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

Publisher Website
Google Scholar
Abstract
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.