Vibrational Spectra and Structure of Oxalyl Chloride in the Crystalline and Fluid States

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 Moller-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-zeta (cc-pVQZ) and quintuple-zeta (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[Double Bond]C-C[Double Bond]O dihedral angles of 81.9 degrees , 79.4 degrees , and 83.4 degrees , 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[Double Bond]C-C[Double Bond]O torsional mode is exceedingly flat. The existence of the gauche conformation is mainly due to the minimization of steric repulsion.