The effect of the torsional barrier height on the acceleration of intramolecular vibrational relaxation (IVR) by molecular flexibility

Previous work by the present authors (Can. J. Chem. 72, 652 (1994)) pointed out the acceleration of IVR in flexible molecules when the prepared vibration is close to the centre of flexibility (COF). A COF was defined as a bond about which hindered internal rotation can occur thereby giving rise to molecular flexibility. Here, it is shown that the rate of IVR is inversely correlated with the height of the barrier to internal rotation for systems in which the prepared vibration is adjacent to the COF. In ethanol and hydrogen peroxide the barriers are low ( 380 cm −1 ) and the relaxation of the adjacent O-H vibration is fast (4 to 26 ps). On the other hand, higher barrier torsions (1100 to 1700 cm −1 ) adjacent to the chromophore in 1-butyne, 2-fluoroethanol, and 1,2-difluoroethane give rise to much longer IVR lifetimes (270 to 565 ps). Most of the IVR lifetimes used in this paper were derived from discretely resolved spectra in which a bright state transition is fragmented into a clump of molecular eigenstates; the remainder are from rotationally selected double resonance spectra. An algorithm is described for the derivation of consistent IVR lifetimes for coupling cases ranging from intermediate down to the very sparse limit where only a few perturbing states are explicitly observed in the spectrum