OXALYL HALIDES - ANALYSIS OF 4100 A ABSORPTION SYSTEM OF OXALYL CHLORIDE

The weak vapor-phase absorption spectrum of oxalyl chloride in the 4130-3800 Å region has been examined under high resolution, and assigned to the a ̃3Au ← X ̃1Ag transition resulting from the same n → π* electron promotion as the stronger A ̃1Au ← X ̃1Ag transition that lies 2800 cm-1 to higher energies. The overall patterns, intensity distributions and excited state frequencies for the two systems are very similar. The a ̃3Au ← X ̃1Ag origin is at 24370.2 cm-1. The upper state vibrational frequencies ν′1 - ν′5, of ag symmetry, are identified as 1508, 1028.2, 634.6, 401.1 and 281.1 cm-1 respectively, and sequences intervals of +78 cm-1 (ν7(au) sequence) are prominent. Other sequences intervals of (+32), (-71) and (-11) cm-1 are identified as being in ν6(au), ν11(bu) and ν12(bu) respectively. The observed spectrum is consistent with an electronically allowed (but spin forbidden) transition between planar, trans, electronic states of the molecule which differ only slightly in their geometries. © 1968.