Experimental Quantification of Halogen⋅⋅⋅Arene van der Waals Contacts

Abstract

Crystallographic and computational studies suggest the occurrence of favourable interactions between polarizable arenes and halogen atoms. However, the systematic experimental quantification of halogen⋅⋅⋅arene interactions in solution has been hindered by the large variance in the steric demands of the halogens. Here we have synthesized molecular balances to quantify halogen⋅⋅⋅arene contacts in 17 solvents and solvent mixtures using ¹ H NMR spectroscopy. Calculations indicate that favourable halogen⋅⋅⋅arene interactions arise from London dispersion in the gas phase. In contrast, comparison of our experimental measurements with partitioned SAPT0 energies indicate that dispersion is sufficiently attenuated by the solvent that the halogen⋅⋅⋅arene interaction trend was instead aligned with increasing exchange repulsion as the halogen increased in size (ΔG_X ⋅⋅⋅_Ph =0 to +1.5 kJ mol^-1 ). Halogen⋅⋅⋅arene contacts were slightly less disfavoured in solvents with higher solvophobicities and lower polarizabilities, but strikingly, were always less favoured than CH₃ ⋅⋅⋅arene contacts (ΔG_Me ⋅⋅⋅_Ph =0 to -1.4 kJ mol^-1 ).

Keywords: Halogens; Molecular Balances; Molecular Recognition; Noncovalent Interactions; π Interactions.