Kangnian Fan

The harmonic force field and vibrational spectra of pyrrole

The complete harmonic vibrational force field of pyrrole has been calculated by the ab initio gradient method at the Hartree-Fock level using the 4–21 basis set. The force field was then scaled with a set of six factors transferred from benzene, and the vibrational spectrum of pyrrole was calculated. This a priori prediction, made with no reference to observations on pyrrole, agreed with the known experimental fundamental frequencies with a mean deviation of 12 cm-1 for in-plane modes and 20 cm-1 for out-of-plane modes except for the NH wagging and NH stretch. A new set of ten scale factors was next obtained by direct fitting of the computed force field to the observed pyrrole spectrum, producing the best force field obtainable by combined use of the theoretical and experimental information. This force field reproduced the entire pyrrole spectrum with mean deviations of 4·2 cm-1 (in-plane) and 5·9 cm-1 (out-of-plane). The spectra of three deuterated forms of pyrrole were also computed. Infrared absorption...

A study of the vibrational behavior of imidazole by the ab initio gradient method

Abstract The complete harmonic vibrational force field of imidazole has been computed at the Hartree—Fock level using the 4–21 basis set of Gaussian orbitals. The harmonic constants were scaled by using scale factors previously derived by fitting the computed force field of benzene to the observed benzene vibrational spectrum. The resulting scaled force field was then used to predict the vibrational spectrum of imidazole. The NH wagging and NH stretching frequencies, which have no counterpart in benzene, were poorly predicted, but the mean deviation between experiment and prediction was only 9 cm−1 for the other in-plane (A′) vibrations and 27 cm−1 for the out-of-plane (A″) vibrations. In order to fit the NH modes and to obtain a better fit for the out-of-plane vibrations, a new set of scale factors, some of which turned out to be identical with those from benzene, was derived by fitting the computed spectrum of imidazole to the observed spectrum. This new set of scale factors was used to predict the vibrational spectra of several deuterated forms of imidazole (−1D, −3D, and −4D). The imidazole scale factors are presumably more accurate in scaling computed spectra of other 5-membered heterocyclic ring compounds. Dipole moment derivatives were also calculated and used to predict infrared intensities which are compared with experimental values. A few uncertainties in the experimental assignment of the imidazole spectrum can be clarified.

Analysis of the vibrational spectra of 1-methyluracil and its isotopic derivatives by ab initio calculations

Abstract The vibrational spectrum of 1-methyluracil trapped in an argon matrix has been analysed based on ab initio Hartree—Fock SCF calculations with a split-valence 4–21 basis set. The directly computed theoretical harmonic force field was scaled with empirical scale factors which were transferred from uracil (except for the methyl vibrational modes) to provide an a priori prediction of fundamental frequencies and intensities. The average deviations between experiment and prediction were 9.8 cm−1 for the in-plane vibrations and 18.3 cm−1 for the ring out-of-plane modes. After a few corrections of assignment of the observed spectrum, a new set of scale factors was optimized to give the best force field available from combined consideration of the experimental and theoretical information. These scale factors reduced the average deviations to 6.7 cm−1 for the in-plane modes and to 11.7 cm−1 for the out-of-plane ones. The vibrational spectra of seven isotopic derivatives (-2180, -4180, -3d, -5d, -6d, -5, 6d2 and -CD3) of 1-methyluracil were predicted with the force field resulting from the optimized set of scale factors, and compared with the crystal-phase experimental data. A few misassignments in the experimental isotopic spectra have been corrected. Vibrational absorption intensities have been computed and compared with experiment and with an earlier calculation.

Rotational isomerism of acrylic acid

Abstract Scaled quantum-mechanical force fields have been calculated for the s-cis and s-trans conformations of acrylic acid. The directly computed force fields, obtained at the Hartree-Fock 4–21 basis set level, were scaled with a small set of scale factors previously derived to account for the residual error in calculations at this level. The spectra of the two species were then computed and compared with spectra observed in an inert matrix at 25 K. Two sets of observed bands, distinguishable by their differing intensity behavior under ultraviolet irradiation, could be identified as corresponding to the two rotational conformers. Although the spectral differences resulting from the conformational difference were rather subtle, they could be unequivocally identified by comparison with the computational results.

Analysis of the vibrational spectrum of 6-methyluracil by ab initio calculation

Abstract The harmonic vibrational force field of 6-methyluracil has been computed at the ab initio Hartree-Fock level using a 4/2-21 basis set of Gaussian orbitals. A set of scaling factors, optimized for 1-methyluracil, was transferred unchanged and used to scale the force field computed for this closely related molecule. Dipole-moment derivatives were also computed. The spectrum of fundamental frequencies and absorption intensities was computed from the scaled force field and compared with previous experimental assignments of the gas-phase and matrix spectra. Mean deviations between computed frequencies and the matrix data were 8.2 cm −1 for the inplane modes and 14.4 cm −1 for the out-of-plane ones. One transition, the v 21 CO wagging motion, is believed to have been previously misassigned.

