LASER-INDUCED PHOSPHORESCENCE SPECTROSCOPY IN SUPERSONIC JETS. THE LOWEST TRIPLET STATES OF GLYOXAL, METHYLGLYOXAL, AND BIACETYL$^{1}$
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Date
1986
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Ohio State University
Abstract
We report the first structural study of the lowest triplet states of three $\alpha$-dicarbonyls is (glyoxal, methylglyoxal, and biacetyl) using the technique of laser-induced phosphorescence (LIP) spectroscopy in supersonic jets. The $O_{0}^{0}$ band is the strongest band in the LIP spectrum of glyoxal. But the $T_{1} - S_{0}$ 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 $V_{3} = 115 \pm cm^{-1}$ 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 $(V_{3} = 269 cm^{-1})$ and first excited singlet $(V_{3} = 190 cm^{-1})$ states. Possible reasons for these differences are discussed.
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$^{1}$ Work supported by NSF (CHE-8402996). Address of Spangler: MS-G738, LANL, Los Alamos, NM 87545. Address of Pratt: Department of chemistry, University of Pittsburgh, Pittsburgh, PA 15260.
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