Non-additivity of methyl group in the single-electron halogen bond of CH3-BrH complex

Ab initio calculations have been performed at the UMP2/aug-cc-pVDZ and UMP2/aug-cc-pVTZ levels to study the non-additivity of methyl group in the single-electron halogen bond. The strength of the interaction in the CH3-BrH, CH3CH2-BrH, (CH3)2CH-BrH, and (CH3)3C-BrH complexes has been analyzed in terms of the geometries, energies, frequency shifts, stabilization energies, charges, and topological parameters. tert-Butyl radical forms the strongest halogen bond, followed by iso-propyl radical and then ethyl radical, methyl radical forms the weakest halogen bond. The shortening of the C···Br distance and increase of the binding energy in the (CH3)2CH-BrH and (CH3)3C-BrH complexes are more than two and three times those in the CH3CH2-BrH complex, respectively. A positive non-additivity is present among methyl groups. Such conclusions have also been estimated with natural bond orbital (NBO) and atom in molecules (AIM) analyses. The orbital interaction and charge transfer play an important role in the non-additivity of methyl groups.