Abstract
We present the results of a computation for the lowest excited triplet and singlet states of the pyrimidine bases and of some of their derivatives by the SCF open shell theory and CI method on SCF virtual orbitals. It is shown that excitation to the lowest singlet S 1 and triplet T 1 states mostly affects the bond C5-C6 and that triplet excitation is almost completely localized on this bond. The latter effect leads to exceptionally strong weakening of the bond C5-C6 in the triplet state T 1. It is concluded that under otherwise equal circumstances the triplet state T 1 presents more favourable conditions for dimerization reaction than the lowest excited singlet state S 1 does. Correlation between the degree of the triplet excitation localization on the bond C5-C6 and the easiness of the photodimerization is established. An explanation of the experimental facts concerned with the formation of the thymine free radicals in DNA is also suggested.
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