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Conformational Properties of cis- and trans-N-Cyclopropylformamide Studied by Microwave Spectroscopy and Quantum Chemical Calculations Svein Samdal, Harald Møllendal, and Jean-Claude Guillemin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b00542 • Publication Date (Web): 16 Mar 2015 Downloaded from http://pubs.acs.org on March 20, 2015

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The Journal of Physical Chemistry

Conformational

Properties

of

cis-

and

trans-N-

Cyclopropylformamide Studied by Microwave Spectroscopy and Quantum Chemical Calculations

Svein Samdal,ϯ Harald Møllendal,*,ϯ and Jean-Claude Guillemin‡

ϯCentre

for Theoretical and Computational Chemistry (CTCC), Department of Chemistry,

University of Oslo, P. O. Box 1033 Blindern, NO-0315 Oslo, Norway

‡Institut

des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes,

CNRS, UMR 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France

Received: January 19, 2015

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ABSTRACT:

The

microwave

spectra

of

cis-

and

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trans-N-cyclopropylformamide,

C3H5NHC(=O)H, have been investigated in the 31 – 123 GHz spectral region at room temperature. Rotational isomerism about the Cring‒N bond is possible for both cis and trans. MP2/cc-pVTZ and CCSD/cc-pVTZ calculations indicate that there are two conformers in the case of cis, called Cis I and Cis II, while only one rotamer, denoted Trans, exists for trans-Ncyclopropylformamide. The quantum chemical methods predict that Cis I has an electronic energy that is 8 – 9 kJ/mol higher than the energy of Cis II. The CCSD H‒Cring‒N‒H dihedral angle is 0.0º in Cis I, 93.0º in Cis II and 79.9º in Trans. The CCSD and MP2 calculations predict a slightly non-planar structure for the amide moiety in both Trans and Cis II, whereas Cis I is computed to have a planar amide group bisecting the cyclopropyl ring. Surprisingly, the MP2 and CCSD methods predict practically the same energy for Trans and Cis II. The spectra of Cis II in the ground state and in two vibrationally excited states were assigned, while the spectrum of Cis I was not found presumably because of a low Boltzmann population due to a relatively large energy difference (8 – 9 kJ/mol). The spectra of the ground vibrational state and seven vibrationally excited states of Trans, were assigned. Vibrational frequencies of several of the excited state of both Cis II and Trans were determined by relative intensity measurements. The experimental and CCSD rotational constants are in satisfactory agreement. The MP2 values of the quartic centrifugal distortion constants of both species are in relatively poor agreement with their experimental counterparts. The MP2 vibration-rotation constants and sextic centrifugal distortion constants have little resemblance with the corresponding experimental values.

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Key words: Rotational constants, centrifugal distortion constants, vibration-rotation constants, vibrationally excited states, MP2 calculations, CCSD calculations, vibrational frequencies, radiofrequency-microwave double resonance spectroscopy

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INTRODUCTION

The amide group is a crucial structural element in proteins and many other biomolecules. The chemical and physical properties of amides are consequently of great interest both for chemistry and for biology. Not surprisingly, a large review literature exists for this functional group.1-12 Amides are often crystals or liquids with low sublimation or vapor pressures, which is at least partly due to extensive ubiquitous intermolecular hydrogen bonding in condensed phases of amides. Hydrogen bonding is relatively strong in most members with this functional group and has a significant influence on structural and conformational properties of crystalline and liquid amides. X-ray crystallography has provided a large number of accurate structures of solid amides. However, structural and conformational properties of gaseous monomeric amides are much less well known. This is mainly due to their low volatility and the fact that they often decompose upon heating, which make them less prone to experimental gas-phase studies. It is desirable to investigate monomeric amides in the gas phase to obtain the best possible insight in the true, unperturbed structural and conformational properties of this important class of compounds. Such investigations can best be performed using microwave (MW) spectroscopy or gas electron diffraction (GED) in combination with advanced quantum chemical calculations. We have for these reasons for a long time taken an interest in gas-phase studies of them. Our previous MW and GED studies include cis- and trans-N-vinylformamide (H2C=CHNHC(=O)H),13 acetamide

(CH3CONH2),14

2-fluoroacetamide

(CH2FCONH2),15,16

2-chloroacetamide

(CH2ClCONH2),16,17 2-iodoacetamide (CH2ICONH2),18 2,2-difluoroacetamide (CF2HCONH2)19 2,2-dichloroacetamide (CHCl2CONH2),20 2-chloro-2,2-difluoroacetamide (CF2ClCONH2),21 2,2,2-trifluoroacetamide (CF3CONH2),22 2,2,2-trichloroacetamide (CCl3CONH2),23 propionamide 4 ACS Paragon Plus Environment

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(CH3CH2CONH2),24 formic hydrazide (H2NNHCHO),25 acrylamide (H2C=CHCONH2),26 methyl carbamate (CH3OCONH2),27,28 methoxyacetamide (CH3OCH2CONH2),29 and 2-azetidinone.30,31 Experimental contributions from other laboratories include, for example, the prototype formamide (HCONH2),32-34 urea (H2NCONH2),35 cis- and trans- N-methyl formamide (HCONHCH3),36 N,N-dimethylformamide (HCON(CH3)2),37 C‒N‒C‒O-cis-N-ethylformamide (HCONHCH2CH3),38 O‒N‒C‒O-trans-methoxyformamide (HCONHOCH3),39 C‒N‒C‒O-cisformanilide

