Structural information from methyl internal rotation spectroscopy.
The fundamental quantum mechanics, group theory, and spectroscopy of methyl torsional structure accompanying electronic transitions is presented. The origin of barriers to internal rotation and the interaction of the methyl with the pi system via hyperconjugation are discussed. Because of the relationship between the methyl barrier and the pi system, measurement of the CH3 properties provides structural information about the molecule. In para'-substituted p-methyl-t-stilbenes, barriers in the S1 state show a strong dependence on the substituent, substituent conformation, and involvement of the substituent in hydrogen bonding interaction. The methyl torsional barrier reflects these changes despite the distance of the substitution site, 10 atoms away.