Surprising enhancing effect of methyl group on the strength of O•••XF and S•••XF (X=Cl and Br) halogen bonds

The role of methyl group in H2O center dot center dot center dot XF and H2S center dot center dot center dot XF (X=Cl and Br) halogen-bonded complexes has been investigated with quantum chemical calculations. The halogen bond in the H2O center dot center dot center dot XF complexes is stronger than that in the H2S center dot center dot center dot XF complexes. However, the S center dot center dot center dot X halogen bond is stronger than the O center dot center dot center dot X one with the increase of methyl number. The result shows that the methyl group in the halogen acceptor has a positive contribution to the formation of halogen bond and there is a positive nonadditivity of methyl groups. Surprisingly, the methyl groups in dimethyl sulfide causes an increase of 150% for the interaction energy of S center dot center dot center dot Cl halogen bond. The natural bond orbital analyses have been performed to unveil the mechanism of the methyl group in the halogen bonding formation. (c) 2010 American Institute of Physics. [doi:10.1063/1.3479398]