The S1←S0 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-h(1) (MPA-h(1), (CH3)(2) CHCHO)) and 2-methylpropanal-d(1) (MPA-d(1), (CH3)(2) CHCDO)) 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-h(1) and MPA-d(1) conformers in the ground electronic state (S-0) to vibrational levels of conformers 1 and 3 in the lowest singlet excited electronic state (S-1). According to our estimates, the origins (0(0)(0)) of the 1(S-1) <- cis (S-0) and 3(S-1) <- cis (S-0) and also 1(S-1) <- gauche (S-0) and 3(S-1) gauche (S-0) electronic transitions were situated at 29147 and 29177, 29391 and 29417 cm(-1), respectively, for MPA-h(1) and at 29226 and 29240, 29480 and 29500 cm(-1) for MPA-d(1). The structure of conformers 1 and 3 in the S 1 state was shown to differ from the structure of the cis and gauche conformers in the S-0 state by the angle of rotation of the (CH3)(2)CH- 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-h(1) and MPA-d(1) 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 degrees/+/-32 degrees for MPA-h(1) and MPA-d(1), respectively.