229 Charge Distribution and the Propensity for Halogen Bonding in Group 14 Halomethane Analogues

Thursday, 22 October 2009: 10:25 AM
203 (Puerto Rico Convention Center)
Bernard K. Wittmaack , Department of Chemistry, University of Richmond, University of Richmond, VA
Kelling J. Donald , Department of Chemistry, University of Richmond, University of Richmond, VA
Computed (MP2) NBO population analysis data reveal that for some chloro- and bromo-, and that for all the iodomethane CH4-nXn; X=Cl, Br, I molecules, the halogen atom possesses a positive atomic charge, with the central carbon atom having a negative charge. Contact has been made previously between this charge density depletion at X and the halogen bonding phenomenon, in which (like H in hydrogen bonding) the halide acts as the Lewis acid. We investigate the charge distribution in four series of group 14 halomethane analogues MH4-nXn; X=F, Cl, Br, I (M=C, Si, Ge, Sn, and Pb). We find that for all of these systems the halides are significantly more negative than they are in the halomethane molecules. However, the charges on the iodides, are quite small, especially for n=4. This suggests that the latter systems are the analogues most likely to exhibit halogen bonding. A question we ask, therefore, is will the halides in the heavier group 14 analogues of halomethane also exhibit some halogen bonding, even if it is weaker than when M=C. We answer this by looking for the σ-hole (a region of positive electrostatic potential) on the iodides in MX4 (M=Si, Ge, Sn, and Pb). We investigate, as well, the effect of F substitution on the size of the σ-hole in MHF2I, MH2FI, MFI3, MF3I, MHI3, and MH3I. We find that the greater the number of electron withdrawing halogens (i.e. fluorine), the larger the size of the region of positive electrostatic potential (the σ-hole) on the iodine atom. In this regard the NBO charges may be misleading; the atomic charges of the iodine atoms do not neatly correlate with the size of the σ-hole on the iodine atom, and can hardly be used a direct indicator of the propensity for halogen bonding.