An experimental potential energy surface for internal rotation in glyoxal

Abstract

The torsional potential energy surface (TPES) for internal rotation of the CHO group in glyoxal CHOCHO has been derived experimentally by fitting observed energies for the torsional vibration ν7 in both trans- and cis-glyoxal to those calculated with the hindered rotation formalism of Lewis, Malloy, Chao, and Laane. The experimental energies were obtained from S1 → S0 single vibronic level fluorescence (SVLF) spectra of jet-cooled glyoxal. SVLF from the trans levels 73 and 74 plus the cis levels 00 and 51 yield the torsional vibrational energies of all 7n trans levels with n≤14 (except for 713) and that of the cis level 72. The energies of odd trans 7n levels with n≥7 as well as spectroscopic values of any cis 7n level were previously unknown. The best derived TPES fits the observed trans and cis levels to within 0.4 cm−1 except for two cases where the mismatch is 1.0 cm−1. The TPES is defined by the potential energy 2V=∑6n=1Vn (1−cos nφ) where φ is the torsional angle. For the best TPES, coefficients are (in cm−1) V1=1719.4, V2=1063.5, V3=−53.2, V4=−81.9, V5=21.3 and V6=2.9. For this TPES, the energy separation between the trans and cis potential wells is 1688 cm−1, the barrier to trans → cis internal rotation is 2077 cm−1 and the barrier to cis → trans rotation is 389 cm−1. As one moves from the trans well at φ=0°, the torsional barrier occurs at 110°.