1–C2–H8 C2–C1–H6 C2–C1–H5 H5–C1–Cl3 H6–C1–Cl3 H7–C2–Cl4 H8–C2–Cl4

111.663 109.213 111.663 109.213 106.349 106.964 106.964 106.349

111.860 109.222 109.180 112.012 106.878 106.165 106.626 106.264

the total atomic charges (calculated by Natural Bond Orbital Analysis) observed with 6-311++G(d,p) basis set are given in Table 4. As can be seen from Table 4, the combined effect of the weak noncovalent interactions leads to a redistribution of the electron density in the formation of tg-dimer.

t-conformer

Observed frequencies tg-dimer g-conformer 644 649 Q(C-Cl) 667 671 Q(C-Cl)

1265 b(CCH)+d(HCCl) 1271

653 661 673 685 1230 1234

The complexation energy was calculated as the difference between optimized full energies of the dimer and the monomer calculated with 6-31G(d,p) and 6-311++G(d,p) basis sets (Table 4) and corrected for Basis Set Superposition Error (BSSE) [42]. The calculated complexation energy was found to be 1.59 and 1.52 kcal mol1 for 6-31G(d,p) and 6-311++G(d,p), respectively. These values are in qualitative agreement with the experimental value of complexation enthalpy of 4.2 ± 0.4 kcal mol1 which was obtained for the condensed phase. In Table 5, some of the calculated (without scaling factor) and observed vibrational frequencies for t- and g-conformations (monomers) and the tg-dimer of DCE are shown. The calculated

Table 4 Sum of electronic and zero-point energies E(a.e.) in 6-31G(d,p) and 6-311++G(d,p) basis sets and total atomic charges (calculated by Natural Bond Orbital Analysis)in 6-311++G(d,p) basis set.

g-conformer (monomer)

t-conformer (monomer)

tg-dimer (see Fig. 7)

E = 999.040954 (6-311++G(d,p)) E = 998.963307 (6-31G(d,p)) Total atomic charges: C1 0.432938 C2 0.432943 H3 0.173730 H4 0.181919 H5 0.173731 Cl6 0.077290 Cl7 0.077289 H8 0.181921 E = 999.043422 (6-311++G(d,p)) E = 998,965815 (6-31G(d,p)) Total atomic charges: C1 0.502250 C2 0.502248 H3 0.180190 H4 0.180189 Cl5 0.141869 H6 0.180190 H7 0.180189 Cl8 0.141870 E = 1998.086800 (6-311++G(d,p)) E = 1997.931234 (6-31G(d,p)) Total atomic charges: C1 0.066132 C2 0.825502 Cl3 0.023354 Cl4 0.073563 H5 0.098281 H6 0.225630 H7 0.185864 H8 0.208300 C9 0.537757 C10 0.270912 Cl11 0.136356 Cl12 0.064487 H13 0.077590 H14 0.192491 H15 0.127477 H16 0.201353



values are in good agreement with the experimental data and correctly predict the trends in vibrational frequencies upon the dimerization: i.e., decreasing the frequency from 661 to 653 cm1 and from 685 to 673 cm1, as well as increasing the frequency from 1230 to 1234 cm1. Formation of intermolecular bonds and the related redistribution of the electron density is accompanied by weakening (or strengthening) of intramolecular bonds, which results in an increase (or decrease) of vibrational frequencies in the spectrum of the tg-dimer. Vibrations with the frequencies of 661 and 685 cm1 are assigned to the stretching vibrations Q(C–Cl) of g-conformer (Table 5). The calculation shows that the electron density in the complex is redistributed (Table 4) and the length of C–Cl bond increases from 1.810 to 1.813 Å, which probably leads to the lowerfrequency shift. The band at 1230 cm1 (b(CCH) and d(HCCl) of t-monomer) shifts to 1234 cm1 in the tg-dimer (Table 5). Redistribution of the electron density upon the dimerization results in changes of the bond lengths and also the bond angles. In that case it is difficult to find a correlation between these changes and the tendency of the shift of the vibrational frequency in the spectra. Acknowledgments We gratefully acknowledge financial support from the Ministry of education and science of the Russian Federation (Grant No. 2012-5.2-16-552-0002-190). The authors would also like to acknowledge Andrei Stolov for his help in preparing the manuscript. Dr. A.A. Stolov took this opportunity to send his best wishes to professor Ben van der Veken, in who’s lab he spent a fruitful year during 1990s. References [1] P. Hobza, P. Structure, Chem. Rev. 99 (1999) 3247–3276. [2] A.A. Nafikova, R.M. Aminova, A.V. Aganov, V.S. Reznik, J. Struct. Chem. 48 (2007) 71–85. [3] G.R. Desiraju, T. Steiner, The Weak Hydrogen Bond in Structural Chemistry and Biology, Oxford University, New York, 1999. [4] J. Klimes, D.R. Bowler, A. Michaelides, Phys. Rev. B 83 (2011) 195131. [5] Kyoung-Tae Youm, J. Ko, Moo-Jin Jun, Polyhedron 25 (2006) 2717–2720. [6] G.R. Desiraju, Crystal Engineering: The design of Organic Solids, Elsevier, New York, 1989. [7] Q. Gan, F. Li, G.P. Li, B. Kauffmann, J.F. Xiang, I. Huc, H. Jiang, Chem. Commun. 46 (2010) 297–299. [8] Y.X. Lu, Y. Wang, Z.J. Xu, X.H. Yan, X.M. Luo, H.L. Jiang, W.L. Zhu, J. Phys. Chem. B 113 (2009) 12615–12621. [9] B.Y. Lu, Z.M. Li, Y.Y. Zhu, X. Zhao, Z.T. Li, Tetrahedron 68 (2012) 8857–8862. [10] B.Y. Lu, Z.M. Li, Y.Y. Zhu, X. Zhao, Z.T. Li, Chem. Phys. Lett. 423 (2006) 131–137.

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