The structure of 2-methylpropanal molecule in the S1 lowest excited singlet electronic state: theoretical and experimental studies

Abstract

We have obtained and analyzed the S1 ← S0 fluorescence excitation spectrum of jet-cooled 2-methylpropanal ((CH3)2CHCHO). In addition, the ab initio calculations of the 2-methylpropanal structure in the ground (S0) and lowest excited singlet (S1) electronic states have been carried out by the CASPT2/def2-TZVPP method. The calculations have shown that the S1 ← S0 electronic excitation of the 2-methylpropanal conformers (cis and gauche) causes large conformational changes: the isopropyl group rotates around the central C–C bond and the carbonyl fragment CCHO becomes non-planar. The potential energy surface of 2-methylpropanal in the S1 state has the six minima, corresponding to three pairs of mirror symmetry conformers: 1ab, 2ab, and 3ab. In the S1 ← S0 jet-cooled spectra of (CH3)2CHCHO and (CH3)2CHCDO three systems of vibronic transitions from the gauche (S0) conformer to vibrational levels of the 1, 2, and 3 conformers (S1) with the origins ( $$ {0}_0^0 $$ ) at 29,887; 30,198; and 29,963 cm−1 for (CH3)2CHCHO and 29,926; 30,221; and 29,990 cm−1 for (CH3)2CHCDO have been identified. The high activity of torsional and inversional vibrations of the 1, 2, and 3 conformers has been found for both molecules. Intensities of bands in torsional sequences pass through a maximum; it confirms the theoretical prediction about the minima shifts between the gauche and 1, 2, and 3 conformers along the torsional coordinate. Using the experimental inversional energy levels, the 1↔3 and 2a↔2b potential functions of inversion have been determined for both molecules. The corresponding calculated and experimental data are in satisfactory agreement.