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 dispersed fluorescence spectra of o-, m- and p-tolunitrile were measured in supersonic jets. Low-frequency bands due to internal rotation of the methyl group were observed in m- and p-tolunitrile. Observed band positions and relative intensities of the internal rotational bands were reproduced by a calculation using a free rotor basis set. From the analysis, the potential curve of the internal rotation was determined in both 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. In contrast, no low-frequency band was found in o-tolunitrile. It is concluded that the potential curve in o-tolunitrile does not change in going from S0 to S1. The change of the barrier height by electronic excitation in tolunitriles differs greatly from that observed in other toluene derivatives. It is suggested that the electronic properties of a substituent are important for the methyl rotation in the excited state.