The Nature of Halogen···Halide Synthons: Theoretical and Crystallographic Studies

Two types of halogen···halide synthons are investigated on the basis of theoretical and crystallographic studies;
the simple halogen···halide synthons and the charge assisted halogen···halide synthons. The former interactions
were investigated theoretically (ab initio) by studying the energy of interaction of a halide anion with a
halocarbon species as a function of Y···X- separation distance and the C−Y···X- angle in a series of complexes
(R−Y···X-, R = methyl, phenyl, acetyl or pyridyl; Y = F, Cl, Br, or I; X- = F-, Cl-, Br-, or I-). The
theoretical study of the latter interaction type was investigated in only one system, the [(4BP)Cl]2 dimer,
(4BP = 4-bromopyrdinium cation). Crystal structure determinations, to complement the latter theoretical
calculations, were performed on 13 n-chloropyridinium and n-bromopyridinium halide salts (n = 2−4). The
theoretical and crystallographic studies indicate that these interactions are controlled by electrostatics and are
characterized by linear C−Y···X- angles and separation distances less than the sum of van der Waals radius
(rvdW) of the halogen atom and the ionic radii of the halide anion. The strength of these contacts from
calculations varies from weak or absent, e.g., H3C−Cl···I-, to very strong, e.g., HCC−I···F- (energy of
interaction ca. −153 kJ/mol). The strengths of these contacts are influenced by four factors:  (a) the type of
the halide anion; (b) the type of the halogen atom; (c) the hybridization of the ipso carbon; (d) the nature of
the functional groups. The calculations also show that charge assisted halogen···halide synthons have a
comparable strength to simple halogen···halide synthons. The nature of these contacts is explained on the
basis of an electrostatic model.