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The structural stability and the vibrational spectra of the anticancer drug tamoxifen and triphenylethylene were investigated by the DFT B3LYP/6-311G (d,p) calculations. Tamoxifen and triphenylethylene were predicted to exist predominantly as non-planar structures. The vibrational frequencies and the 1 H and 13 C NMR chemical shifts of the low energy structures of tamoxifen and triphenylethylene were computed at the DFT B3LYP level of theory. Complete vibrational assignments were provided by combined theoretical and experimental data of tamoxifen and triphenylethylene. The 1 H and 13 C NMR spectra of both molecules were interpreted by experimental and DFT calculated chemical shifts of the two molecules. The RMSD between experimental and theoretical 1 H and 13 C chemical shifts for tamoxifen is 0.29 and 4.72 ppm, whereas for triphenylethylene, it is 0.16 and 2.70 ppm, respectively.

The structural stability and the vibrational spectra of the anticancer drug melphalan and its parent compounds 3-phenylpropionic acid and l -phenylalanine were investigated by the DFT B3LYP/6-311G** calculations. Melphalan and its fundamental compounds were predicted to exist predominantly in non-planar structures. The vibrational frequencies of the low energy structures of melphalan, 3-phenylpropionic acid, and phenylalanine were computed at the DFT B3LYP level of theory. Complete vibrational assignments of the normal modes of melphalan, 3-phenylpropionic acid, and phenylalanine were provided by combined theoretical and experimental data of the molecules. The experimental infrared spectra of phenylalanine and melphalan show a significantly different pattern of the C O stretching mode as compared to those of normal carboxylic acids. A comparison of the 3700–2000 cm −1 infrared spectral region of the three molecules suggests the presence of similar intermolecular H-bonding in their condensed phases. The observed infrared and Raman spectra are consistent with the presence of one predominant melphalan conformation at room temperature.

The structural stability of mandelic acid and 3-phenyllactic acid was investigated by Density Functional B3LYP calculations with the 6-311G ∗∗ basis set. The two acids were predicted to have non-planar forms with cisoid α-hydroxy moiety with respect to the C O group as their lowest energy structures. From the SCRF = SMD calculations the stability of both acids is predicted to be significantly dependent on the dielectric constant of the solvent. The infrared and Raman spectra of l -mandelic and l -3-phenyllactic acids were compared to the corresponding solid state spectra of the dl -racemic mixtures. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of the molecules. The 1 H and 13 C NMR spectra of both acids were interpreted by experimental and DFT calculated chemical shifts of the two acids. The RMSD between experimental and theoretical 1 H and 13 C chemical shifts for mandelic acid is 0.961 and 6.119 ppm, whereas for 3-phenyllactic acid, it is 0.611 and 5.214 ppm, respectively.

The structure, vibrational and 1 H and 13 C NMR spectra of the local anesthetic drug lidocaine were investigated by the B3LYP/6-311G ∗∗ calculations. The molecule was predicted to have the non-planar cis (NCCN ∼ 0°) structures being about 2–6 kcal/mol lower in energy than the corresponding trans (NCCN ∼ 180°) forms. The calculated NCCN (9.6°) and CNCC (−132.2°) torsional angles were in a good qualitative agreement with the reported X-ray angles (3.1 and 13.0°, −102.67 and −77.9° , respectively, for H-bonded dimers). The Gibbs energy of solution of lidocaine in formamide, water, dimethylsulfoxide, acetonitrile, methanol, ethanol and chloroform solutions was estimated at the B3LYP level. The predicted affinity of lidocaine toward the alcohols, acetonitrile and chloroform solutions was in excellent agreement with the reported experimental solubility of the drug in organic solvents. The analysis of the observed vibrational spectra is consistent with the presence of lidocaine in only one conformation at room temperature. The 1 H and 13 C NMR spectra of lidocaine were interpreted by experimental and DFT calculated chemical shifts of the drug. The RMSD between experimental and theoretical 1 H and 13 C chemical shifts for lidocaine is 0.47 and 8.26 ppm, respectively.

The structural stability, vibrational, 1 H and 13 C NMR spectra of valeric and valproic acids were investigated by the B3LYP calculations with the 6-311G ** basis set. Valeric acid is predicted to exist predominantly in the planar cis form (80% abundance). Valproic acid is predicted to have an equilibrium mixture of 68% gauche - 1 and 32% gauche - 2 structures at 298.15 K. The spectral feature of the O H stretching mode in the infrared spectra of both acids suggests the presence of strong H-bonding in the condensed phase of valeric acid and weak H-bonding in the case of valproic acid. The harmonic and anharmonic vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of the molecules. Not all of the calculated anharmonic wavenumbers showed a consistent trend with the observed wavenumbers. The 1 H and 13 C NMR spectra of both acids were interpreted by experimental and DFT calculated chemical shifts of the two acids. The RMSD between experimental and theoretical 1 H and 13 C chemical shifts for valeric acid is 1.8 and 3.8 ppm, whereas for valproic acid, it is 1.4 and 4.5 ppm, respectively.

2,3-diphenylquinoxaline (DPQ) and 2,3-diphenylquinoxaline N,N′-dioxide (DPQDO) (also known as 2,3-diphenylquinoxaline 1,4-dioxide) were synthesized, and their infrared and Raman spectra were measured. The structural stability and the vibrational spectra of the two quinoxaline molecules were investigated at the DFT-B3LYP and ab initio MP2 levels of calculation with the 6-311G ** basis set. From the calculations, the two molecules were predicted to exist in a non-planar structure with nearly C 2 symmetry. The quinoxaline and the two phenyl rings of the parent compound were calculated to have tilting angles of about 12°, 42° and 42° out of the plane of the molecule at the DFT level as compared to the X-ray angles of 12°, 37° and 53°. The non-planar structure was predicted to be about 5 and 11 kcal/mol lower in energy than the perpendicular structures in the case of the parent and the dioxide, repectively. Complete vibrational assignments of the infrared and Raman spectra of DPQ and DPQDO molecules were provided by combining experimental and theoretical data.

