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)) po inted out the acceleration of IVR in flexible molecules when the prepa red vibration is close to the centre of flexibility (COF). A COF was d efined as a bond about which hindered internal rotation can occur ther eby giving rise to molecular flexibility. Here, it is shown that the r ate of IVR is inversely correlated with the height of the barrier to i nternal rotation for systems in which the prepared vibration is adjace nt to the COF. In ethanol and hydrogen peroxide the barriers are low ( approximate to 380 cm(-1)) and the relaxation of the adjacent O - H vi bration is fast (4 to 26 ps). On the other hand, higher barrier torsio ns (1100 to 1700 cm(-1)) adjacent to the chromophore in 1-butyne, 2-fl uoroethanol, and 1,2-difluoroethane give rise to much longer IVR lifet imes (270 to 565 ps). Most of the IVR lifetimes used in this paper wer e derived from discretely resolved spectra in which a bright state tra nsition is fragmented into a clump of molecular eigenstates; the remai nder are from rotationally selected double resonance spectra. An algor ithm is described for the derivation of consistent IVR lifetimes for c oupling cases ranging from intermediate down to the very sparse limit where only a few perturbing states are explicitly observed in the spec trum.