Transient Resonance Raman and ab Initio MO Calculation Studies of the Structures and Vibrational Assignments of the T1 State and the Anion Radical of Coumarin and Its Isotopically Substituted Analogues

Abstract

Transient resonance Raman and absorption spectra of the anion radical CM•- and lowest excited triplet state 3CM* of coumarin were measured. Vibrational assignments of the ground state S0, CM•-, and 3CM* were performed based on both the frequency shifts on isotopic (18O, 13C, and deuterium) substitutions and normal-coordinate calculations using the force constants obtained by ab initio molecular orbital computations. The vibrational assignments and ab initio MO calculations provided much information on the structures of S0, CM•-, and 3CM*. It is concluded that the C(3)C(4) bond of the pyrone ring is lengthened markedly in CM•- and drastically in 3CM*. This suggests that the C(3)C(4) bond is one of the reactive sites of coumarin and furocoumarins in their photochemical reactions, which is in good accord with the formation of cyclobutane-type adducts with pyrimidine bases, particularly with thymine, through the C(3)C(4) bond of furocoumarins and the C(5)C(6) bond of pyrimidine bases. The CO bond was found to be moderately lengthened both in CM•- and 3CM*. The moderate lengthening of the CO bond of 3CM* is consistent with the π−π* character of 3CM*.