We have calculated the vibrational spectra of isoamyl alcohol and of tert-amyl alcohol using Density Functional Theory (DFT) with the Becke-3 Lee Yang Parr (B3LYP) functional and a 6-311+G** atomic basis set. The energies of the conformers were also calculated with ab initio Perturbation Theory of second (MP2) and fourth order restricted to single, double and quadruple excitations (MP4 SDQ) with the same basis set. We found rather complicated equilibria of four conformations in each case, counting only those with appreciable abundancies. PED data were compared with GAUSSVIEW animations. The calculated wavenumbers agree rather well with the experimental ones when the gauche-trans conformer is assumed as the most important one for isoamyl alcohol, and the gauchegauche one for tert-amyl alcohol. However, some of the experimental bands had to be assigned also to other conformers, indicating their presence in the equilibrium mixture. Due to sterical reasons both the CO and the OH bonds appear to be weaker in the tertiary alcohol, considering the wavenumbers of the CO and OH bond stretching vibrations. The bond lengths point into the same direction, however, the OH bond in the tertiary alcohol is only slightly longer than that in the primary alcohol.

The structural stability of 4,4′-methylenedianiline (MDA), 4,4′-methylenebis(N,N-dimethylaniline) (MBDMA) and their parent diphenylmethane (DPM) was investigated by the DFT-B3LYP and ab initio MP2 calculations with the 6-311G ** basis set. From the calculations the three diphenylmethanes were predicted to exist predominantly in a non-planar structure with a near C 2 molecular symmetry in agreement with X-ray studies. The CCCC angles in DPM and the two dianilines were calculated to be nearly equal of about 60° as compared to X-ray angles of 17° and 67° for MDA. The NH 2 inversion barriers in MDA was estimated to be about 1–5 kcal/mol. On comparison of the Raman spectra of the three compounds, the line intensity and position of the characteristic ring breathing mode were shown to be sensitive to the amino substitution. The vibrational wavenumbers of the optimized structures of the two dianilines were computed at the DFT-B3LYP level of theory. Tentative vibrational assignments were made on the basis of combined DFT and experimental FT-IR and FT-Raman data of MDA and MBDMA.

The complex conformational profile and the vibrational spectra of carbohydrazide (1,3-diaminourea) and thiocarbohydrazide were investigated by the DFT-B3LYP and ab initio MP2 and MP4(SDQ) levels of theory using the 6-311G ** basis set. The planar syn - cis - anti - trans structure of the two molecules with C s symmetry was predicted to have an imaginary frequency. The corresponding non-planar structure ( C 1 symmetry) was predicted to be the lowest energy structure and to have a positive real frequency as the real minima of the molecules. The stability of the non-planar structure agrees with the X-ray study of carbohydrazide where significant molecular distortions from planarity were reported for the lowest energy form as a result of strong intermolecular hydrogen bonding in the crystal. The vibrational wavenumbers of the lowest energy non-planar conformer of carbohydrazide and thiocarbohydrazide were computed at the B3LYP level and complete vibrational assignments were provided on the basis of the combined DFT-B3LYP results, normal coordinate calculations and experimental infrared and Raman data.

The possible conformations in the most stable structure of cyclophosphamide were investigated at the DFT-B3LYP and MP2/6-311G** levels of calculation. The axial structure is calculated to be lower in energy than the equatorial form of cyclophosphamide by 5 (MP2) to 6 (DFT) kcal mol-1 because of a very weak anomeric effect and weak conjugation. Further it is the same structure as the one found in the X-ray investigations both of the hydrate and of the anhydrous form. The computed DFT vibrational frequencies of the axial structure were used to analyze the infrared and Raman spectra using normal-coordinate calculations to provide reliable assignments to the normal modes of the molecule.

The structural stability of 1,3-diphenylurea and 1,3-diphenylacetone was investigated by the DFT-B3LYP and ab initio MP2 calculations using the 6-311G(**) basis set. From full energy optimization at the MP2 level of theory the near-planar cis-trans form of 1,3-diphenylurea was predicted to be about 1.5kcal/mol lower in energy than the X-ray near-planar cis-cis conformer. At the DFT-B3LYP level of theory the near-planar cis-trans and cis-cis forms were predicted to have a very comparable relative stability. The dibenzyl ketone was predicted by both levels to exist predominantly in a gauche-gauche conformation. On comparison the DFT near-planar structures of each of the two molecules were found to be consistent with the reported X-ray structures. The analysis of the infrared and Raman spectra of 1,3-diphenylurea suggests the presence of a second form at room temperature. The vibrational frequencies of the conformers most consistent with X-ray data of diphenylurea and dibenzyl ketone were computed at the B3LYP level and tentative vibrational assignments of their normal modes were provided on the basis of combined experimental and computed data.

We have studied the structural properties and vibrational spectra of Bisphenol S (4,4′-sulfonyldi-phenol) with the help of Density Functional Calculations using a 6-311G∗∗ basis set. In addition we recorded experimental spectra in our laboratory. With the help of the GaussView program we could provide a complete assignment of the vibrational lines to the calculated normal modes of the molecule. The calculated and experimental vibrational spectra agree rather well with each other and we found the properties of Bisphenol S to be rather similar to the sulfa drug dapsone studied earlier. Interestingly, the electron donating power of an OH aubstituent at a phenyl ring seems to be less effective than that of an NH2 (in dapsone) substituent, because in Bisphenol S the relative intensity of the ring breathing line in the Raman spectrum appears to be much larger than in dapsone. We reach this conclusion about the electron donating powers of susbtituents, because in an early study on chlorine substituted anilines we have found that the larger the electron density in the ring becomes, the more the relative intensity of that line diminishes.

The structural stability of 2-phenyl- and 2-phenoxyethanols were investigated at the DFT-B3LYP/6-311G**, MP2 and MP4(SDQ) levels of theory. From the calculations at the three levels of theory 2-phenylethanol and 2-phenoxyethanol were predicted to exist predominantly in non-planar gauche conformations. For 2-phenylethanol the lowest energy Gg1 structure was predicted to be stabilized by an interaction between the hydroxyl H atom and the phenyl ring. For 2-phenoxyethanol the Ggg1 structure was predicted to be strongly stabilized by dipolar interactions between the hydroxyl H atom and the phenoxy O atom of the alcohol. For both alcohols the planar trans structure with minimum steric interactions between the CH(2) groups was predicted to be significantly higher in energy than the ground state gauche structure of the alcohols. The dipolar interactions are reported to play more important role than steric ones in stabilizing the molecules. The vibrational frequencies of each of the two alcohols in its lowest energy gauche structure were computed at the B3LYP level and tentative vibrational assignments were made for their normal modes on the basis of the calculated and experimental data.

