ACCELERATION OF INTRAMOLECULAR VIBRATIONAL REDISTRIBUTION BY METHYL INTERNAL-ROTATION .2. A COMPARISON OF M-FLUOROTOLUENE AND P-FLUOROTOLUENE

Comparisons of rate constants for intramolecular vibrational redistribution (IVR) within the S-1 manifolds of aromatic molecules with and without methyl internal, rotation have been extended to meta-fluorotoluene (mFT). The IVR rate constant, as determined by the method of O-2 fluorescence quenching (chemical timing), is 8.2 X 10(11) s(-1) for the zero-order level 3(1) with epsilon(vib) approximate to 1250 cm(-1). This rate is approximately;10 times greater than that for the comparable level in para-fluorotoluene (pFT). The IVR rate in pFT in turn exceeds that for a comparable level in para-difluorobenzene (pDFB) by a factor of 40. We consider these differences using the Fermi golden rule formulation of the IVR rate constant. The enhanced value for mFT vs pFT may be a consequence of increased coupling strength between the internal rotation and molecular vibration plus a combination of factors that increase the coupled state density. This result is in contrast to the comparison between pFT and pDFB, where NR acceleration is attributed primarily to the increased density of coupled states provided by introducing the internal rotation-vibration interaction.