The extremely flat torsional potential energy surface of oxalyl chloride


The conformational behavior of oxalyl chloride has been investigated using ab initio Hartree-Fock (HF) and second-order Møller-Plesset (MP2) perturbation theories, and the coupled-cluster singles and doubles method appended with a perturbative inclusion of connected triple excitations [CCSD(T)]. Correlation consistent polarized valence quadruple- ζ (cc-pVQZ) and quintuple- ζ (cc-pV5Z) basis sets were used in this research. At the cc-pVQZ and cc-pV5Z HF levels, there is no stationary point corresponding to a stable gauche conformer. On the other hand, at the cc-pVQZ and cc-pV5Z MP2 levels and with the cc-pVQZ CCSD(T) method, the gauche conformer of oxalyl chloride was found at O=C-C=O dihedral angles of 81.9°, 79.4°, and 83.4°, respectively. At the cc-pV5Z MP2 level, the energy barrier from trans to gauche was predicted to be 0.74 kcal mol-1 and that from gauche to trans to be 0.09 kcal mol-1. Thus, the potential-energy surface along the O=C-C=O torsional mode is exceedingly flat. The existence of the gauche conformation is mainly due to the minimization of steric repulsion. © 2005 American Institute of Physics.

Publication Title

Journal of Chemical Physics