The structural stability of 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylacetic acid were investigated using the DFT-B3LYP and the ab initio MP2 methods together with the 6-311G** basis set. The two molecules were predicted at the DFT and MP2 levels of calculation to have the non-planar (np) forms as the lowest energy conformer of the acids. Another very low lying conformer of the former acid in the gas phase seems to play no role in the condensed phase. The observed spectral intensities of the acids were consistent with the np conformation being the predominant form at room temperature. The vibrational wavenumbers were computed at the B3LYP level of theory and vibrational assignments were provided on the basis of combined theroretical and experimental infrared and Raman data of the molecules. As reported earlier in the case of chlorine substituted anilines, the ring breathing mode, which is expected to have a very high intensity in the Raman spectra of substituted benzenes, diminishes in intensity with increasing electron density in the ring (activation for electrophilic attacks).

The complex conformational behavior of 3-phenylprop-2-en-1-ol (cinnamyl alcohol) and its saturated analogue 3-phenyl-1-propanol were investigated at the DFT-B3LYP/6-311G ** , MP2 and MP4(SDQ) levels of theory. The unsaturated 3-phenylprop-2-en-1-ol was predicted to exist in Cg and Gg1 conformational mixture as a result of competitive conjugation and hyperconjugation interactions in the molecule. The saturated 3-phenyl-1-propanol was predicted to exist predominantly in a Ggg structure as a result of predominant steric hindrances in the alcohol. Only the one predominant form was identified in the infrared and Raman spectra of both alcohols. The excellent agreement between the calculated wavenumbers and the observed ones in the infrared and Raman spectra supports the conclusion that each of the two alcohols is present in one predominant form in the condensed phases. The vibrational frequencies of 3-phenylprop-2-en-1-ol and 3-phenyl-1-propanol in their lowest energy forms were computed at the B3LYP level and tentative vibrational assignments were provided on the basis of combined calculated and experimental data.

We have performed density functional calculations with the B3LYP functional and a 6-311G** basis set to obtain the vibrational spectra in harmonic approximation of the anti-leprosy drug Dapsone and the parent compound diphenylsulfone. Although the chemical difference between the two molecules is not that pronounced (Dapsone has amino groups in the para positions in the phenyl rings), Dapsone is an active drug, while to our knowledge diphenylsulfone shows no medical activity. We compared the theoretical results to experimental vibrational spectra found in the literature. With the help of the program GAUSSVIEW we were able to assign the experimentally found spectral lines to specific atomic motions. The remarkable difference between the two molecules, regarding their structural behavior, is that the drug Dapsone has a more flexible structure of the phenyl ring than the parent molecule has. This might contribute to a greater ability of the drug to fit into receptor sites in a cell membrane although one has to be well aware that this plays most propably only a minor role in the drug activity of Dapsone

The structure of phenoxyacetohydrazide was investigated at the B3LYP/6-311G ** , MP2/6-311G ** and MP4(SDQ)/6-311G ** levels of theory and compared to the corresponding structure obtained from the X-ray study. At the DFT-B3LYP level, the planar Ttcp conformation of phenoxyacetohydrazide was predicted to be about 0.7 kcal/mol lower in energy than the non-planar Tgcnp structure. At the MP2 and the MP4(SDQ) levels, a reverse conformational behavior was reported where the planar form was predicted to be about 1.3 and 0.5 kcal/mol lower in energy than the non-planar form, respectively. A drastic difference in the COCC torsional angle of about 90° was predicted between the X-ray and the computed ground state structure. The X-ray structure (near-planar Ttcnp ) of the molecule was then optimized at the three levels of theory. At the DFT-B3LYP level, the near-planar Ttcnp structure was predicted to turn into the planar Ttcp structure. At the two Moller–Plesset levels the Ttcnp structure was found to be in excellent agreement with the X-ray structure but being 0.5–1.3 kcal/mol higher in energy than the computed lowest energy Tgcnp form. The computed vibrational frequencies of the near-planar structure at the B3LYP/6-311G ** level of calculation were combined with experimental infrared and Raman data to provide tentative vibrational assignments of the molecule.

The structural stability of 2,4-dichlorophenoxyacetic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G** basis set. From the calculations at both levels of theory the Cgcpp structure was predicted to be the lowest energy minimum for the acid. The DFT and the MP2 levels disagreed about the nature of the second stable structure of 2,4-dichlorophenoxyacetic acid. At the DFT-B3LYP level of calculation the planar Tttp (transoid O=C-O-H) and the non-planar Tgcpp (cisoid O=C-O-H) forms were predicted to be 0.7 and 1.5 kcal/mol, respectively higher in energy than the Cgcpp conformation. At the MP2 level the two high energy Tttp and Tgcpp forms were predicted to be 2.7 and 1.4 kcal/mol, respectively higher in energy than the ground state Cgcpp structure. The Tgcpp form was adopted as the second possible structure of 2,4-dichlorophenoxyacetic acid on the basis of the fact that the Møller-Plesset calculations account better than the DFT ones for the non-bonding Ocdots, three dots, centeredH interactions. The vibrational frequencies of the lowest energy Cgcpp conformer were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of normal coordinate analysis and experimental infrared and Raman data.

The structural stability of phenoxyacetic acid and 2,3,4,5,6-pentafluorphenoxyacetic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G** basis set. For the parent acid the calculations were extended to the MP4(SDQ) level of theory. At the DFT-B3LYP level of calculation the planar Tttp (transoid O C O H) was predicted to be about 0.5 and 1.3 kcal/mol lower in energy than the non-planar Cgcpp and Tgcpp (cisoid O C O H) forms, respectively. At the MP2 and the MP4(SDQ) levels the Cgcpp form was predicted to be about 0.8 and 1.4 kcal/mol lower in energy than the Tgcpp and the Tttcp structures, respectively. On the basis of the Moller–Plesset calculations the Cgcpp and the Tgcpp conformations were adopted as the low and high energy structures of phenoxyacetic acid. The observed spectral intensities of phenoxyacetic acid were consistence with the Cgcpp conformation being the predominant form of the acid at room temperature. At the DFT and MP2 levels of theory 2,3,4,5,6-pentafluorophenoxyacetic acid was predicted to exist predominantly in the Cgcpp structure. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of both molecules.

