Influence of C-H•••O interactions on the conformation of methyl groups quantified from neutron diffraction data

The effect of C-H ... O interactions on the torsional conformation of methyl groups bonded to C(sp(3)) is analyzed in a crystal correlation study based on neutron diffraction data. It is found that methyl groups involved in intermolecular C-H ... O contacts are systematically rotated out of the perfectly staggered conformation so as to improve the C-H ... O geometry. The effect, however, is small with rotation angles of only a few degrees. For C-H ... O interactions with H ... O distances < 2.8 Angstrom, the torsion angles X-C-C ... O and X-C-C-H are well correlated, with the linear regression line having a slope of 0.23(3). The related effect for hydrogen bonds of hydroxy groups bonded to C(sp(3)) is described for comparison, and the opposite effect of repulsive C-H ... H-C contacts is also shown.