ACCELERATION OF INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION OF METHYL INTERNAL-ROTATION - A CHEMICAL TIMING STUDY OF P-FLUOROTOLUENE AND P-FLUOROTOLUENE-D3

Time-integrated, frequency-resolved fluorescence spectroscopy has been used to determine rates of intramolecular vibrational energy redistri bution (IVR) from the vibrational levels 3(1) (epsilon(vib) almost-equ al-to 1200 cm-1) and 3(1)5(1) (epsilon(vib) almost-equal-to 2000 cm-1) in both p-fluorotoluene and p-fluorotoluene-d3 for comparison with ea ch other and with comparable levels in p-difluorobenzene. Methyl subst itution increases the rate of IVR by roughly two orders of magnitude, while deuteration of the methyl rotor produces at most a small (two- t o fourfold) further increase in the rate of IVR. It is argued that the IVR response to methyl substitution is a consequence of the methyl in ternal rotation without significant influence from the methyl vibratio ns. The increased IVR rate is predominantly a reflection of the large number of additional states that can couple through the exchange of en ergy between ring vibration and internal rotation. The difference, if any, between the protonated and deuterated methyl rotor species probab ly arises from subtle differences in the level structures and coupling strengths of the two systems. Fermi golden rule modeling of the relat ive IVR rates is built on these propositions. It accounts for much of the IVR rate increase associated with the methyl substitution as well as for the near equivalence of the -CH3 and -CD3 IVR rates.