The diperoxide 3,6-diphenyl-1,2,4,5-tetroxane was synthesized and its infrared and Raman spectra were measured. The structural stability and the vibrational spectra of 1,2,4,5-tetroxane and 3,6-diphenyl-1,2,4,5-tetroxane were investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G∗∗ basis set. From the calculations, the chair form of 1,2,4,5-tetroxane was predicted to be about 9, 20, and 50 kcal/mol lower in energy than the twist, boat, and planar structures, respectively. The ring inversion in 1,2,4,5-tetroxane is concluded to take place most probably through the twist structure of the ring. In the case of 3,6-diphenyl-1,2,4,5-tetroxane, the planar-equatorial (Peq) conformation was predicted by both levels of theory to be the lowest energy and predominant form of the molecule. The planar ⇋ perpendicular and axial ⇋ equatorial interconversion barriers were calculated to be about 2 and 12 kcal/mol, respectively. Reliable vibrational assignments were provided for the spectra of 1,2,4,5-tetroxane and 3,6-diphenyl-1,2,4,5-tetroxane by combining experimental and theoretical data of the two molecules.

Our interest in conjugation effects in substituted phenyl compounds has turned our attention to the highly reactive compounds phenyl ketene, azide, and isocyanate, which due to their reactivity are of utmost importance in organic synthesis. We performed local density functional theory (DFT) calculations using a 6-311G** basis set to calculate the structures and potential functions of the internal rotation. Further for the minimum structures we computed the vibrational infrared and Raman spectra of the three molecules. In order to confirm that DFT works rather well in these systems we performed the geometry optimizations also using ab initio Moeller–Plesset perturbation theory of second order (MP2) in the same basis set. As expected there exist just two minimum structures for each of the molecules which both correspond to planar structures and are identical due to the symmetry of the phenyl ring. The transitions states (TS) of the internal rotations are the perpendicular ones. We expect conjugation to play no major role in these molecules since extensive conjugation effects would imply a large reduction of the aromatic character of the phenyl ring which in turn would greatly destabilize the systems. However, although the rotational barriers appear to be rather small in these systems conjugation must play at least some role in stabilizing the planar ground state. As detailed later, the relative heights of the rotational barriers can all be explained naturally. Experimental vibrational spectra could be obtained only for phenyl isocyanate and azide, but not for the ketene because of the high reactivity of this molecule. Since in the former cases the calculated spectra agree fairly well with the measured ones, we present those of the other molecule as theoretical prediction, which could be useful to detect spectroscopically in a reaction mixture residual reactant. On the basis of potential energy distribution (PED) calculations we present a complete assignment of the vibrational lines to symmetry coordinates, where, for example, ring breathing must show up with rather large intensities in the Raman spectra of the molecules.

The structural stability of aniline, 2,4,6-trichloroaniline and 2,3,5,6-tetrachloroaniline was investigated by DFT-B3LYP and ab initio MP2 and MP4(SDQ) calculations with the 6-311G∗∗ basis set. From the calculations the three molecules were predicted to exist predominantly in a symmetric near-planar structure. The NH2 inversion barrier was estimated from the MP2/6-311G∗∗ level of theory to be about 9.6 kJ/mol for aniline, 5.9 kJ/mol for the trichloro and 4.6 kJ/mol for the tetrachloro derivatives. The line intensities of the ring breathing and the C Cl stretching modes were shown to have a great dependence on the number of chlorine atoms on the benzene ring. The relative change in Raman line intensity of the C Cl stretching mode was explained on the basis of the inductive effect of the chlorine atoms on the benzene ring. The vibrational frequencies were computed at the DFT-B3LYP level and the infrared and Raman spectra for each molecule were calculated. Complete vibrational assignments were made on the basis of normal coordinate analyses and potential energy distributions for the two chloroanilines.

The complex internal rotations and conformational equilibria in oxiraneethanol were investigated at the DFT-B3LYP/6-311G** level of theory. Four minima were predicted in the CCOH potential energy scans of the molecule to have relative energies of about 2 kcal/mol or less and all were calculated to have real frequencies upon full optimization of structural parameters and the calculation of the Gibb’s free-energies at the DFT level of calculation. At the DFT-B3LYP, the MP2 and the MP4(SDQ) levels of theory, the G1gg1 conformation, predicted to be the lowest energy conformation for oxiraneethanol, was in excellent agreement with the rotational microwave study. The equilibrium mixture was calculated to be about 47% G1gg1 , 32% Cg1g , 15% Gg1t and 6% G1g1g at the B3LYP/6-311G** level of theory at 298.15 K. Solvent study corroborated the presence of the high energy Cg1g form in the liquid phase of oxiraneethanol. The vibrational frequencies of oxiraneethanol in its two stable forms were computed at the B3LYP level and vibrational assignments were made for the two lowest energy G1gg1 and Cg1g forms on the basis of calculated and experimental data of the molecule.

The structural stability of 1-aminocyclopropanecarboxylic acid was investigated at DFT-BLYP, DFT-B3LYP and ab initio MP2 levels of theory with 6-311G(**) basis set. The molecule was predicted at the three levels of calculation to exist in cis-syn--trans-syn conformational equilibrium. The equilibrium constants for the conformational interconversion were calculated and found to correspond to an equilibrium mixture of about 35% cis-syn and 65% trans-syn conformations at 298.15 K. The cis-trans rotational barrier and the NH(2)syn-anti inversion barrier were estimated to be about 6 and 7 kcal/mol, respectively. The O-H rotational barrier was calculated to be of the order 12-14 kcal/mol. The vibrational frequencies of the two stable conformers of the acid were computed at the BLYP and the B3LYP levels of theory. Only vibrational assignments were provided for the low energy trans-syn conformer on the basis of normal coordinate analysis and experimental data. No clear evidence was found for the presence of the second high energy cis-syn structure in the spectra of the molecule.

The structural stability and C-N internal rotations of phenylurea and phenylthiourea were investigated by DFT-B3LYP and ab initio MP2 and MP4//MP2 calculations with 6-311G** and/or 6-311+G** basis sets. The complex multirotor internal rotations in phenylurea and phenylthiourea were investigated at the B3LYP/6-311+G** level of theory from which several clear minima were predicted in the calculated potential energy scans of both molecules. For phenylurea two minima that correspond to non-planar- (CNCC dihedral angle of about 45 degrees ) cis (CNCO dihedral angle is near 0 degrees ) and trans (CNCO dihedral angle is near 180 degrees ) structures were predicted to have real frequency. For phenylthiourea only the non-planar-trans structure was predicted to be the low energy minimum for the molecule. The vibrational frequencies of the lowest energy non-planar-trans conformer of each of the two molecules were computed at the B3LYP level and tentative vibrational assignments were provided on the basis of normal coordinate analysis and experimental infrared and Raman data.

