Knowledge Bank

Past studies have shown that the replacement of a fluorine in p-difluorobenzene with a methyl group to make p-fluorotoluene has a big effect on the IVR rate among ring modes. It is now clear that the nearly free internal rotation of the methyl group is responsible for the nearly hundred-fold acceleration of the rate. The studies have been extended to explorations of IVR rates with a deuterated rotor and, most importantly, with the rotor on its way around the ring, in the meta and ortho positions. The internal rotation potentials are quite different for these positions. Deuteration of the rotor in the para molecule has relatively little effect on the rate. On the other hand, moving the rotor from para to meta boosts the rate further, by about an order of magnitude. The consequences of a move to the ortho position will, we hope, be revealed at the Symposium. Chemical timing is used to extract these IVR rates from the $S1-S0$ fluorescence spectroscopy. The method is based on observations of fluorescence at short times after $S1$ excitation by fluorescence quenching with molecular oxygen. Some comments will be made about the causes of the IVR response to rotor position and to deuteration.