Knowledge Bank

Several recent studies have shown that the contormational preferences of methyl groups adjacent to double bonds in the ground states of organic molecules are reversed on electronic $excitation.1$ In this report, we describe the results of phosphorescence and fluorescence excitation experiments in a supersonic jet that were designed to explore the origins of these effects. $S1$ acetophenone exhibits the expected change, with the methyl group being staggered with respect to the carbonyl bond rather than eclipsed, as in the ground state. But $T1$ acetophenone exhibits no such change, a fact that can be attributed to subtle differences in the orbital character of the two excited states. In contrast, both $S1$ and $T1$ 2-methylpyrazine have methyl group conformations that are identical to the ground state. Analyses of these results, in terms of the usual potential functions for hindered methyl rotors, provide considerable insight into the electronic factors that are responsible for these effects.