The C N internal rotations in formohydrazide OHC NH NH 2 and formohydroxamic acid OHC NH OH were investigated at the B3LYP/6-311+G** and MP2/6-311+G** levels of theory. The C N rotational barrier in the molecules was calculated to be about 28–30 kcal/mol. The energies of the molecules were calculated at the B3LYP, MP2, MP4(SDTQ) and CCSD(T) levels of theory with both 6-311G** and 6-311+G** basis sets. From the calculations at all the levels formohydroxamic acid was predicted to exist predominantly in a non-planar near- cis conformation at ambient temperature. From all the calculations formohydrazide was predicted to have a planar cis–syn (C O and N N bonds eclipse each other and NH 2 moiety is syn to C N bond) conformation as the lowest energy structure. The NH 2 inversion barrier in formohydrazide was predicted to be about 5–7 kcal/mol. The vibrational frequencies of the cis–syn formohydrazide were computed at the B3LYP/6-311+G** level and normal coordinate calculations were carried out. Complete vibrational assignments were made on the basis of normal coordinate analyses and experimental infrared and Raman data of the molecule.

The structural stability and ring inversion of 2-chloro-1,3,2-dioxaphospholane 2-oxide and 2-sulfide were investigated by the DFT-B3LYP and the ab initio MP2 calculations with 6-311+G ∗∗ basis set. From the calculations the two molecules were predicted at the two levels of theory to exist predominantly in the twist conformation. The effect of the mutual interactions between the two oxygen atoms in the ring and the -PXCl moiety on the inversion of the five-membered hetero-ring of the two molecules and hence on determining their conformational stability was thoroughly explained. The repulsive interaction between the lone-pairs at the two in-ring oxygen atoms was concluded to be superior to the non-bonded overlap between the occupied lone-pair orbitals at the in-ring O atoms with the empty, antibonding orbitals of both the P Cl and the P X (X is O or S) bonds. The vibrational wavenumbers were calculated and the potential energy distributions PED among the symmetry coordinates of the normal modes were computed for the low energy structure of the molecules. Complete vibrational assignments were provided on the basis of the calculated PED values. The experimental infrared and Raman spectra of the two molecules were compared to the calculated ones.

The structural stability and internal rotations in cyclopropanecarboxylic acid and cyclopropanecarboxamide were investigated by the DFT-B3LYP and the ab initio MP2 calculations using 6-311G** and 6-311+G** basis sets. The computations were extended to the MP4//MP2/6-311G** and CCSD(T)//MP2/6-311G** single-point calculations. From the calculations the molecules were predicted to exist predominantly in the cis (C O group eclipses the cyclopropane ring) with a cis – trans barrier of about 4–6 kcal/mol. The OCOH torsional barrier in the acid was estimated to be about 12–13 kcal/mol while the corresponding OCNH torsional barrier in the amide was calculated to be about 20 kcal/mol. The equilibrium constant k for the cis ⇔ trans interconversion in cyclopropanecarboxylic acid was calculated to be 0.1729 at 298.15 K that corresponds to an equilibrium mixture of about 85% cis and 15% trans . The vibrational frequencies were computed at the DFT-B3LYP level. Normal coordinate calculations were carried out and potential energy distributions were calculated for the low energy cis conformer of the molecules. Complete vibrational assignments were made on the basis of normal coordinate calculations and comparison with experimental data of the molecules.

The structures and conformational stability of vinylphosphonic and vinylthiophosphonic acids were investigated using calculations mostly at DFT/6-311G** level and ab initio ones at MP2/6-311G** level. From the calculations the molecules were predicted to exist in a nonplanar near-cis (nc) to trans -gauche (tg) conformational equilibrium with nc (phosphonic oxygen or sulfur nearly eclipses the vinyl group) being the predominant conformer at ambient temperature, at least on the basis of Gibbs free energy (in the case of the thio compound the potential energy of tg is slightly lower than that of nc). The antisymmetric potential function for the internal rotation was determined for each one of the molecules. However, when starting optimizations from a structure too close to the full cis symmetry, the cis form results as an extremum on the potential hyper-surface. Consequently cis was reported in the literature several times as minimum. However, when we calculated the vibrational frequencies the cis form turned out to have an imaginary frequency, and thus cis is a local maximum along one of the normal coordinates. The true minimum, with only real frequencies, actually is an nc one with a very low barrier to rotation. The rotational angle out of cis is rather small; however, HOPX dihedral angles are quite away from real cis. This holds true not only with DFT but also with MP2. The vibrational frequencies were computed and the spectra were plotted as a prediction. Normal coordinate calculations were carried out and potential energy distributions were calculated for the molecules in the nc and the tg conformations. From our results and their analysis, we conclude, in agreement with results from the literature based on localized orbitals, that conjugation effects are absent – or at least negligible – as compared with electrostatic or steric ones in determining the structures of the stable conformers in the vinyl derivatives.

The conformational stability of 2,2,3,3,3-pentafluoro-1-propanol was investigated by the DFT-B3LYP/6-311+G** and the ab initio MP2/6-311+G** calculations. The calculated potential energy curves of 2,2,3,3,3-pentafluoro-1-propanol at both levels of theory were consistent with three distinct minima that correspond to Trans-gauche-gauche (Tgg), trans-trans-gauche (Ttg) and trans-gauche-gauche(-) (Tgg1) conformers in the order of decreasing relative stability. The equilibrium constants for the conformational interconversion of 2,2,3,3,3-pentafluoro-1-propanol were calculated and found to correspond to an equilibrium mixture of about 46% Tgg, 43% Ttg and 11% Tgg1 conformations at 298.15K. The calculated (%Ttg/%Tgg) ratio of 0.93 is consistent with the 0.85 ratio of the observed intensities of the 772 and 794 cm(-1) lines in the Raman spectrum of the liquid. The nature of the high energy Ttg conformation was verified by solvent experiments using formamide (epsilon = 109.5) and acetonitrile (epsilon = 37). The vibrational frequencies of the molecule in its stable forms were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental data of the molecule.

