Theoretical study of the interaction mechanism of single-electron halogen bond complexes H3C⋯Br-Y (Y = H, CN, NC, CCH, C2H3)

Abstract

The characteristics and structures of single-electron halogen bond complexes [H3C⋯Br-Y (Y = H, CCH, CN, NC, C2H3)] have been investigated by theoretical calculation methods. The geometries were optimized and frequencies calculated at the B3LYP/6-311++G** level. The interaction energies were corrected for basis set superposition error (BSSE) and the wavefunctions obtained by the natural bond orbital (NBO) and atom in molecule (AIM) analyses at the MP2/6-311++G** level. For each H3C⋯Br-Y complex, a single-electron Br bond is formed between the unpaired electron of the CH3 (electron donor) radical and the Br atom of Br-Y (electron acceptor); this kind of single-electron bromine bond also possesses the character of a “three-electron bond”. Due to the formation of the single-electron Br bond, the C-H bonds of the CH3 radical bend away from the Br-Y moiety and the Br-Y bond elongates, giving red-shifted single-electron Br bond complexes. The effects of substituents, hybridization of the carbon atom, and solvent on the properties of the complexes have been investigated. The strengths of single-electron hydrogen bonds, single-electron halogen bonds and single-electron lithium bonds have been compared. In addition, the single-electron halogen bond system is discussed in the light of the first three criteria for hydrogen bonding proposed by Popelier.