STRUCTURAL EFFECTS ON THE ISOMERIZATION DYNAMICS OF TRANS-STILBENES -IVR, MICROCANONICAL REACTION-RATES, AND THE NATURE OF THE TRANSITION-STATE

Picosecond time-resolved fluorescence spectroscopy has been used for s tudying the intramolecular dynamics of trans-stilbene (TS) derivatives . Nonradiative decay rate constants (k(nr)) of jet-cooled 4-methoxy-tr ans-stilbene (MS), 4,4'-dimethoxy-trans-stilbene (DMS), 2-phenylindene (PI), 4,4'-dihydroxy-trans-stilbene (DHS), and 1-(trans-beta-styryl)c yclohexene (SCH) have been obtained from time-resolved fluorescence me asurements with wide-band detection from photoselected vibrational sta tes in the S-1 manifold. Time- and frequency-resolved fluorescence hav e been measured to characterize intramolecular vibrational energy redi stribution (IVR) in the first three of these molecules. Statistical no nadiabatic RRKM theory is applied to model the energy dependence of k( nr) and extract the reaction barriers. Dispersed fluorescence is measu red and analyzed in order to determine the substitution effect on vibr ational frequencies most significant to the RRKM calculations. While k (nr) is almost invariant to pam substitution of a methoxy group, with similar isomerization thresholds for MS and TS (E(0)(MS) similar to E( 0)(TS)), a dramatic slowing of the rate is observed in both disubstitu ted derivatives, and reaction barriers of similar to 1800 and 1700 cm( -1) are derived for DMS and DHS, respectively. The altered conjugation of the system in SCH produces a large blue shift of the 0(0)(0) and r esults in a lowering of the isomerization barrier (E(0)(SCH) < 740 cm( -1)). The twisting around the ethylenic double bond (C-e-C-e) is preve nted upon intramolecular bridging in PI, but another nonradiative chan nel, attributed to twisting around the C-e-C-phi single bond, is model ed by RRKM theory with E(0)(PI) similar to 1770 cm(-1). From the elect ronic structure effects on both E(0) and the shift of the S-1 transiti on, we elucidate the nature of the transition state, as zwitterionic, and we correlate substitution effects with the HOMO/LUMO (frontier orb ital) description. The results are relevant to studies in polar soluti ons and to the dimensionality of the reaction coordinate in the dynami cs of barrier crossing.