The nature of Halogen•••Halide synthons:: Theoretical and crystallographic studies

Two types of halogen center dot center dot center dot halide synthons are investigated on the basis of theoretical and crystallographic studies; the simple halogen center dot center dot center dot halide synthons and the charge assisted halogen center dot center dot center dot 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 center dot center dot center dot X- separation distance and the C-Y center dot center dot center dot X- angle in a series of complexes (R-Y center dot center dot center dot 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 center dot center dot center dot X- angles and separation distances less than the sum of van der Waals radius (r(vdW)) 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 center dot center dot center dot I-, to very strong, e.g., HCC-I center dot center dot center dot 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 center dot center dot center dot halide synthons have a comparable strength to simple halogen center dot center dot center dot halide synthons. The nature of these contacts is explained on the basis of an electrostatic model.