The Torsional Potential of Dimethyl Peroxide: Still a Difficult Case for Theory

The torsional potential about the O−O single bond of dimethyl peroxide, CH 3 OOCH 3, was investigated with the aid of large-scale ab initio calculations performed at different levels of Møller−Plesset perturbation theory and coupled-cluster expansions. Additionally, several density functional approaches were applied. For comparative purposes, the torsional potentials of methyl hydroperoxide, CH 3 OOH, and hydrogen peroxide, HOOH, were calculated at the same levels of approximation. In the already well-investigated case of HOOH and also for CH 3 OOH excellent agreement with the experimentally determined structures and barrier heights can be achieved at the coupled-cluster CCSD(T) level with the application of extended polarized basis sets augmented with diffuse functions. However, in the case of dimethyl peroxide, the peculiar shape of the computed CCSD(T)/cc-pVTZ torsional potential, with an exceedingly shallow region ranging from 110 to 250°, with two skew minima at about 115 and 245° and with a trans minimum at 180°, deviates significantly from that of the experimentally derived torsional potential, which has a barrier at 180° separating the two distinctly deeper skew minima at 120 and 240°. The difficulties encountered in reaching a reasonably converged result with respect to further basis set extension are discussed. It is also shown that the results of density functional theory (DFT) and Møller−Plesset second-order (MP2) calculations differ considerably from the Møller−Plesset higher-order and CCSD(T) results.