Prevalence of the alkyl/phenyl-folded conformation in benzylic compounds C6H5CH2-X-R (X = O, CH2, CO, S, SO, SO2):: Significance of the CH/π interaction as evidenced by high-level ab initio MO calculations

Ab initio MO calculations were carried out to examine the conformational energies of various benzylic compounds C6H5CH2XR (X = O, CH2, CO, S, SO, SO2; R = CH3, C2H5, iC(3)H(7), tC(4)H(9)) at the MP2/6-311G(d,p)//MP2/6-31G(d) level. Rotamers with R/Ph in gauche relationship are generally more stable than the R/Ph anti rotamers. In these stable geometries, the interatomic distance in the interaction of alpha- or beta-CH in the alkyl group and the ipso-carbon atom of the phenyl ring is short. The computational results are consistent with experimental data from supersonic molecular jet spectroscopy on 3-n-propyltoluene and NMR and crystallographic data on structurally related ketones, sulfoxides, and sulfones. In view of this, the alkyl/phenyl-congested conformation of these compounds has been suggested to be a general phenomenon, rather than an exception. The attractive CH/pi interaction has been suggested to be a dominant factor in determining the conformation of simple aralkyl compounds.