Torsional symmetry dependence of S-1 dynamics in molecules that undergo methyl internal rotation

The single-rovibronic-level fluorescence of ''intermediate-case" molecules that undergo methyl internal rotation is strongly influenced by the torsion al symmetry of the lowest excited singlet state (S-1). The most dramatic ex ample of such symmetry dependence comes from our recent finding that the in tensities of the e-e transitions in the high-resolution S-1 <-- S-0 fluores cence excitation spectra of jet-cooled acetaldehyde become very weak relati ve to the a-a transitions at higher beam temperatures. In this study, we ra tionalize this remarkable torsional symmetry dependence of electronic relax ation in acetaldehyde on the basis of internal-overall rotation coupling th at leads to symmetry-selective increase in the density of states for single t-tripler coupling. Related observations by others on aliphatic carbonyls a nd diazabenzenes are also discussed within the context of the coupling betw een the internal and overall rotation. (C) 1999 John Wiley & Sons, Inc.