N.N. Yakovlev

Determination of the potential functions of molecular inversion from experimental data: Current state and problems

Various approximations of the method of determining two-well potential functions of molecular inversion from experimental data (geometrical parameters and inversion level energies) are considered. The potential of the method is illustrated by reference to carbonyl molecules in the lowest excited states. Some of the current problems are discussed.

The S 1 ← S 0 vibronic spectra and structure of the (CH3)2 CHCHO and (CH3)2 CHCDO 2-methylpropanal molecules

A multipass cell with an optical path up to 120 m long was used to measure the vibronic absorption spectra of 2-methylpropanal-h1 (MPA-h1, (CH3)2CHCHO)) and 2-methylpropanal-d1 (MPA-d1, (CH3)2CHCDO)) over the frequency range 28200–31600 cm−1. The most intense spectral lines were assigned to transitions from vibrational levels of the cis and gauche MPA-h1 and MPA-d1 conformers in the ground electronic state (S0) to vibrational levels of conformers 1 and 3 in the lowest singlet excited electronic state (S1). According to our estimates, the origins (000) of the 1S1) ← cis(S0) and 3(S1) ← cis(S0) and also 1(S1) ← gauche(S0) and 3(S1) ← gauche(S0) electronic transitions were situated at 29147 and 29177, 29391 and 29417 cm−1, respectively, for MPA-h1 and at 29226 and 29240, 29480 and 29500 cm−1 for MPA-d1. The structure of conformers 1 and 3 in the S1 state was shown to differ from the structure of the cis and gauche conformers in the S0 state by the angle of rotation of the (CH3)2CH-isopropyl top and “pyramidal distortion” of the CCHO/CCDO carbonyl fragment. A series of fundamental frequencies of MPA conformers in different electronic states were found. The potential functions of inversion were determined for the conformer 1-conformer 3 pairs of MPA-h1 and MPA-d1 from the experimental energy levels of inversion vibrations. The potential barriers to inversion and equilibrium displacements of the CH/CD bond out of the CCO plane were found to be 735/675 cm−1 and ±34°/±32° for MPA-h1 and MPA-d1, respectively.

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.

S 1←S 0 vibronic spectra and inversion potentials in theS 1 states of acetyl and formyl halides

The potential functions of inversion in the S1 states are determined from the inversion transitions (out-of-plane C=O vibrations) found in the S1←S0 vibronic spectra of acetyl halide molecules. The high intensities of the hot inversion transitions are explained by the high probabilities of these transitions. The literature values of the inversion potentials (equilibrium values of angles formed by the C=O bonds deviating from the planes) of formyl halide molecules in the S1 states are refined. The inversion potentials of formyl and acetyl halides are similar.