Knowledge Bank

The hindered internal rotation of methyl groups continues to be a subject of intense interest. We report here a study of the barriers to such motion in the 1- and 2-methylnaphthalenes, and their dependence on both the vibrational and electronic state of the molecule to which the rotor is attached. New methods for determining absolute barrier heights based on an analysis of the rotationally resolved $S1-S0$ spectra, will be described. The results of these analyses show, unambiguously, that the magnitudes of the torsional barriers are extraordinarily sensitive to electronic excitation, to the position of the methyl group substitution, and to the presence or absence of vibrational excitation in the naphthalene framework. A rationalization of these results using simple MO theory will be presented.