A. V. Abramenkov

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.

On the second rotational isomeric form of butadiene-1,3

Abstract Eleven lines of the “hot” band progression of the torsional overtone 2 ν 13 have been measured in the Raman spectra of cis,cis -1,4- d 2 -butadiene-1,3 and d 6 -butadiene-1,3. The potential of Carreira ( J. Chem. Phys. 62 , 3851–3854 (1975)) ( V 1 = 600, V 2 = 2068, V 3 = 273, and V 4 = −49 cm −1 ) and that of Bock et al. ( J. Chem. Soc. Perkin II , 26–34 (1979)) ( V 1 = 347.3, V 2 = 1790.0, V 3 = 612.8, and V 4 = 179.8 cm −1 ) have been used to interpret the results obtained. For the accepted expansion of the Pitzer function F ( φ ), it is shown that the potential of Bock et al. reproduces the frequencies of the torsional overtones of these two molecules and h 6 -butadiene-1,3 more closely. The improtance of data on isotopic molecules in the calculation of potential curves from spectroscopic information is emphasized.

One- and Two-Dimensional Torsion-Inversion Models for the Fluoral Molecule (CF3CHO) in Excited Electronic States

The structure of the conformationally nonrigid fluoral molecule (CF3CHO) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was studied by ab initio quantum-chemical methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecule in these electronic states were determined. The calculations demonstrated that the electronic excitation causes substantial changes in the molecular structure involving the rotation of the CF3 top and the deviation of the CCHO carbonyl fragment from planarity. The quantum-mechanical problems for large-amplitude vibrations, namely, for the torsional vibration in the S0 state and the torsional and inversion vibrations (nonplanar carbonyl fragment) in the T1 and S1 states, were solved in the one- and two-dimensional approximations. A comparison of the results of calculations revealed the correlation between the torsional and inversion motions.

S1←S0 Vibronic spectrum and structure of 2,2-difluoroethanal in the S1 state

The vibronic spectrum of the 2,2-difluoroethanal vapor was recorded using a multipass optical cell with an optical length of at least 140 m. The spectrum in the region of 300—364 nm was assigned to the S1←S0 electronic transition (from the ground S0 to the first excited singlet S1 electronic state); the vibrational structure of the spectrum was analyzed. The spectrum bands were assigned to two systems of vibronic transitions, namely, transitions between the levels of the cis-conformer (S0) and of the S1 conformers, with the origins (000 transitions between the zero vibrational levels of conformers) at 29192 and 29087 cm–1, respectively. Analysis of the spectrum showed that the S1←S0 electronic excitation of the cis-conformer was followed by rotation of the CHF2 top and pyramidal distortion of the carbonyl fragment. A number of fundamental frequencies were found for S1 conformers, in particular, torsion and inversion energy levels. The experimental data are in satisfactory agreement with the results of quantum-chemical calculations for the 2,2-difluoroethanal molecule in the S0 and S1 states.

Ab initio study of the structure and conformations of 2-fluoroethanal in the ground and lowest excited electronic states

The structure of the conformationally flexible 2-fluoroethanal molecule (CH2FCHO, FE) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was investigated by ab initio quantum-chemical methods. The FE molecule in the S0 state was found to exist as two conformers, viz., as cis (the F—C—C—O angle is 0°) and trans (the F—C—C—O angle is 180°) conformers. On going both to the T1 and S1 states, the FE molecule undergoes substantial structural changes, in particular, the CH2F top is rotated with respect to the core and the carbonyl CCHO fragment becomes nonplanar. The potential energy surfaces for the T1 and S1 states are qualitatively similar, viz., six minima in each of the excited states of FE correspond to three pairs of mirror-symmetrical conformers. Based on the potential energy surfaces calculated for the FE molecule in the T1 and S1 states, the one-dimensional problems on the torsion and inversion nuclear motions as well as the two-dimensional torsion-inversion problems were solved.