The nature of halogen ... halogen synthons: Crystallographic and theoretical studies

A study of the halogen center dot center dot center dot halogen contacts in organic compounds using ab initio calculations and the results of previously reported crystallographic studies show that these interactions are controlled by electrostatics. These contacts can be represented by the geometric parameters of the C-X-1 center dot center dot center dot X-2-C moieties (where theta(1) = C-X-1 center dot center dot center dot X-2 and theta(1) = X-1 center dot center dot center dot X-2-C; theta = X-1 center dot center dot center dot X-2 distance). The distributions of the contacts within the sum of van der Waals radii (r(vdW)) versus theta(i) (theta(1)=theta(2)) show a maximum at theta approximate to 150 degrees for X=Cl, Br, and I. This maximum is not seen in the distribution of F center dot center dot center dot F contacts. These results are in good agreement with our ab initio calculations. The theoretical results show that the position of the maximum depends on three factors: 1) The type of halogen atom, 2) the hybridization of the ipso carbon atom, and 3) the nature of the other atoms that are bonded to the ipso carbon atom apart from the halogen atom. Calculations show that the strength of these contacts decreases in the following order: I center dot center dot center dot I > Br center dot center dot center dot Br > Cl center dot center dot center dot Cl. Their relative strengths decrease as a function of the hybridization of the ipso carbon atom in the following order: sp(2)> sp > sp(3). Attaching an electronegative atom to the carbon atom strengthens the halogen center dot center dot center dot halogen contacts. An electrostatic model is proposed based on two assumptions: 1) The presence of a positive electrostatic end cap on the halogen atom (except for fluorine) and 2) the electronic charge is anisotropically distributed around the halogen atom.