(C6H5NHCHO),40,41

acetamide,42-45

C‒N‒C‒O-cis-N-methylacetamide

(CH3CONHCH3),46 C‒N‒C‒O-cis-N-methylpropionamide (CH3CONHCH2CH3)47 C‒N‒C‒Otrans-acetanilide (CH3CONHC6H5),48 and alaninamide (H2NCHCONH2).49 Rotation isomerism is possible in several of the compounds listed above, but acrylamide is the only example of an amide where the MW spectra of more than one rotamer have been assigned.26 Amide studies are hereby extended to include the first MW investigation of the spectra of cis- and trans-N-cyclopropylformamide, (C3H5NHC(=O)H), where cis and trans refer to the orientation of the Cring‒N‒C‒O dihedral angle, which is about 0º in cis and approximately 180º in trans. A nitrogen-15 NMR study of the composition of cis- and trans-N-cyclopropylformamide produced by heating ethyl formate and aminocyclopropane showed that 18 % cis and 82 % trans are formed in this manner.50 The fact that trans predominates was ascribed to steric repulsion between the cyclopropyl and carbonyl groups, which was assumed to be much more important in cis than in trans due to the close proximity of the two groups.50 Rotation about the carbon-nitrogen single bond joining the cyclopropyl and formamide moieties may in principle produce rotational isomerism, but this question was not dealt with in the NMR work.50 Quantum chemical calculations reported below found that two conformers may exist for cis-, whereas only one conformer was predicted for trans-N-cyclopropylformamide. The 5 ACS Paragon Plus Environment

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three forms have been denoted Cis I, Cis II and Trans. Models of them, with atom numbering indicated on Cis I, are shown in Figure 1. The dihedral angle associated with the H6‒C2‒N9‒ H10 chain of atoms may conveniently be used to describe rotational isomerism in the two isomers of N-cyclopropylformamide. In Cis I, this dihedral angle is exactly 0º, while this angle is 93.0º in Cis II and 79.9º in Trans according to CCSD/cc-pVTZ computations discussed below. MW spectroscopy is an ideal method to investigate rotational isomerism due to its superior accuracy and resolution and this method was therefore chosen for this study. The MW work has been augmented by advanced quantum chemical calculations, which provide both spectroscopic parameters that are useful for the assignment of the MW spectrum and also make available valuable information not obtainable from experiment.

EXPERIMENTAL SECTION

Synthesis and Spectroscopic Experiments. N-cyclopropylformamide, which was synthesized and purified as described by Gate et al,51 was found to have a cis/trans ratio that was very dependent on the solvent. The percentage cis determined by 1H NMR was in D2O: 15%, CD3OD: 16%, DMSO-d6: 16%, (CD3)2CO: 21%, CD3CN: 22%, C6D6: 31%, CDCl3 36%. Ncyclopropylformamide is a colorless liquid with a vapor pressure of roughly 30 Pa at room temperature. Fumes of this liquid were admitted to the MW cell. The amount of cis and trans seemed to be roughly equal in the gas phase according to the MW spectrum. The compound was kept in a refrigerator at roughly 4º C when not in use. No decomposition or polymerization of the sample was observed.

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The MW spectrum was recorded at room temperature at a pressure of 5 – 10 Pa using the Stark-modulation spectrometer of the University of Oslo described in details elsewhere.52 It is only mentioned here that this device has a resolution of about 0.5 MHz and measures the frequency of isolated transitions with an estimated accuracy of ≈ 0.10 MHz. The spectrum was investigated in the whole 31 – 123 GHz frequency interval. Radio-frequency microwave doubleresonance experiments (RFMWDR), similar to those of Wodarczyk and Wilson53 were also undertaken. This technique makes it possible to assign unambiguously particular transitions. The RFMWDR equipment is described elsewhere.52

RESULTS AND DISCUSSION Quantum Chemical Calculations. The present frozen-core MP254 and CCSD55-58 calculations were performed employing the Gaussian 0959 and Molpro60 programs running on the Abel cluster in Oslo. Dunning’s61 correlation-consistent cc-pVTZ triple- basis set was used in the calculations. The CCSD computations were undertaken using Molpro, while Gaussian 09 was used for the MP2 calculations. The default convergence criteria of the two computer programs were observed. MP2/cc-pVTZ potential functions for rotation about the C2‒N9 bond were calculated for the cis and trans forms in order to localize possible conformer(s). The H6‒C2‒N9‒H10 dihedral angle was stepped in 10º intervals in these calculations while all other structural parameters were allowed to vary freely. The resulting potential functions are drawn in Figure 2 (cis) and Figure 3 (trans). The two functions are remarkably different. The cis function has two minima, while the trans function has only one minimum. The barrier heights are significantly lower in cis

