Theoretical investigation on the substituent effects of the C–H/π interaction

The substituent effects of C–H/π non-covalent interactions between the C–H bond of alkane/alkene/alkyne/alkane halide and π orbitals of aromatic compounds were studied using the theoretical calculation methods at the CCSD(T)/aug-cc-pvdz level. The natural bond orbital (NBO) analysis implies the charge transfer from the π-system to the C–H bond in the C–H/π interaction. Therefore, either electron-donating group on the π-system or the electron-withdrawing group on the C–H system could enhance the C–H/π interaction. Meanwhile, the substituent effect of the π-system is weaker than that of the C–H systems. There is a good linear correlation between the stabilization energy (energy difference between the C–H/π complex and the separated reactants) and the reaction enthalpy (for the formation of the C–H/π complex) of the alkane/alkene/alkyne-aromatic systems. For the C–H/π interactions between two aromatic systems, good linear correlations were observed between the σp (C–H/π bond pair substituent constant) and the stabilization energy or the r(CH…η), i.e., the distance from H atom to the center of benzene ring.