Insights into the strength and nature of carbene•••halogen bond interactions: a theoretical perspective

Halogen-bonding, a noncovalent interaction between a halogen atom X in one molecule and a negative site in another, plays critical roles in fields as diverse as molecular biology, drug design and material engineering. In this work, we have examined the strength and origin of halogen bonds between carbene CH2 and XCCY molecules, where X = Cl, Br, I, and Y = H, F, COF, COOH, CF3, NO2, CN, NH2, CH3, OH. These calculations have been carried out using M06-2X, MP2 and CCSD(T) methods, through analyses of surface electrostatic potentials V (S)(r) and intermolecular interaction energies. Not surprisingly, the strength of the halogen bonds in the CH2 center dot center dot center dot XCCY complexes depend on the polarizability of the halogen X and the electron-withdrawing power of the Y group. It is revealed that for a given carbene center dot center dot center dot X interaction, the electrostatic term is slightly larger (i.e., more negative) than the dispersion term. Comparing the data for the chlorine, bromine and iodine substituted CH2 center dot center dot center dot XCCY systems, it can be seen that both the polarization and dispersion components of the interaction energy increase with increasing halogen size. One can see that increasing the size and positive nature of a halogen's sigma-hole markedly enhances the electrostatic contribution of the halogen-bonding interaction.