Electrostatic models of hydrogen-bonded dimers: a donor—acceptor scale for hydrogen halides and pseudohalides

Abstract

A previously developed electrostatic model is applied to the homo- and hetero-dimers of the HX series (X=F,Cl,Br,I,CN). Charge distributions are taken from ab initio SCF calculations on the monomers, and molecular size is modelled by van der Waals' spheres on the heavy atoms. The model reproduces experimental angular geometries, where known, and predicts, for example, that HI⋯HCl and HI⋯HBr have geometries similar to the known HI⋯HF, with HI acting as proton acceptor. All fifteen possible complexes can be rationalised by ordering the monomers by their power as H-bond acceptors: HCN > HI > HF > HCl > HBr. The stable isomer HX⋯HY has HX > HY in this series. Used in conjunction with the Legon—Millen rules for angular geometry, this scale appears to give a complete qualitative description of HX dimers. Experimental tests are suggested.