Tuning sigma-holes: charge redistribution in the heavy (group 14) analogues of simple and mixed halomethanes can impose strong propensities for halogen bonding.

Halogen bonding between halide sites (in substituted organic molecules or inorganic halides) and Lewis bases is a rapidly progressing area of exploration. Investigations of this phenomenon have improved our understanding of weak intermolecular interactions and suggested new possibilities in supramolecular chemistry and crystal engineering. The capacity for halogen bonding is investigated at the MP2(full) level of theory for 100 compounds, including all 80 MH(4-n)X(n) systems (M = C, Si, Ge, Sn, and Pb; X = F, Cl, Br, and I). The charge redistribution in these molecules and the (in)stability of the sigma-hole at X as a function of M and n are catalogued and examined. For the mixed MH(3-m)F(m)I compounds, we identify a complicated dependence of the relative halogen bond strengths on M and m. For m = 0, for example, the H(3)C-I----NH(3) halogen bond is 6.6 times stronger than the H(3)Pb-I----NH(3) bond. When m = 3, however, the F(3)Pb-I----NH(3) bond is shorter and approximately 1.6 times stronger than the F(3)C-I----NH(3) bond. This substituent-induced reversal in the relative strengths of halogen bond energies is explained.