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(approximately 12 and 7 kJ/mol) compared to about 20 and 12 kJ/mol in trans. The different potential functions are not surprising given that the highly electronegative oxygen atom (O12) will be in rather close contact with the cyclopropyl ring during most of the rotation about the C2‒ N9 bond in cis, whereas the electropositive hydrogen atom (H13) takes this role in trans. This should indeed result in different non-bonded interactions in the two cases resulting in dissimilar potential functions. Optimized MP2 structures, dipole moments, harmonic and anharmonic vibrational frequencies, quartic and sextic Watson centrifugal distortion constants,62 vibration-rotation constants (the 's),63 and the differences between the equilibrium and the effective ground-state rotational constants were computed for the two conformers corresponding to the two minima of the cis form and the one minimum of the trans configuration. The centrifugal distortion constants and the 's were calculated observing the precautions of McKean et al.64 The results are given in Tables S1 (Cis I), S3 (Cis II), and S7 (Trans) of the Supporting Information. The MP2 H6‒C2‒N9‒H10 dihedral angle is 0.0º in Cis I (Table S1) and 92.9º in Cis II (Table S3). The amide moiety is planar in Cis I (Table S1) and bisects the cyclopropyl ring. This group is slightly non-planar in Cis II (Table S3) and in Trans (Table S7). The electronic energy difference between Cis I and Cis II is 8.61 kJ/mol, with Cis II as the lower-energy conformer. Correction for zero-point vibrational effects (Table S1 and S3) yields 8.87 kJ/mol for this energy difference. The H6‒C2‒N9‒H10 dihedral angles of the two maxima (transition states) of the cis potential function (Figure 2) were calculated to be 42.5º (Table S5) and exactly 180.0º (Table S6), respectively. The corresponding electronic energies are 12.03 and 6.99 kJ/mol above the energy of the Cis II rotamer.

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In trans, the MP2 minimum of the potential function occurs at 80.4º for the H6‒C2‒N9‒ H10 dihedral angle, 12.5º smaller than in Cis II (92.9º; see above). The maxima are located at 0º, 19.54 kJ/mol higher than the electronic energy of the Trans conformer (Table S9), and at 180º, 11.44 kJ/mol higher (Table S10). The energy of Cis II corrected for harmonic vibrational zeropoint energy (Table 3) is 0.69 kJ/mol lower than the corresponding energy of Trans (Table S7), a remarkably small energy difference. CCSD/cc-pVTZ computations of optimized structures, dipole moments and electronic energies of the cis and trans conformers were carried out. Unfortunately, it is not possible to calculate vibrational frequencies and centrifugal constants at the CCSD level given our present computational resources. The full CCSD structures of Cis I, Cis II, and Trans are listed in Tables S2, S4, and S8 of the Supporting Information, respectively. A selection of important structural parameters of the three forms is repeated in Table 1, which also contains the rotational constants calculated from these structures. The MP2 quartic centrifugal distortion constants in the S-reduction form62 taken from Tables S1, S3, and S7, and the CCSD principal inertial axis dipole moment components are also given in Table 1. The CCSD results warrant comments: Cis II is predicted to be 9.54 kJ/mol lower in electronic energy than Cis I (from entries in Tables S2 and S4). This energy difference is more than 1 kJ/mol larger than the MP2 electronic energy difference (see above). The CCSD H6‒C2‒ N9‒H10 dihedral angles (Table 1) are 0º in Cis I, just as in the MP2 case, 93.0º in Cis II, and 79.9º in Trans. The two last values are close to the MP2 results above. The electronic energy of Cis II, is only 0.17 kJ/mol lower than the energy of Trans, similar to 0.69 kJ/mol found above in the MP2 calculations. This surprisingly small energy difference suggests that cis and trans should be formed in about equal amounts provided the reaction was thermodynamically controlled. 9 ACS Paragon Plus Environment

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However, the previous NMR work reported that only 18% cis was formed in the synthetic procedure they chose,50 while 15 – 36 % were found in the solvents mentioned above. It is concluded that factors other than the energy difference definitely play a role for the cis/trans composition of this compound. The equilibrium structure of the amide group is generally non-planar.65 Formamide, the prototypical amide, is planar, but this is an exception.65 The formamide moiety of the title compound is planar and bisects the cyclopropyl ring in Cis I according to CCSD. It is slightly non-planar in Cis II, and rotated 93.0º about the C2‒N9 bond relative to its position in Cis I. The rotation about the C2‒N9 is 79.9º in Trans (Table 1), in good agreement with the MP2 findings. The amide group is non-planar in Trans too. Moreover, the equilibrium C=O bond length is 120.97 pm and the C‒N bond length is 135.47 pm in formamide,65 about the same as the CCSD predictions for the corresponding bond lengths in the three forms of N-cyclopropylformamide (Table 1). In the cyclopropyl moiety, the C1‒C3 bond length varies between 151.5 (Cis I) and 150.5 pm (Trans), while C1‒C2 and C2‒ C3 vary between 149.8 and 150.5 pm. The C‒C equilibrium bond length in cyclopropane is 150.30(10) pm66 for comparison. Microwave Spectrum and Assignment of the Ground-State Spectrum of Trans. Survey spectra revealed a dense spectrum with absorption lines occurring every few MHz throughout the entire MW region. A prominent feature of the spectrum was lumps of strong transitions occurring at intervals of about 3.71 GHz. This behavior is typical for a-type R-branch transitions of an asymmetric rotor having Ray's asymmetry parameter67  close to ‒1, as well as a relatively large dipole moment component along the a-inertial axis. It is seen from Table 1 that