An ab initio calculation of the vibrational force field and spectrum of pyrazole

Abstract The complete harmonic vibrational force field of pyrazole has been computed at the Hartree—Fock level with the 4–21 basis set of Gaussian orbitals. The harmonic force field was scaled by using 6 scale factors previously derived by fitting the computed force field of benzene to the observed benzene vibrational spectrum. The resulting scaled force field was then used to predict the vibrational spectrum of pyrazole. The frequency predictions have also been done by using the 9 scale factors (including those for the NH motions) previously optimized for imidazole. The predicted values for the fundamentals ν12 and ν19, for which no experimental data are available, are totally consistent in the above two procedures. In order to obtain the best possible force field, a new set of 10 scale factors, some of which turned out to be identical with those from imidazole, was derived by fitting the computed spectrum of pyrazole to the observed pyrazole spectrum. Dipole-moment derivatives were also calculated and used to predict infrared intensities which are compared with experimental values.

Infrared and ab initio studies of molecules containing SiH bonds: Part I. A correlation between vis(SiH) and re (SiH)

Abstract Isolated SiH stretching frequencies v is (SiH) are reported for the species SiHDX 2 (X = F or Cl), SiHD 2 NMe 2 , SiHD 2 OMe, (SiHD 2 NMe and (SiHD 2 ) 2 S. IR spectra in the gas and solid phases are also shown for CHD 2 OSiH 3 . Splittings of v as (CH 3 ) in (CH 3 ) 2 )SiH 2 and (CH 3 ) 3 SiH are reported, from which v is (CH) values are deduced. Evidence for the structures of these molecules is discussed. Ab initio structures have been determined using a 4–21G/3-3-21G basis set for the above molecules, and for SiH 4 , SiH 3 F, SiHF 3 , SiH 3 Cl, SiHCl 3 , SiH 3 Me, SiH 2 Me 2 , SiHMe 3 , SiH 3 CN, SiH 3 CCH, SiH 3 SiH 3 , SiH 3 CH 2 CH 3 , SiH 3 CH 3 Cl, SiH 3 CHCH 2 and CH 3 CH 2 CH 3 . The r e (SiH) and r e (CH) values are tabulated. A good correlation is found between v is (SiH) and r e (SiH) r e (SiH)(A) = 1.9089 − 1.998 × 10 −4 v is (SiH) (cm −1 ) Plots for SiH 2 Cl 2 , and SiHCl 3 reveal large anomalies, but SiH 3 CN and SiH 3 CCH behave normally. Results are compared with similar ones for CH bonds. The CH bond length asymmetry in Me 2 SiH 2 and Me 3 SiH revealed by the v as (CH 3 ) splittings is confirmed by the ab initio calculations. The effect of conformation on the β effect of methyl on SiH and CH bonds is explored.

The effect of methylation on the structure of uracil

Abstract Equilibrium geometries of uracil, 1-methyluracil and 3-methyluracil (in which the methyl group is attached to nitrogen), 5-methyluracil (thymine) and 6-methyluracil (in which the methyl group is attached to carbon), 1,3-dimethyluracil and 5,6-dimethyluracil have been determined by ab initio Hartree-Fock calculations with the split-valence 4-21G basis set. For the methylated derivatives, calculations are made for different conformations corresponding to different orientations of the methyl groups. The conformational energy differences are small, indicating a very low barrier to internal rotation, except for 5- and 6-methyluracils in which there is a preference of 1–2kcal mol −1 for the conformer with the methyl CH bond eclipsing the double bond of the ring. The structural differences between the methylated uracils and the parent molecule are analyzed. Angular deformations within the ring induced by substitution of a methyl group for hydrogen follow, to a rough approximation, the trends established earlier for benzene derivatives on the basis of X-ray studies. Deviations occur due to the difference between nitrogen and carbon in the ring, with deformations being more pronounced for N- than for C-substituted uracils. The methyl groups, in general, show a distinct tilt away from an adjacent carbonyl group, indicating a repulsive interaction. Mulliken population analysis shows the electronegative methyl group withdraws charge mainly from the atom to which it is attached and, to at least as great an extent, from the adjacent ring atoms. The results are compared with those obtained earlier by the semiempirical MINDO 3 method and also by different experimental techniques including X-ray, neutron, and electron diffraction. These other studies have given much information on the structure of the compounds, but because of their nature they have not been able to analyze detailed structural variations induced by the methyl group substitution.

Infrared and ab initio studies of compounds containing SiH bonds: Vibration-internal rotation spectra and silyl group geometry in PhSiHD2

Abstract Internal rotation structure has been observed in the νSiH band of PhSiHD 2 . The main features have been analyzed conventionally to obtain A ″ − B ″ = 1.6817 (20) cm −1 . Assuming probable values for B ″ and r 0 SiH, the HSiH angle is found to be 108.9 (2)°. Other features of the spectrum, including an apparent large negative D K value, require theoretical study. Ab initio calculations of the full geometry of the molecule in both orthogonal and planar conformations indicate a small change (0.0013 A) in the SiH bond length during internal rotation, and quite large changes in HSiH angle. Changes in the CC distances are similar to those calculated earlier in toluene.