The structures of carbamoyl azide and ketene were investigated by the Density Functional B3LYP level of theory and ab initio second-, third- and fourth-order Moller–Plesset MP2, MP3 and MP4 calculations with the 6-311G ∗∗ and/or the 6-311+G ∗∗ basis sets. The amino NH 2 group of both molecules was predicted at the DFT-B3LYP level to adopt the planar structure, while at the Moller–Plesset levels of theory to have the non-planar conformation. The vibrational frequencies of the two molecules in their non-planar ground state conformation were computed at the MP2/6-311G ∗∗ level of theory. Normal coordinate calculations were then carried out and the potential energy distributions (PEDs) among the symmetry coordinates of the normal vibrational modes of the molecule were calculated. Complete vibrational assignments were provided on the basis of normal coordinate calculations and comparison with reported experimental data of the azide. Better assignment for the low frequency modes was provided by the scaled MP2 wavenumbers than the calculated ones by the B3LYP level of calculation for carbamoyl azide.

The ring puckering in the four-membered 3-chloro-1,3-thiaphosphetane 3-oxide, its 3-sulfide derivative and 1,3-dithietane 1,1-dioxide hetero rings were investigated by DFT and ab initio MP2 calculations using a 6-311+G ∗∗ basis set. From the calculations at both levels of theory the asymmetric ring puckering potential energy curves in the two phosphetanes were consistent with an almost single minimum that corresponds to a pseudo - axial configuration (P Cl bond is in axial position) with a puckering angle of about 15–20°. In the case of the dithietane dioxide the symmetric ring puckering potential was consistent at the B3LYP level with a flat minimum that corresponds to a planar ring but at the MP2 level with a double minimum with a very low barrier of about 185 cal/mol to ring planarity. The potential functions that describe the ring puckering in the three molecules were derived at the B3LYP/6-311+G ∗∗ and the MP2/6-311+G ∗∗ levels of calculations. The vibrational wavenumbers were calculated and the potential energy distributions PED among the symmetry coordinates of the normal modes were computed for the stable conformers of each of the molecules. Complete vibrational assignments were provided on the basis of the calculated PED values. The experimental infrared and Raman spectra of the two phosphetane molecules were simulated and compared to the calculated ones.

The structure and conformational stability of dichloro(vinyl)phosphine and dichloro(phenyl)- phosphine oxides and sulfides were investigated using calculations at the DFT/6-311G** and ab initio ones at the MP2/6-311G** level. We know from our previous results that the addition of diffuse functions to a valence triple zeta basis with polarization functions might lead to an unbalanced basis, which performs even worse than the smaller basis without diffuse functions, as it is the case for the 6-311++G** basis set in the Gaussian program. For large energy differences between conformers, DFT works very well, in some cases even better than MP3 or MP4. The vinyl derivatives were predicted to exist in a cis/gauche conformational equilibrium with cis (the PX bond, X being oxygen or sulfur eclipses the vinyl groups) being the predominant conformer at ambient temperature. In the phenyl case case the two planar forms are equivalent minima. The asymmetric potential function for the internal rotation was determined for each of the molecules. The vibrational frequencies were computed and the spectra, where possible, were compared with the experimental ones. Normal coordinate calculations were carried out and potential energy distributions were calculated for the molecules in the cis and gauche conformations (in the vinyl case, planar one for phenyl), providing a complete assignment of the vibrational lines to symmetry coordinates in the molecules. From our results and their analysis we conclude, in agreement with literature results based on localized orbitals, that conjugation effects are absent - or at least negligible - as compared with electrostatic ones in determining the structures of the stable conformers in both the vinyl and the phenyl derivatives. The P-O bond should be a highly polarized triple bond, as confirmed by analysis of Mulliken populations. The polarization turned out to be much less in the sufides due to the much smaller electronegativity of sulfur as compared with oxygen.

The conformational stability and the three rotor internal rotations in 3-fluoro-1-propanol were investigated by the DFT-B3LYP/6-311 + G** and the ab initio MP2/6-311 + G** levels of theory. The calculated potential energy curves of the molecule at both levels of theory were consistent with complex conformational equilibria of about 12 minima, all of which were predicted to have real frequencies at both the B3LYP and the MP2 levels. The lowest energy minimum in the potential curves of 3-fluoro-1-propanol was predicted to correspond to the Gauche–gauche–trans (Ggt) conformer in excellent agreement with microwave and electron diffraction results. The equilibrium constants for the conformational interconversion of the molecule were calculated and found to correspond to an equilibrium mixture of about 33% Ggt, 14% Ggg1 and 13% Gg1g and about 43% Ggt, 12% Ggg1 and 10% Gg1g distribution by the B3LYP/6-311 + G** and the MP2/6-311 + G** calculations, respectively, at 298.15 K. The vibrational frequencies of each molecule in its three stable forms were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental data of the molecule.

The structural stability of halosulfonic acids X–SO2–OH (X = F, Cl and Br) were investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. The potential energy curve for the XSOH internal rotation around S–O bond was consistent with one minimum that corresponds to non-linear structure with XSOH torsional angle of about 80°. The vibrational frequencies were computed at DFT-B3LYP level for the stable non-planar structure of the three molecules. Normal coordinate calculations were then carried out and the potential energy distributions (PED) were calculated for the molecules. On the basis of PED values and comparison with experimental data reliable assignments were provided for normal modes of fluoro-, chloro- and bromosulfonic acids.

In recent literature it was reported that the valence triple zeta basis set augmented by polarization functions is not too reliable for vinyl monohalo- and dihalomethanes and -silanes, the halogen being fluorine and chlorine. The major conclusion was that a valence triple zeta basis is too small to be augmented by polarization functions in a balanced way, at least on vinylmonofluoromethane. Thus we decided to apply the 6-311++G** basis set to the complete series of methanes, silanes and germanes (the latter ones are just added for completeness because no experimental data are available for them and, moreover, we published them already previously) and to compare the results to experimental data available in the literature to see whether the failures of this basis set show up in the complete series of molecules. In the literature we found five such molecules and the information which of the conformers is the most stable. Indeed we found that predictions on the relative stability of conformers in those systems with this basis set and MP2 as well as DFT are with a 60:40 chance, three being correct predictions and two being incorrect ones out of the five. However, since the energy differences are rather small in these systems and due to the fact that - as a consequence of twofold degeneracy of the gauche conformer on the potential curve of the torsional vibration - the abundances of the conformers in equilibrium do not change too much, we decided to calculate also vibrational spectra for three examples and to compare them also to experiment. It is reported that besides the failures in total energy (we have chosen two examples where predictions of the nature of the stable conformer are correct, and one where it is not), the vibrational spectra are rather well reproduced, especially when experimental energies are used to calculate abundances in equilibrium in the case where the prediction of the stable conformer failed.

