Geometries of the ground, first triplet and first singlet excited states of a hydrohydroxide of uracil have been optimized using the SINDO 1 SCF molecular orbital method in conjunction with configuration interaction. The first triplet and first singlet transitions of the molecule are found tc/be of (π-sp*) type which shows their orbitally forbidden character. Thus an explanation as to why the adducts formed by the addition of H2O, hydroxylamine, bisulphite etc at the CC double bonds of pyrimidines do not absorb in the characteristic 260 nm region has been provided.
Volume 96, 2022
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