Torsional Symmetry Dependence of S_1 Dynamics in Molecules that Undergo Methyl Internal Rotation

摘要

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 (S_1). The most dramatic example of such symmetry dependence comes from our recent finding that the intensities of the e-e transitions in the high-resolution S_1 implied by S_0 fluorescence excitation spectra of jet-cooled acetaldehyde become very weak relative to the a-a transitions at higher beam temperatures. In this study, we 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.