Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part I: Reanalysis of the 3680Â Ã and 4100Â Ã absorption systems of oxalyl chloride

The vapor-phase absorption spectrum of oxalyl chloride in the 3000-4180 Å region has been re-examined at high resolution. Singlet-singlet A˜1Au←X˜1Ag and singlet-triplet a˜3Au←X˜1Ag electronic transitions of the trans-conformer found in the spectrum are in agreement with earlier works [W.J. Balfour, G.W. King, J. Mol. Spectrosc. 26 (1968) 384-397; ibid. 27 (1968) 432-442]. Torsion levels of trans-oxalyl chloride in the ground X˜1Ag and excited A˜1Au and a˜3Au states were found for the first time. Ab initio calculations of structures for conformers of oxalyl chloride in the ground and lowest excited electronic states explain the absence of second conformer transitions in the vibronic absorption spectrum.