Torsion-inversion coupling in the S1 state of acetaldehyde

Abstract

Coupling between the methyl torsion (υ15) and inversion (υ14) modes is used to model the singlet π*←n spectrum of jet-cooled acetaldehyde and its deuterated analogs. The anomalies observed in the two torsion progressions built onto the lowest inversion doublet are accurately reproduced by using a potential of the form 1/2V3[1−cos3(τ−kq)], where k indicates the proportionality of interaction between the torsion and inversion motions. The coupling also results in intensity to torsion–inversion combination levels that, in the absence of coupling, would not be observed. Consequently, many of the observed transitions are simultaneously Franck–Condon allowed and vibronically induced. The weak origin transition and long progression in the methyl torsion mode is also reproduced in the intensity calculation, indicating that the equilibrium position of the methyl rotor has changed from the S0 eclipsed position to a nearly staggered geometry in the S1 state. The calculated potential energy surface and Franck–Condon intensity calculations predict an S1 equilibrium geometry in which the methyl rotor is staggered with respect to the CO bond by 54° and where the acetyl hydrogen is bent out of the CCO plane by 35°.