文摘
CASPT2//CASSCF and B3LYP methodologies have been used to study the excited-state properties andphotochemical isomerizations of p-, m-, and o-methylbenzonitriles and methylanisoles. Calculations showthat the biradical mechanism is the most favored channel for the photoinduced interconversion of p-, m- ando-methylbenzonitriles, both dynamically and thermodynamically. The formation of biradical as a keyintermediate is highly selective, and only the biradicals with a turned-up cyano-substituted carbon are involvedin photoisomerization. Methylanisole isomers are inactive relative to methylbenzonitriles at 254 nm. Suchremarkable activity difference between methylbenzonitrile and methylanisole in photochemistry arises fromthe accessibility of the S1/S0 conical intersection as well as the stability of prefulvene biradicals. Formethylanisoles, the S1/S0 precursor and the reactive biradicals are inaccessible at 254 nm, which should bethe origin of inactivity. The results suggest that the conical intersection accessibility plays a crucial role inthe photochemistry of substituted benzenes at 254 nm.