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⋯XF and H2S⋯XF (X=Cl and Br) halogen-bonded complexes has been investigated with quantum chemical calculations. The halogen bond in the H2O⋯XF complexes is stronger than that in the H2S⋯XF complexes. However, the S⋯X halogen bond is stronger than the O⋯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⋯Cl halogen bond. The natural bond orbital analyses have been performed to unveil the mechanism of the methyl group in the halogen bonding formation.