Torsional symmetry dependence of S1 dynamics in molecules that under methyl internal rotation

the single-rovibronic-level fluorescence of intermediate-case molecules that undergo methyl internal rotation is strongly influenced by the torsional symmetry of the lowest excited singlet state (S1). The most dramatic example of such symmetry dependence comes from the recent finding that the intensities of the e-e transitions in the high-resolution S1 left-arrow S0 fluorescence excitation spectra of jet-cooled acetaldehyde become very weak relative to the a-a transitions at higher beam temperatures. In this study, they rationalize this remarkable torsional symmetry dependence of electronic relaxation in acetaldehyde on the basis of internal-overall rotation coupling that leads to symmetry-selective increase in the density of states for singlet-triplet coupling. Related observations by others on aliphatic carbonyls and diazabenzenes are also discussed within the context of the coupling between the internal and overall rotation