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this is indeed the case for the Trans conformer, where B + C = 3.7117 GHz,  = ‒0.977, and a = 3.94 D. The assignment of the spectrum of the ground vibrational-state aR-transitions was thus straightforward due to these precise predictions. The assignments of several selected transitions were checked by RFMWDR experiments providing unequivocal proofs of their assignments.53 The RFMWDR spectrum of a portion of the 204 ← 194 shown in Figure 4 is a typical example, displaying the spectra of the ground state and of four vibrationally excited states. The 14N nucleus has a small quadrupole moment, but none of the transitions displayed clearly resolved quadrupole splittings caused by this nucleus. The spectrum was least-squares fitted to Watson's S-reduced Hamiltonian in the Irrepresentation using Sørensen's program Rotfit.68 A total of 413 aR-transitions with Jmax = 32 and K‒1max = 25 listed in Table S11 of the Supporting Information were employed to obtain the spectroscopic constants listed in Table 2. It was possible to determine all quartic centrifugal distortion constants but DK, which was preset at the MP2 value, 65.1 kHz, (Table 7S) in the least squares fit. Two of the sextic distortion constants, HJK and HKJ were also fitted. Further sextic constants with standard deviations less than their absolute values could not be obtained and they were therefore preset at zero in the least-squares fit. The hypothetical frequencies of b- and c-type transitions can be quite accurately predicted using the spectroscopic constants shown in Table 2. However, no unambiguous assignments could be made presumably because these transitions are much weaker than the a-type transitions due to the fact that a is much larger than the two other dipole moment components (Table 1).

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The high spectral density with the occurrence of frequent overlapping transitions and Stark lobes was another severe obstacle for obtaining definite assignments of b- and c-type lines. Equilibrium (re) rotational constants are normally larger than effective (r0) rotational constants. The MP2 method (Table S7) predicts that the equilibrium values of the A, B, and C rotational constants of Trans are larger than the observed values by 111.66, 15.72, and 13.54 MHz, respectively, which is typical for the performance of this method. Comparison of the CCSD rotational constants (Table 1) with the ground-state constants (Table 2) shows that the CCSD A rotational constant 227.2 MHz larger than the ground-state equivalent, while B and C are smaller by 3.3 and 1.8 MHz. This is an indication that although the CCSD structure of Trans is very likely close to the equilibrium structure, there is still room for improvement. Unfortunately, more comprehensive quantum chemical calculations than those reported here could not be undertaken due to the limited resources available. The experimental quartic centrifugal distortion constants (Table 2) deviate from the MP2 constants (Table 1) by about 13% for DJ, 3% for DJK, and 2% for d1, while d2 deviates by much more, but this constant has a relatively much larger uncertainty than DJ, DJK, and d1. The MP2 sextic HJK and HKJ constants listed in Table S7 deviate very much from their experimental counterparts (Table 2). The reason for this is assumed to be the poor quality of the MP2 anharmonic part of the force field, which is discussed further below in connection with the vibration-rotation constants of the vibrationally excited states. Vibrationally Excited States of Trans. The MP2 calculations (Table S7) predict that the five lowest anharmonic fundamental vibrational frequencies are 90, 147, 201, 382, and 452 cm‒1. Further fundamentals are above 550 cm‒1. The Boltzmann factors relative to the ground

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vibrational state are thus 0.64, 0.49, 0.37, 0.15, and 0.11 for these vibrations. Spectra of several vibrationally excited states were therefore expected to be present with notable intensities. This was indeed observed. RFMWDR spectra revealed the existence of at least 7 vibrationally excited state spectra in addition to the ground vibrational state spectrum. The first assignments of transitions of vibrationally excited states were obtained from the RFMWDR spectra such as the one shown in Figure 4 and subsequently extended to include additional transitions from the Stark spectrum. A total of seven vibrationally excited states were ultimately assigned. The aR-spectra are listed in Tables S12 – S18 of the Supporting Information and the spectroscopic constants are collected in Table 2. The quartic centrifugal distortion constants, with the exception of DK, were determined for most of the excited-state spectra. d1 and d2 were not obtained for three excited states and were preset at the ground-state values in the least-squares fit. It is not always obvious to which vibrationally excited state each of these excited-state spectra belongs. Comparison of experimental and theoretical -vibration-rotation63 constants as well as vibrational frequencies can be useful for making assignments. In Table 3, we have listed the -constants of the seven excited-state spectra. These values have been found by subtraction of the excited-state rotational constants from their ground-state counterparts. Relative intensity measurements yielded 82(20) cm‒1 for the strongest excited state listed in Table S12. This value is close to the MP2 result (90 cm‒1, Table S8) for the lowest bending vibration. The -values of Table 3 for this vibration are A = 55.3(17), B = 0.8179(48), and C = ‒2.0183(45) MHz, in quite poor agreement with the MP2 (Table S7) results, 85.02, ‒0.86, and ‒3.47 MHz, respectively.

