C. W. Bock

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.

An AB initio structural investigation of 1,3-butadiene, isoprene and 2,3-dimethyl-1,3-butadiene rotamers

Abstract The variation in the torsional angles of the high energy gauche conformers of 1,3-butadiene, 2-methyl-1, 3-butadiene (isoprene) and 2,3-dimethyl-1,3-butadieneare investigated using geometry optimizations at the HF/6-31G and HF/6-31G * levels. In each case, comparisons are made with the corresponding cis structures. The effects of electron correlation are explored in the case of 1,3-butadiene by including geometry optimizations at the MP2/6-31G * level for both the gauche and cis conformers and for isoprene with single point MP2 and MP3/6-31G * //HF/6-31G * calculations. In all cases, the gauche conformers are found to be more stable than the corresponding cis conformers.

The butadiene-1,3 internal rotation potential function obtained from ab initio calculation and experimental data

Abstract Values of the Pitzer function F (φ), coefficients of its Fourier expansion, and coefficients of the potential energy expansion are obtained from ab initio HF/6-311G ** calculations of the potential curve for internal rotation and the optimized geometry of butadiene-1,3 (Ch. W. Bock et al., Theor. Chim. Acta, 64 (1984) 293). A correction of the theoretical coefficients in the potential energy expansion is performed on the basis of the “hot” band progression frequencies of the trans -form torsional overtones and the ab initio calculated values of the gauche -form torsional frequency-1,3 and its two isotopomers (C 4 D 6 and cis , cis -1,4-d 2 -butadiene-1,3). The coefficients in the potential energy expansion are found to be: V 1 = 484.9, V 2 = 1349.1 V 3 = 837.2 V 4 = −139.3, V 5 = −9.8, V 6 = −77.5, V 7 = 25.8, and V 8 = 13.5. The advantage of using an ab initio potential curve as a starting approximation in such investigations is 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.

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.

A theoretical study of the structure and spectrum of 1,3,5-hexatriene

Abstract Ab initio STO-3G calculations are performed on tTt- and tCt-1,3,5-hexatriene, including a partial geometry optimization on the tTt-isomer. These results are compared with the conclusions of Traetteberg based on an analysis of electron diffraction data. Some significant discrepancies are found, particularly with respect to the relative lengths of the central and terminal double bonds. In addition, a CNDO/S spectral analysis is performed at several different geometries and is compared with experiment.

COMMENT ON THE AB INITIO VIBRATIONAL ANALYSIS OF THE ROTATIONAL ISOMERS OF ACROLEIN

Due to the publication of a number of contradictory assignments of the vibrational wave numbers of rotational isomers of Acrolein in the ground electronic state, the analysis of their vibrational spectra is repeated based on the previously calculated scaled ab initio force fields. With the use of the reported results that predicted the force fields of trans-acrolein in the 1(n,π*) and 3(n, π*) states at the CASSCF/cc-pVTZ level, the experimental vibrational bands are analyzed in these excited electronic states based on well-established regularities. It is noted that in the assignment of the calculated vibrational wave numbers of the molecule, the isotopic shifts in the ground and excited electronic states 1(n, π*) and 3(n, π*) are taken into account. The previously considered calculated potential curves of the internal rotation of acrolein in combination with the data on the difference in the enthalpies (ΔH0) of conformers allow a choice to be made in favor of one of the variants of the torsional vibration wave numbers that have been reported in the literature.