The S1 ← S0 fluorescence excitation spectrum and structure of propanal in the S1 excited electronic state

Abstract

We have obtained and analyzed the S₁ ← S₀ fluorescence excitation spectra of jet-cooled propanal-h₁ (CH₃CH₂CHO) and -d₁ (CH₃CH₂CDO). Using the results of theoretical studies of the structure of propanal molecule in the S₁ lowest excited singlet electronic state, we have assigned the bands of both spectra to the vibronic transitions of the cis conformer (in the S₀ ground electronic state) to the 1 and 3 conformers (in the S₁ state) differed by the angle of the C₂H₅ ethyl group rotation around the central C–C bond. The origins of the 1 ← cis and 3 ← cis electronic transitions have been observed at 29 997 and 30 075 cm⁻¹ for propanal-h₁ and at 30 040 and 30 115 cm⁻¹ for propanal-d₁, respectively. The high activity of torsional (C₂H₅ ethyl groups) and inversional (CCHO/CCDO carbonyl fragments) vibrations and the intensity distribution of the bands in torsional sequences (passing through maximum) are in agreement with the theoretical prediction that the S₁ ← S₀ electronic excitation of the cis conformer causes (after geometrical relaxation) the pyramidalization of carbonyl fragments and the rotation of ethyl groups around the central C–C bond. A number of energy levels have been found for torsional and inversional vibrations, and also fundamentals of ν₁₀ (CCO bend) and ν₁₃ (CCC bend) for the both 1 and 3 conformers of propanal-h₁ and -d₁ have been found. Then the “experimental” potential functions of inversion for the pair of the 1 and 3 conformers have been determined. The heights of potential barriers to inversion and the angle values corresponding to the minima of potential functions of inversion are 900 cm⁻¹ and 35° for propanal-h₁ and 820 cm⁻¹ and 34° for propanal-d₁, respectively.