The Role of CH•••O Coulombic Interactions in Determining Rotameric Conformations of Phenyl Substituted 1,3-Dioxanes and Tetrahydropyrans

The rotameric conformations of the phenyl ring in both the axial and the equatorial conformers of phenyl substituted 1,3-dioxanes and tetrahydropyrans are compared with those of the corresponding phenylcyclohexanes at the MP2/6-311+G* level. The compounds with an axial phenyl commonly adopt a conformation in which the plane of the aromatic ring is perpendicular to the benzylic C-H bond. However, axial 5-phenyl-1,3-dioxane adopts a "parallel" conformation that allows an ortho hydrogen to be proximate to the two ring oxygens, leading to attractive CH center dot center dot center dot O interactions. Stabilizing Coulombic interactions of this sort are found with many of the oxygen-containing six-membered rings that were investigated.