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The spectrum of what is assumed to be the second excited state of this mode (Table S13) was also assigned. αB and αC are almost twice as large as those of the first excited state (Table 3). However, αA (63.1(40) MHz) is not much different from that of the first excited state (55.3(17) MHz). It is therefore concluded that the lowest bending vibration is very anharmonic. An excited-state spectrum assumed to be the first excited state of the torsion about the C2‒N9 bond is shown in Table S14. Its frequency is 138(25) cm‒1 according to relative intensity measurements, compared to 147 cm‒1 (MP2 result) for the lowest torsional vibration. The αvalues of Table 3 (‒36.9(20), 5.7674(44), and 2.7268(42) MHz) are again in poor agreement with theory (‒93.05, 7.05, and 4.43 MHz; Table S7). Very tentative assignments are suggested for the second, third, and fourth excited states of this vibration, whose spectra are listed in Tables S15 – S17. The vibration-rotation constants of these modes (Table 3) indicate that the torsional mode has a very anharmonic nature. We have also assigned the spectrum (Table S18) of what we believe is the first excited state of the lowest bending vibration calculated to have a frequency of 201 cm‒1, compared to ca 210 cm‒1 from relative intensity measurements. There is poor agreement between the MP2 (last column of Table 2) and the corresponding experimental α-values in Table S7 in this case as well. The overall poor performance of MP2/cc-pVTZ calculations of vibration-rotation αvalues seen in the case of Trans indicates that this computational method, which is very demanding since computations of third derivatives are involved, is still too primitive. Unfortunately, calculations at higher levels of theory of these constants are beyond our present computational possibilities.

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Searches for the Spectrum of Cis I. The energy of this rotamer is 8 – 9 kJ/mol higher than the energy of Cis II according to the quantum chemical calculations above. This indicates that this form should be present in a very low concentration (Boltzmann factor less than 0.04 relative to the ground vibrational state of Cis II, which means that the gas consists of more than about 96 % of Cis II and less than approximately 4 % Cis I). However, the rather sizable b = 3.6 D should produce a relatively strong spectrum. A search for the b-type spectrum of this form was performed but no assignments could be made. It is concluded that Cis I in all likelihood is a high-energy form of cis-N-cyclopropylformamide. Assignment of the Spectrum of Cis II. The spectroscopic constants in Table 1 were used to predict the spectrum of aR-transitions. Several of these were easily assigned in the RFMWDR spectrum in the same manner as described above for Trans. The frequencies of further transitions of this category could now be predicted fairly accurately and they were assigned in a straightforward manner and included in the least-squares fit. a-type lines with a maximum value of Jmax = 27 (Table S19) were ultimately assigned and used to predict the frequencies of b-type lines with J < 20. These transitions were found close to their predicted frequencies. Additional btype lines including higher and higher values of J were gradually assigned and included in the fit. Ultimately, 836 transitions of the a- and b-type varieties with Jmax = 85 and K‒1max = 20 listed in Table 19S of the Supporting Information were used to determine the spectroscopic constants displayed in Table 4. Transitions with J > 85 were too weak due to an unfavorable Boltzmann factor and could not be assigned. c-type lines, whose hypothetical frequencies can be very accurately predicted from the spectroscopic constants in Table 4, were searched for but not assigned. This is consistent with a small c predicted to be only 0.31 D (Table 1) producing insufficient intensities. 15 ACS Paragon Plus Environment

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It is seen from Table 4 that very accurate values have been obtained for the rotational and quartic centrifugal distortion constants. Five of the seven sextic centrifugal distortion constants, HJ, HJK, HKJ, hJ, and hJK, were determined. Values of HK and hK significantly different from zero could not be obtained and these constants were preset at zero in the least-squares fit. A comparison of the rotational constants reveals that the CCSD constants (Table 1) are larger than the experimental equivalents (Table 4) by 0.1, 21.5, and 9.6 MHz in the cases A, B, and C, respectively. The MP2 method (Table 3S) predicts 66.10, 18.88, and 14.01 MHz for these differences and it is concluded that the CCSD structure of Cis II (Table 1) is subject to the same rather minor deficiencies discussed above for Trans. The experimental quartic centrifugal distortion constants (Table 4) are all larger than the MP2 constants (Table 1). The smallest difference is seen for DJ (7.7 %) and the largest for d2 (46.1 %), a disappointing result. It comes as no surprise that the experimental sextic centrifugal distortion constants (Table 4) deviate so much from their MP2 equivalents to render a comparison meaningless. Vibrationally Excited States of Cis II. The MP2 anharmonic frequencies of the lowest vibrational fundamentals of this species are 55, 176, 225, 368, 407, and 504 cm‒1 (Table S3). RFMWDR spectra revealed the existence of several vibrationally excited state spectra in addition to the spectrum of the ground vibrational state. The two strongest of these were assigned in the same manner as described above for the ground vibrational state spectrum. 580 transitions with Jmax = 65 were assigned for what is assumed to be the first excited state of the torsion about the C2‒N9 bond, while 366 transitions with Jmax = 39 were assigned for the first excited state of what is supposed to be the lowest bending vibration. The spectra are listed in Tables S20 and S21,

