Hydrogen-Bond-Like Nature of the CH/π Interaction as Evidenced by Crystallographic Database Analyses and Ab Initio Molecular Orbital Calculations

Abstract

Abstract Spatial distribution of CH hydrogen atoms with reference to a six-membered carbon aromatic ring was analyzed in the crystal structures deposited in the Cambridge Structural Database (CSD). The crystal data showed the propensity that the CH hydrogen atoms lie above the center of the aromatic ring to form interatomic CH/π contacts. Investigation of the effects of the CH proton acidity on the strength and the structure of the CH/π interaction has demonstrated that the distance between the hydrogen atom and the π plane (Dpln) decreases with increase of the CH proton acidity (Dpln: CCH3 ≅ sp2-CH > sp-CH ≅ Cl2CH2 > Cl3CH), and that the C–H···π access angle (α) tends to approach 180° in the same order. Further, a negative correlation has been found between Dpln and α. The directional preferences are in accord with the property of conventional hydrogen bond, indicating a hydrogen-bond-like character of the CH/π interaction. In order to elucidate the intrinsic nature of the CH/π interaction, ab initio calculations [MP2/6-311++G(d,p)] were carried out for methane/benzene, ethylene/benzene, and acetylene/benzene complexes. Potential surfaces obtained for the three model supramolecules are consistent with the results from the CSD analyses.