Computational study of oxalyl, thioyl and selenoyl dichloride: Failure of post-HF single-reference wavefunction theory

The molecular structure of oxalyl chloride has received significant interest in the area of spectroscopy. In this paper, we show that C2h trans-oxalyl chloride serves as another example of the failure of post-HF single-reference wavefunction theory. Although HF and B3LYP predict C2h trans-oxalyl chloride to be a minimum at all basis sets investigated, MP2(FC), MP2(Full), CISD and CCSD predict trans-oxalyl chloride to be a maximum with a number of basis sets. Using the polarizable continuum model (PCM), we have also optimized trans-oxalyl chloride with five different solvents at MP2(FC)/6-31+G(d) and MP2(Full)/6-31+G(d). MP2(FC)/6-31+G(d) predicts trans-oxalyl chloride to be a minimum in the gas phase but a maximum with all five solvents while, MP2(Full)/6-31+G(d) predicts the reverse. For the sulfur and selenium analogs of oxalyl chloride the only minimum is the gauche C2 structure and there is no evidence of post-HF single-reference wavefunction failure in these cases.