ELECTRONIC-SPECTRA OF O-TOLUNITRILE, M-TOLUNITRILE AND P-TOLUNITRILE - SUBSTITUENT EFFECT ON INTERNAL-ROTATION OF THE METHYL-GROUP

The S1 <-- S0 fluorescence excitation spectra and the S1 --> S0 disper sed fluorescence spectra of o-, m- and p-tolunitrile were measured in supersonic jets. Low-frequency bands due to internal rotation of the m ethyl group were observed in m- and p-tolunitrile. Observed band posit ions and relative intensities of the internal rotational bands were re produced by a calculation using a free rotor basis set. From the analy sis, the potential curve of the internal rotation was determined in bo th S1 and S0. It was found that the barrier height increases in going from S0 to S1 in m-tolunitrile, while it decreases in p-tolunitrile. I n contrast, no low-frequency band was found in o-tolunitrile. It is co ncluded that the potential curve in o-tolunitrile does not change in g oing from S0 to S1. The change of the barrier height by electronic exc itation in tolunitriles differs greatly from that observed in other to luene derivatives. It is suggested that the electronic properties of a substituent are important for the methyl rotation in the excited stat e.