An ab initio study of the structure and methyl rotational barriers of methylphosphonic dihalides

The molecular structure and the barriers to methyl rotation of symmetric methylphosphonic dihalides (CH3POX2, X=F, Cl, Br) and of the corresponding non-halogenated molecule (methyl phosphine oxide, X=H) were determined through ab initio calculations at the MP2/6-31+G(2d,p) level. The four molecules adopt the staggered conformation, with skeletal angles decreasing in the order OPCˆ>OPXˆ>XPCˆ>XPXˆ. The strong electron-withdrawing effect of the halogen atoms decreases the electron density on the oxygen atom, and increases the double bond character of the phosphoryl group. This, in turn, leads to an increase of the hyperconjugative interactions between this group and the two C-H bonds lying out of the OPC plane, and results in a slight lengthening of these bonds relative to the in-plane C-H bond. Methyl rotations were followed by means of intrinsic reaction coordinate calculations (IRC), and the calculated energy barriers were 1.61, 2.41, 2.77 kcal/mol for the difluoride, dichloride and dibromide, respectively, and 2.01 kcal/mol for methyl phosphine oxide. Structural evidences suggest that hyperconjugation stabilizes the transition structure for methyl rotation in the difluoride molecule, which explains the comparatively low value of its rotational barrier.