Laser-induced phosphorescence spectroscopy in supersonic jets. The lowest triplet states of glyoxal, methylglyoxal, and biacetyl

We report the first structural study of the lowest triplet states of three α-dicarbonyls (glyoxal, methylglyoxal, and biacetyl) using the technique of laser-induced phosphorescence (LIP) spectroscopy in supersonic jets. At the level of vibrational resolution, 3Au glyoxal appears to have a geometry very similar to that of the ground state. But the T1←S0 transitions of methylglyoxal and biacetyl each exhibit strong progressions in the torsional vibrations of the methyl groups, showing that these molecules undergo a conformational change on excitation to the lowest triplet state. A Franck–Condon analysis of the methylglyoxal spectrum, with proper consideration for nuclear spin statistics, yields a methyl barrier of V3=115±5 cm−1 in this state. This value has been confirmed by a direct measurement of the tunneling splitting of A and E torsional levels. The hindering potential in the lowest triplet state of methylglyoxal is substantially different from those in the ground (V3=269 cm−1) and first excited singlet (V3=190 cm−1) states. Possible reasons for these differences are discussed.