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 3680 Å and 4100 Å 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 over(A, ̃)¹ A_u ← over(X, ̃)¹ A_g and singlet-triplet over(a, ̃)³ A_u ← over(X, ̃)¹ A_g 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 over(X, ̃)¹ A_g and excited over(A, ̃)¹ A_u and over(a, ̃)³ A_u 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.