nπ* transitions in sulfur derivatives of cyclobutanedione. Tetramethyl-3-thio-1,3-cyclobutanedione-h12 and -d12

Triplet and singlet A2(nπ*/sub CS/) states, and a 3A2(nπ*/sub CO/) state, have been identified in the spectrum of tetramethyl-3-thio-1,3- cyclobutanedione crystals, at ca. 17050, 18350, and 27430 cm-1. As in related compounds, the triplet-singlet transitions appear to borrow intensity from 1A1 reverse arrow 1A1 transitions; and CH/CD stretching vibrations in the peripheral methyl groups are vibronically active in the singlet-singlet transition, suggesting extensive delocalization of the effects of excitation. Bands in the visible region are unexpectedly broad