Halogen bonding (XB) most frequently is explained by the attraction of electron rich species to the areas of positive potentials on the surface of halogen atoms (referred to as σ-holes). This suggests a strong correlation between strength of halogen bonding and the magnitude of the maximum positive potentials, Vmax, on their surfaces. Due to the polarizability of halogen substituents, the Vmax in the XB complexes might deviate substantially from that in the individual molecules. However, while theoretical studies stressed the importance of polarization, most of the previous studies correlated XB strength with the Vmax values in the separate molecules and the experimental corroborations of the effects of polarizations were lacking. In our current work, we evaluate the effects of polarization on halogen and hydrogen bonding (HB) of various halomethanes via the comparison of formation constants of XB and HB complexes with the values of Vmax in the isolated and polarized molecules. Figure 1(left) shows that there is essentially no correlation between the values of formation constants of XB and HB complexes of various anions with CHBr3 or CHI3 and the values of Vmax(ind) on surfaces of halogen or hydrogen atoms in the isolated molecules. However, the presence of anions substantially change the values of Vmax. The latter is highly correlated with the formation constants of XB and HB complexes. Thus, the σ-hole model allowed us to explain variation and prevalence of certain type (HB or XB) complexes involving trihalomethane in the acetonitrile solutions - provided that the polarization of the electrophile by nucleophile in their complex is taken into account. This finding underscores the critical role of polarization in halogen bonding strength.

Evaluating the role of polarization in halogen bonding

2019 Fall National Meeting

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