STRUCTURAL INFORMATION FROM METHYL INTERNAL-ROTATION SPECTROSCOPY

The fundamental quantum mechanics, group theory, and spectroscopy of m ethyl torsional structure accompanying electronic transitions is prese nted. The origin of barriers to internal rotation and the interaction of the methyl with the pi system via hyperconjugation are discussed. B ecause of the relationship between the methyl barrier and the pi syste m, measurement of the CH3 properties provides structural information a bout the molecule. In para'-substituted p-methyl-t-stilbenes, barriers in the S-1 state show a strong dependence on the substituent, substit uent conformation, and involvement of the substituent in hydrogen bond ing interaction. The methyl torsional barrier reflects these changes d espite the distance of the substitution site, 10 atoms away.