The structure, vibrational and NMR spectra of the local anesthetic drug lidocaine hydrochloride monohydrate salt were investigated by B3LYP/6-311G(∗∗) calculations. The lidocaine·HCl·H2O salt is predicted to have the gauche structure as the predominant form at ambient temperature with NCCN and CNCC torsional angles of 110° and -123° as compared to 10° and -64°, respectively in the base lidocaine. The repulsive interaction between the two N-H bonds destabilized the gauche structure of lidocaine·HCl·H2O salt. The analysis of the observed vibrational spectra is consistent with the presence of the lidocaine salt in only one gauche conformation at room temperature. The (1)H and (13)C NMR spectra of lidocaine·HCl·H2O were interpreted by experimental and DFT calculated chemical shifts of the lidocaine salt. The RMSD between experimental and theoretical (1)H and (13)C chemical shifts for lidocaine·HCl·H2O is 2.32 and 8.21ppm, respectively.

The structural stability of sulfamoyl halides NH(2)-SO(2)X (X is F, Cl and Br) were investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. From the calculations the molecules were predicted to exist only in the anti (XS bond is anti with respect to nitrogen lone pair) conformation with the possibility of very low abundance of the syn (SO(2) and NH(2) groups eclipse each other) form of only the fluoride. The equilibrium constant for the syn--anti conformational conversion of sulfamoyl fluoride was calculated to be 0.0172 that corresponds to an equilibrium mixture of about 2% syn and 98% anti at 298.15K. The vibrational frequencies were computed at DFT-B3LYP level for the stable anti conformer of the d(0) and d(2) (ND(2)-SO(2)X) deuterated species of the three molecules. Normal coordinate calculations were then carried out and the potential energy distributions were calculated for the molecules.

The conformational and structural stability of imidoylketene O C CH–CH NH were investigated by DFT-B3LYP and ab initio MP2 calculations with the 6-311+G** basis set. From the calculations imidoylketene was predicted to exist predominantly in a mixture of trans – anti (the CCCN dihedral angle is 180°) and cis – anti (the CCCN dihedral angle is 0°) conformations with the trans – anti being the lower energy form. The two anti conformations were predicted to have a comparable relative stability with the C–C rotational barrier of about 9–10 kcal/mol at DFT-B3LYP and MP2 calculations. The equilibrium constanat for the trans ⇔ cis conformational conversion of the two anti forms of imidoylketene was calculated to be 0.4432 kcal/mol that corresponds to an equilibrium mixture of about 31% cis – anti and 69% trans – anti at 300 K. The vibrational frequencies were computed at the DFT-B3LYP level and the infrared and Raman spectra of the molecule were calculated. Complete vibrational assignments were made on the basis of normal coordinate analyses.

We studied the ring puckering potential in thietane-1,1-dioxide with different methods, using a suitable basis set, 6-311+G**. We obtained a barrier to ring puckering of 153 cal/mol with the DFT/B3LYP (Becke3 exchange–Lee, Yang, Parr correlation functional) method, ∼60% too small compared with experiment. However, using MP2, MP3, and MP4 we obtained values around 200% too large. The MP series turned out to converge far too slowly to the experimental barrier value, showing no sign of convergence even at MP4, while higher orders are out of our reach for such a system. Obviously, of all methods used, DFT worked best despite some shortcomings. The barrier corresponds to 77 K; thus, there should be rapid interconversion over the barrier at room temperature, a phenomenon actually observed at room temperature, the measured barrier corresponding to 201 K. Thus, we decided to use the DFT/6-311+G** calculations to predict reasonable vibrational spectra and assignments of the lines found. The ring puckering mode is found at 80 cm−1 in DFT in good agreement with the experimental value of 78.3 cm−1 for this vibration. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Wolfgang Fo rner*, Hassan M. Badawi, and Zaki S. Seddigi Department of Chemistry, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia *Author to whom correspondence should be addressed; e-mail: forner@kfupm.edu.sa Received: 7 March 2005 / Accepted: 27 March 2005 / Published: 4 October 2005 Abstract: The structural stability of haloselenonyl azides was investigated by quantum mechanical Moller-Plesset perturbation theory of second order and density functional theory calculations. The 6-311+G** basis set was used to include polarization and diffuse functions in the calculations at the DFT-B3LYP level. The potential scans for the rotation of the -NNN rotor were calculated and found to be consistent with a single minimum that corresponds to a gauche conformation (-NNN moiety nearly eclipses one of the two selenonyl Se=O bonds) for the three halogens at ambient temperature. The structural parameters for the minima calculated by MP2 and DFT turned out to be very similar. The vibrational modes, infrared and Raman intensities as well as depolarization ratios were calculated at DFT-B3LYP/6-311+G** level for the three molecules in their gauche conformations. The potential energy distributions among symmetry coordinates of the normal modes of the mo-lecules in their gauche conformation were then computed from normal coordinate analyses. Keywords: MP2 and DFT calculations; structural stability; vibrational modes and assign-ments; haloselenonyl azides. Introduction The conformational behavior of fluorosulfonyl isocyanate F-SO

The structural stability of vinylamine (aminoethylene) CH 2 CH–NH 2 and keteneamine (aminoketene) O C CH–NH 2 were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311+G** basis set. From the calculations vinylamine was predicted to exist predominantly in a non-planar structure consistent with an earlier microwave results, while keteneamine in a mixture of anti and syn conformations, with the anti being the low energy conformer at ambient temperature. The equilibrium constant for the anti ⇔ syn interconversion in keteneamine was estimated to be 0.4456 at 300 K. From the calculations the NH 2 inversion barrier was estimated to be about 0.5 and 1.6 kcal/mol for vinylamine and about 2.3 and 3.3 for keteneamine at DFT-B3LYP/6-311+G** and MP2/6-311+G** levels, respectively as compared to about 1.0 kcal/mol (356 cm −1 ) obtained from a microwave data of vinylamine. The vibrational frequencies were computed at the DFT-B3LYP level and the infrared and Raman spectra for each molecule and its deuterated species. Complete vibrational assignments were made on the basis of normal coordinate analyses and isotopic substitution for vinylamine and keteneamine.