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respectively, of the Supporting Information and the spectroscopic constants are listed in Table 4. The same five sextic centrifugal distortion constants as determined for the ground state were also obtained for the first excited torsional state. Only quartic centrifugal distortion constants are given for the first excited bending vibration. Inclusion of sextic constants was attempted in the fitting of this spectrum, but relatively large standard deviations were obtained and it was concluded that sextic constants of physical significance cannot be obtained from the spectroscopic material in Table S21. Relative intensity measurements yielded 74(20) cm‒1 for the torsion and 176(25) cm‒1 for the lowest bending vibration compared to 55 and 176 cm‒1, respectively (Table S3). The rotationvibration constants calculated from the entries of Table 4 (not given here) were very different from those shown in Table S3 and it is concluded that the MP2 calculations are too primitive to reproduce well the -constants in this case, just as in the case of Trans above. The energy of the first excited state of the lowest bending vibration [176(25) cm‒1] is the highest energy of the cis configuration observed experimentally. The Boltzmann factor of the ground vibrational state of Cis I (discussed above) is 0.08 relative to this excited state. This is another illustration of how weak the spectrum of the hypothetical conformer Cis I is expected to be.

DISCUSSION The reason for the existence of two rotameric forms for cis-N-cyclopropylformamide is probably a complicated compromise of several forces. In Cis I, which has a symmetry plane, the distances between the oxygen atom O12 and the two hydrogen atoms H5 and H8 of the ring are 17 ACS Paragon Plus Environment

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identical. This non-bonded distance is 252.3 pm (Table S2) compared to 260 pm, which is the sum of the Pauling van der Waals radii69 of hydrogen (120 pm) and oxygen (140 pm). This oxygen-hydrogen contact, which may be called a weak intramolecular hydrogen bond, is assumed to stabilize Cis I. There are also repulsive forces that are likely to destabilize Cis I. One such force is the interaction between the two hydrogen atoms H6 and H10. The non-bonded separation between these two atoms is relatively short (227.0 pm; Table S2), while twice the van der Waals radii of hydrogen is 240 pm. Another force that may destabilize Cis I is the interaction between the lone pair electrons of O12 and the pseudo-π electrons70 of the cyclopropyl ring. Interaction between the π-electrons of the amide group and the ring pseudo-π electrons could have a similar effect. The situation in Cis II is quite different from that of Cis I. In Cis II, there is no repulsion between H6 and H10 and possibly attraction between O12 and H6, which are separated by 310 pm (Table S4), as well as a weak intramolecular hydrogen bond between H10 and the pseudo-π electrons along the C1‒C2 bond, because the non-bonded distances between H10 and C2 is 210.5 pm and the non-bonded distance between H10 and C1 is 281.1 pm (Table 4S). The O12 atom is further away from the ring in this case than in Cis I, which may result in less repulsion. It appears that there are more important repulsive forces in Cis I than in Cis II, which may be the reason why Cis II is 8 – 9 kJ/mol lower in energy than Cis I. In the Trans conformer, there are also non-bonded contacts that seem to be of importance. The non-bonded distance between H13 and C2 is 257.7 pm. The distance between H13 and C3 is 287.9 pm, whereas H10 and C1 is separated by 289.8 pm (Table S8). The van der 18 ACS Paragon Plus Environment

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Waals half-thickness of aromatic carbon is 170 pm.69 The sum (290 pm) of this parameter (170 pm) and hydrogen (120 pm) is longer than the aforementioned non-bonded distances and this is indicative of stabilization by internal hydrogen bonding between H10 and H13 on the one side and the pseudo- electrons of the ring on the other. It is again difficult to assess the importance of a possible interaction between the -electrons of the amide group and the pseudo-π electrons of the cyclopropyl ring. However, rotation about the C2‒N9 bond away from the minimum position will presumably decrease the attraction between the pseudo- electrons and the H10 and H13 atoms resulting in the potential-energy function displayed in Figure 3.

ASSOCIATED CONTENT Supporting Information Results of the theoretical calculations, including MP2 and CCSD electronic energies, molecular structures, and rotational constants, dipole moments, MP2 harmonic and anharmonic vibrational frequencies, rotational and centrifugal distortion constants, and rotation-vibration constants. Microwave spectra of the ground and vibrationally excited states. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tel: +47 2285 5674; Fax: +47 2285 5441; E-mail: [email protected]

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ACKNOWLEDGEMENTS We thank Anne Horn for her skillful assistance. This work has been supported by the Research Council of Norway through a Centre of Excellence Grant (Grant No. 179568/V30). It has also received support from the Norwegian Supercomputing Program (NOTUR) through a grant of computer time (Grant No. NN4654K). J.-C.G. thanks the Centre National d’Etudes Spatiales (CNES) for financial support.

