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*.
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