EVIDENCE FOR QUANTIZATION OF THE TRANSITION-STATE FOR CIS-TRANS ISOMERIZATION

Cis-trans isomerization rates of trans,trans-1,3,5,7 octatetraene (OT) on the first excited singlet state (2 (1)A(g)) potential surface have been obtained as a function of vibrational energy by measuring the fl uorescence lifetimes. A stepwise increase in the isomerization rate wi th increasing energy has been observed, which indicates quantization o f the vibrational levels of the transition state for the cis-trans iso merization of a double bond. The energy spacing of 80+/-10 cm(-1) betw een the first two steps tentatively is assigned to an in-plane bending vibration of the transition state.