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Figure 1. Models of three forms of N-cyclopropylformamide found to be minima on the potential energy hypersurface in the MP2/cc-pVTZ calculations. Atom numbering is given on the conformer denoted Cis I. Note that the C2‒N9‒C11‒O12 chain of atoms is cis in both Cis I and Cis II, and trans in the form called Trans. The H6‒C2‒N9‒H10 dihedral angle, which can conveniently be used to characterize the conformational properties, is 0.0º in Cis I, 93.0º in Cis II, and 80.4º in Trans, according to CCSD/cc-pVTZ calculations. 21 ACS Paragon Plus Environment

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12

-1

10

8

Relative energy / kJ mol

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6

4

2

0

0

50

100

150

200

Dihedral angle /degree

Figure 2. MP2 electronic potential energy function for rotation about the C2‒N9 bond of cis-Ncyclopropylformamide. The function was calculated by stepping the H6‒C2‒N9‒H10 dihedral angle in 10º intervals. The global minimum occurs at a value of 92.9º for this angle corresponding to Cis II. The second minimum is found at 0º (Cis I). The electronic energy difference is 8.61 kJ/mol. The function has maxima at 42.5 and 180.0º with energies that are 12.03 and 6.99 kJ/mol higher than the energy of the global minimum.

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-1

20

Relative energy / kJ mol

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15

10

5

0

0

20

40

60

80

100

120

140

160

180

Dihedral angle / degree

Figure 3. MP2 electronic potential energy function for rotation about the C2‒N9 bond of transN-cyclopropylformamide. The function was calculated by stepping the H6‒C2‒N9‒H10 dihedral angle in 10º intervals. The minimum energy occurs at 80.4º for the H6‒C2‒N9‒H10 dihedral angle. The corresponding conformer is called Trans. Maxima occur at 0º and at 180º for this dihedral angle, where the energies are 19.54 and 11.44 kJ/mol, respectively, higher than the energy of Trans.

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-2,0 J = 204

194 b

a

Intensity (arbitrary units)

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c d

-2,5

e

-3,0

74200

74300

74400

74500

Frequency (MHz)

Figure 4. RFMWDR spectrum of the 204 ← 194 transitions of Trans using a RF of 12.60 MHz. The two lines marked a are assumed to belong to the first excited state of the torsional vibration; b represent the ground vibrational state, c and d are the first and second excited states of the lowest bending vibration, while e is tentatively assigned as the first excited state of the second lowest bending vibration.

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Table 1. Selected CCSD/cc-pVTZ Structure Parametersa, Rotational Constants, and Dipole Moments and MP2 Centrifugal Distortion Constants,b of Cis I, Cis II, and Trans Forms of cis- and trans-N-Cyclopropylformamide Conformer:

Cis I

Cis II

Trans

Bond distance (pm) C1–C2

150.1

150.3

150.3

C1–C3

151.5

150.7

150.5

C2–C3

150.1

149.8

150.5

C2–H6

108.0

108.2

108.3

C2–N9

144.0

143.1

142.9

N9–H10

100.2

100.3

100.6

N9–C11

135.2

136.1

136.3

C11–O12

121.3

120.9

120.9

C11–H13

110.1

110.1

110.0

(Table 1 continues next page)

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(Table 1 continued) Angle (deg) C1–C2–N9

122.9

117.4

117.8

C3–C2–N9

122.9

118.1

118.2

H6–C2–N9

110.2

115.4

116.1

C2–N9–H10

116.9

118.7

118.8

C2–N9–C11

126.4

122.3

123.2

H10–N9–C11

116.6

117.5

115.6

N9–C11–O12

125.9

125.2

124.8

N9–C11–H13

111.7

112.0

112.2

O12–C11–H13

122.4

122.8

123.0

Dihedral angle (deg) H6–C2–N9–H10

0.0

93.0

79.9

H6–C2–N9–C11

180.0

–72.8

–81.7

(Table 1 continues next page)

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(Table 1 continued)

C2–N9–C11–O12

0.0

–5.3

169.6

C2–N9–C11–H13

180.0

175.7

–11.7

H10–N9–C11–O12

180.0

–171.2

7.4

H10–N9–C11–H13

0.0

9.8

–173.9

Rotational constants (MHz) A

6832.1

8998.9

12525.5

B

3009.6

2471.6

1917.4

C

2506.0

2111.1

1794.3

Quartic centrifugal distortion constantsb (kHz) DJ

1.30

1.68

DJK

‒0.0513

‒9.84

0.385 ‒4.97

(Table 1 continues next page)

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(Table 1 continued)

DK

0.651

d1

‒0.261

d2

0.0698

33.4

65.1

‒0.421

‒0.0153

‒0.00631

0.00103

Dipole momentc (D) a

0.95

2.09

3.94

b

3.63

2.96

0.18

c

0.0d

0.31

0.76

tot

3.75

3.64

4.01

a

Full CCSD structures are given in Tables S2 (Cis I), Table S4 (Cis II), and Table S8 (Trans) of

the Supporting Information. b S-reduction, Ir-representation.62 c 1 D = 3.33564 × 10‒30 C m. d For symmetry reasons.