The conformational and structural stabilities of nitrosoethylene CH2=CH–N=O, chloronitrosoethylene CH2=CCl–N=O, and Dichloronitrosoethylene CCl2=CH–N=O were investigated by ab initio Moeller–Plesset perturbation theory of second order (MP2) calculations using the 6−311+G** basis set to include electron correlation. From the calculations all three were predicted to exist predominantly in the planar trans structure (C=C and N=O bonds are trans to each other) with high trans-cis rotational barriers of about 9 kcal mol−1 as a result of pronounced conjugation between C=C and N=O bonds. The vibrational frequencies were computed for the three molecules, and also the d 1 and d 2 deuterated variants for the parent molecule at the MP2 level. Normal coordinate analyses were carried out and the potential energy distributions (PED), among the symmetry coordinates of the normal modes of the molecule were computed. Complete vibrational assignments were made on the basis of normal coordinate analyses for the molecules. The two chlorinated derivatives of nitrosoethylene were also investigated in the same way. As expected, we then find high Raman and infrared intensities in all modes that contain a high content of chlorine movements because vibrations of C–Cl bonds lead to large changes in polarizability, as well as to a large change in dipole moment. However, modes involving double bonds also have quite large intensities. An appreciable number of modes in these molecules are more or less pure symmetry coordinates.

The conformational and structural stability of monofluoro-nitrosoethylene CH 2 CF–N O, and difluoro-nitrosoethylene CF 2 CH–N O were investigated by ab initio Moller-Plesset Perturbation Theory of second order (MP2) calculations using the 6-311+G** basis set to include electron correlation. From the calculations the two molecules were predicted to exist predominantly in the planar trans structure with high trans – cis rotational barriers of about 10–11 kcal/mol as a result of pronounced conjugation between C C and N O bonds. The differences in the rotational barriers when going from the parent molecule, nitrosoethylene, to its fluorinated derivatives are explained on the basis of electronegativity. The vibrational frequencies were computed for the two molecules at the MP2 level. Normal coordinate analyses were carried out and the potential energy distributions, PED, among the symmetry coordinates of the normal modes of the molecules were computed. Complete vibrational assignments were made on the basis of normal coordinate analyses for the molecules. As expected we find there low Raman and appreciable infrared intensities in all modes which contain a high content of fluorine movements because vibrations of C–F bonds lead to a small change in polarizability, but to a large change in dipole moment. However, also modes involving double bonds have quite large intensities in both spectra. An appreciable number of modes in these molecules are composed of relatively large numbers of symmetry coordinates, i.e. they are highly mixed.

The structural stability of (trihalomethyl)sulfenyl halides CX3-SX (X is F and Cl) was investigated by DFT-B3LYP and ab initio MP2 calculations using 6-311 + G** basis set. Full energy optimizations were carried out from which the three-fold barrier about C-S bond was calculated to be about 3 kcal mol(-1) in (trifluoromethyl)sulfenyl fluoride and (trifluoromethyl)sulfenyl chloride and about 6 kcal mol(-1) in (trichloromethyl)sulfenyl fluoride and (trichloromethyl)sulfenyl chloride. The vibrational frequencies of the four molecules were computed at the DFT-B3LYP level and the vibrational assignments for the normal modes of the compounds in their ground state structure were made on the basis of normal coordinate calculations and reported experimental data.

The conformational and structural stability of vinyl diazene and vinyl imine were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311+G** basis set. Vinyl diazene was predicted to exist only in the planar trans – anti conformation (CCNN and CNNH dihedral angles are 180°), while vinyl imine to exist in two trans (CCCN dihedral angle is 180°) conformations. The trans – anti (CCNH dihedral angles is 180°) conformation of vinyl imine was predicted to be about 1 kcal/mol lower in energy than the trans – syn (CCNH dihedral angles is 0°) form that is in excellent agreement with an earlier microwave results. The vibrational frequencies were computed at the DFT-B3LYP level and the calculated infrared and Raman spectra of each molecule were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for the two molecules.

The structural stability of carbamothioic acid NH 2 CO–SCl was investigated by DFT-B3LYP and ab initio MP2 calculations using 6-311+G** basis set. Full energy optimizations were carried out and the molecule was predicted to exist predominantly in the trans (C O and S–Cl bonds are anti to each other) conformation. From the calculations, the two-fold NH 2 barrier about the N–C bond was calculated to be about 12 kcal/mol, while, the asymmetric OCSCl barrier about the C–S bond to be about 8 kcal/mol. The vibrational frequencies of the d 0 - (NH 2 COSCl) and d 2 - (ND 2 COSCl) deuterated species of the molecule were computed at the DFT-B3LYP level. Then vibrational assignments for the normal modes of the compound in its stable trans conformation were made on the basis of normal coordinate calculations.

The conformational stability of azidoketene OCCH–NNN was investigated by Density Functional B3LYP and ab initio second-, third- and fourth-order Moller Plesset calculations with the 6-311+G** basis set. Azidoketene was predicted at the DFT-B3LYP level to have the s-cis⇔s-trans conformational equilibrium with the s-trans (O=C=C and NNN moieties are trans to each other) being the predominant low energy form, while at the MP2, MP3 and MP4 levels to have the gauche (O=C=C and NNN moieties are near trans to each other) form as the minimum in the potential curve of the molecule. From full structural optimization of the ground and transition states at the DFT-B3LYP level, the trans–cis barrier was calculated to be about 2 kcal/mol. The vibrational frequencies for the cis and trans conformations were computed for the gauche form at the MP2 level. The calculated infrared and Raman spectra of the molecule were plotted. Complete vibrational assignments were provided on the basis of normal coordinate calculations for the molecule.

The structure of 3,5-dichlorophenylcyanamide c-C6H3Cl2–NHCN was investigated by DFT-B3LYP and ab initio MP2 calculations with the 6-311+G** basis set. The planar to perpendicular rotational barrier was calculated to be of about 5 kcal mol−1 at both levels of calculation. The stability of the planar structure of the molecule was explained on the basis of conjugation effects between the cyanamide–NHCN moiety and the phenyl c-C6H5 ring in agreement with earlier NMR results. The CNC and the HNC bond angles were calculated to be about 120° especially by MP2 calculation, which is consistent with sp 2 (planar –NH–CN group) and not sp 3 (pyramidal –NH–CN group) structure. The vibrational frequencies of the d0, d1 and d3 species of 3,5-dichlorophenylcyanamide and the potential energy distributions among symmetry coordinates of the normal modes of the parent molecule were computed at the DFT-B3LYP level. The calculated infrared and Raman spectra of the molecule were plotted. Complete vibrational assignments were made on the basis of isotopic substitution and normal coordinate calculations. Figure Potential curves for the internal rotation in 3,5-dichlorophenylcyanamide as determined by DFT-B3LYP/6-311+G** (solid) and MP2/6-311+G** (dotted) calculations