S.V. Krasnoshchiokov

Structure and vibrational assignment of gouche-1,3-butadiene

Abstract The completely optimized structures and harmonic force fields of the s-trans(anti) - and gouche -isomers of 1,3-butadiene have been computed at the ab initio Hartree—Fock level using the 6·31G basis set. The gouche dihedral angle was found to be 34.8° from the planar s-cis(syn) -configuration of the 1,3-butadiene molecule. Seven scale factors for correcting the theoretical force constants of the trans -isomer were calculated from the experimental frequencies of light and heavy trans -1,3-butadienes. The correction of the gouche -butadiene force field was then carried out using these scale factors, and the vibrational problems were solved for gouche forms of C 4 H 6 , CD 2 CHCHCD 2 , C 4 D 6 , 13 CH 2 CHCH 13 CH 2 , and 12 CH 2 CHCHCH 2 . The total assignment of the experimental vibrational frequencies of these isotopomers is given. The conclusion drawn is that the quantum mechanical geometry and the scaled quantum mechanical force field correctly simulate the structure and the experimental frequencies of the butadiene gouche -conformer.

An ab initio prediction of structures and vibrational frequencies of high-energy rotamers of glyoxal and acrolein

Abstract Complete optimizations of the structures and calculations of the harmonic force fields for glyoxal and acrolein stable forms in the ground electronic state have been carried out at the RHF/6-31G level. The high-energy conformers are found to be planar for both molecules in agreement with previous calculations. The empirical scale factors correcting the ab initio force field were computed for the trans (anti) conformer of glyoxal. The corresponding scale factors from 1,3-butadiene and the scale factors obtained for trans-glyoxal were then employed to correct the ab initio force fields of the cis (syn) conformer of glyoxal and both rotamers of acrolein. The vibrational frequencies were computed with the scaled quantum mechanical (SQM) force fields and some molecular parameters are also calculated. The assignment of some frequencies is also discussed.

Ab initio structures and vibrational analysis of two planar configurations of 1,3,5-hexatriene

Abstract The geometrical parameters of the tTt -hexatriene-1,3,5 and tCt -hexatriene-1,3,5 molecules have been computed at the RHF/6–31G level. The structures of both molecules are found to be planar in this approximation. The force fields of these molecules were calculated at the HF/6–31G//HF/6–31G level. The vibrational analyses of both molecules and their 2,3,4,5-tetradeuteroanalogues made it possible to refine the assignment of some experimental vibrational frequencies. The calculated mean amplitudes of vibrations of the non-deuterated molecules are compared with the experimental ones.

An ab initio structural and vibrational analysis of gauche,trans,trans- and gauche,cis,trans-hexa-1,3,5-trienes

The results of complete geometry optimizations of the high energy stable gauche,Trans,trans- and gauche,Cis,trans- rotamers of hexa-1,3,5-trienes are reported at the RHF/6-31G//RHF/B-31G level. The angles of rotation around one of the single C-C bonds are found to be 33.7° and 45.5°, respectively. The corresponding harmonic force fields of these molecules are also reported at this level and corrected using scale factors transferred from buta 1,3-diene. Aspecial scale factor was used for the central C=C double bond stretching coordinate to take into account vibronic coupling. The theoretical vibrational frequencies, calculated with the scaled quantum mechanical (SQM) force fields, allow a complete interpretation of the experimental vibrational spectra of these molecules.

Ab initio structures and vibrational analysis of the isoprene conformers

Abstract Complete gradient optimizations of the structures and the calculation of the harmonic force fields of the s- trans ( anti ) and gauche conformers of isoprene (2-methylbuta-1,3-diene) are reported at the RHF/6-31G level. The dihedral angle of the gauche conformer is found to be 41.0° from the planar s- cis ( syn ) form. The force fields obtained are refined using scale factors transferred from analogous calculations for trans -butadiene-1,3 and ethane. The direct vibrational problems are solved for both conformers of isoprene. A complete assignment of the experimental vibrational frequencies is given.

Ab initio analysis of structure and vibrational spectrum of methyl nitrate

Abstract Ab initio calculations of the structure and harmonic force field of methyl nitrate have been computed at the HF/6-31G//HF/6-31G level. The assignment of the experimental frequencies of the CH 3 ONO 2 and CD 3 ONO 2 molecules were made using the unscaled quantum-mechanical force constants, which suggest a reassignment of the δ(NO 2 ) and ϱ r (NO 2 ) vibrations of CD 3 ONO 2 . Taking into account these new vibrational assignments, a refined harmonic force field for methyl nitrate is computed using a least-squares technique.

Transferability of quantum mechanical force field scale factors between conjugated hydrocarbons

The transferability of scale factors (empirical corrections) for quantum mechanical force fields among closely related molecules is tested for the related hydrocarbons, ethylene, 1,3-butadiene, and 1,3,5-hexatriene and their isotopomers. The agreement demonstrated here between the fundamental vibrational frequencies calculated from the scaled quantum mechanical force fields and the experimental frequencies of these molecules confirms this transferability. The advantage of combining quantum mechanical calculations of force constants with empirical scale factors for the accurate prediction of force fields and vibrational frequencies in the analysis of the incomplete experimental spectra of the rotamers of these hydrocarbons is stressed.

All ab initio vibrational study of rotational isomerism in oxalyl fluoride, OCF-CFO, and acryloyl fluoride, OCF-CHCH2

Abstract Results of complete geometry optimizations of the rotational isomers of oxalyl fluoride and acryloyl fluoride at a variety of computational levels are reported. These molecules are found to have planar s-trans (anti) and s-cis (syn) conformations. The RHF/6-31G//RHF/6-31G harmonic force fields of these molecules are calculated as analytical second derivatives, and the vibrational frequencies are obtained from the corresponding scaled quantum mechanical force fields. Some reassignments of fundamental frequencies are suggested.

An ab initio calculation of the structure and scaled quantum mechanical harmonic force field of nitromethane

Abstract A complete optimization of the nitromethane geometry at the HF/6-31G and HF/ 6-31G* (5D) levels for staggered (one CH bond situated in a plane perpendicular to the plane of the heavy atoms) and eclipsed conformations is reported. The geometrical parameters obtained for the staggered conformation are in quite good agreement with the microwave structure (ref. 1). The effect of electron correlation on the energy difference between the two conformers is estimated using single point MP4SDQ/6-31G* calculations at the 6-31G geometry. The harmonic force field for both conformers are calculated at the HF/6-31G//HF/6-31G level. Taking into account the known overestimation of the vibrational frequencies by ∼20% in ab initio calculations, the experimental frequency assignment is confirmed. The six scale factors for the force field were evaluated using the experimental frequencies of nitromethane and nitromethane-d 3 (ref. 2). The vibrational frequencies for the five isotopomers of staggered nitromethane are determined with the scaled quantum mechanical force field (SQMFF). A detailed interpretation of conformational effects in the IR spectrum of crystalline CHD 2 NO 2 is proposed.

Force constants and molecular potential functions in redundant coordinates

Abstract An analysis is given of problems arising in the theory of molecular vibrations in redundant (dependent) coordinates. Any force constant matrix in redundant coordinates is shown to correspond to the potential function, which is transformed into the exact molecular potential energy function when account is taken of connections between coordinates.