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Table 2. Spectroscopic Constantsa of the Ground and Excited States of Trans 2nd ex. tors.b

3rd ex. tors.b

4th ex. tors.b

12335.2(18)

12308.1(17)

12330.1(16)

12471(10)

12182.2(31)

1915.8932(90)

1911.4479(36)

1908.0223(47)

1907.6578(40)

1906.508(27)

1919.7755(79)

1798.1345(38)

1799.9266(87)

1793.3894(34)

1792.9749(48)

1792.4640(35)

1792.900(28)

1802.1603(87)

0.39681(46)

0.43068(71)

0.4370(19)

0.36741(63)

0.38678(79)

0.37449(92)

0.3766(15)

0.4578(16)

DJK (kHz)

‒5.1418(50)

‒5.373(20)

‒5.307(36)

‒4.554(15)

‒4.768(22)

‒3.842(25)

‒5.168(48)

‒4.511(80)

DK (kHz)

65.1c

65.1c

65.1c

65.1c

65.1c

65.1c

65.1c

65.1c

d1 (kHz)

‒0.01561(75)

‒0.0202(11)

‒0.01561c

‒0.0140(13)

‒0.0118(14)

‒0.0080(11)

‒0.01561c

‒0.01561c

d2 (kHz)

0.00948(58)

0.00731(90)

0.00948c

0.0056(13)

0.01035(99)

0.00539(98)

0.00948c

0.00948c

HJK(Hz)

‒0.0126(31)

HKJ(Hz)

0.2572(76)d

vib. state:

ground

1st ex. lowest bend.

2nd ex. lowest bend.

Av (MHz)

12298.34(84)

12243.03(15)

12235.2(39)

Bv (MHz)

1917.7153(26)

1916.3974(40)

Cv (MHz)

1796.1162(25)

DJ (kHz)

1st ex. tors.

1st ex. 2nd lowest bend.b

(Table 2 continues next page)

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(Table 2 continued)

rmse

1.221

1.566

1.380

1.554

1.480

1.464

1.493

1.659

Nf

413

158

88

183

111

89

47

51

a

S-reduction Ir-representation.62 Uncertainties represent one standard deviation. b Tentative assignment; see text. cFixed. d Further sextic constants preset at zero. e Root-mean-square deviation defined as

rms2 = [(obs – calc)/u]2/(N – P), where obs and calc are the observed and calculated frequencies, u is the uncertainty of the observed frequency, N is the number of transitions used in the least-squares fit, and P is the number of spectroscopic constants used in the fit. f Number of transitions used in the fit.

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Table 3. Rotation-Vibration Constants of Transa vib. state:

1st ex. lowest bend.

2nd ex. lowest bend.

1st ex. tors.

2nd ex. tors.

3nd ex. tors.

4th ex. tors.

1st ex. 2nd lowest bend.

A (MHz)

55.3(17)

63.1(40)

‒36.9(20)

‒9.8(19)

‒31.8(18)

‒173(10)

116.1(32)

B (MHz)

0.8179(48)

1.3221(94)

5.7674(44)

9.1930(54)

9.5575(48)

11.207(29)

‒2.5602(83)

C (MHz)

‒2.0183(45)

‒3.8104(91)

2.7268(42)

3.1413(54)

3.6522(43)

3.216(29)

‒6.0468(91)

a

Obtained by subtracting the exited-state rotational constants from the corresponding ground-state rotational constants; see text.

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Table 4. Spectroscopic Constantsa of Cis II vibrational state:

ground

1st ex. torsion

1st ex. lowest bend.

Av (MHz)

8999.0439(27)

9032.6547(38)

9026.6880(79)

Bv (MHz)

2450.13114(99)

2434.8459(15)

2447.79828(95)

Cv (MHz)

2101.4817(10)

2096.4883(16)

2098.85060(88)

DJ (kHz)

1.8145(24)

1.7253

1.6932(8)

DJK (kHz)

‒10.9264(24)

‒10.4418

‒10.4764(47)

DK (kHz)

37.783(29)

37.777(50)

35.40(17)

d1 (kHz)

‒0.483479(62)

‒0.46680(11)

‒0.44663(33)

d 2 (kHz)

‒0.011664(24)

‒0.014028(40)

‒0.00959(15)

HJ (Hz)

‒0.0061(18)

‒0.0109(34)

HJK (Hz)

‒0.03296(49)

‒0.0329(12)

HKJ (Hz)

‒0.2633(90)

‒0.187(30)

hJ (Hz)

‒0.001978(16)

‒0.002142(31)

hJKb (Hz)

‒0.0003707(79)

‒0.000378(16)

(Table 4 continues next page)

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(Table 4 continued)

rmsa

1.436

1.489

1.494

Na

835

580

366

a

Defined in the footnote of Table 2. The spectra are listed in Tables S19 – S20 of the Supporting

Information. b Further sextic constants preset at zero in the least-squares fit.

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Conformational properties of cis- and trans-N-Cyclopropylformamide studied by microwave spectroscopy and quantum chemical calculations.

The microwave spectra of cis- and trans-N-cyclopropylformamide, C3H5NHC(═O)H, have been investigated in the 31-123 GHz spectral region at room tempera...
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