Halogen•••O(nitro) supramolecular synthon in crystal engineering:: A combined crystallographic database and ab initio molecular orbital study

Crystallographic data for 309 C-X ... O(nitro)-C, X = Cl, Br, I, interactions, involving 245 symmetry-independent X atoms, have been investigated out to 0.2 Angstrom above van der Waals radii limits [nu(O) + nu(X)]. A total of 138 (45%) of these interactions are shorter than nu(O) + nu(X), with the degree of interpenetration of the X and O atoms increasing in the order Cl < Br < I. The analysis also shows that: (a) the C-X ... O angle tends to linearity as the X ... O distance shortens, (b) the angle of approach of X to the C-NO2 plane is preferentially less than 45 degrees for Cl, with an increasing tendency to in-plane X approach in the order Cl < Br < I, and (c) the halogen (X) forms either (i) mono-coordinate interactions with one nitro O atom, with X ... O in a cis relationship to the nitro C substituent about the N-O bond, or (ii) approaches both nitro O atoms in a bifurcated manner; the tendency to form such bifurcated motifs increases in the order Cl < Br < I. Only iodine consistently forms short interactions with both nitro O atoms. Ab-initio-based molecular orbital calculations, using intermolecular perturbation theory (IMPT) applied to a nitromethane-1-chloro-2-methylacetylene model dimer, agree with the analysis of experimental crystal structure geometries. The IMPT calculations yield an attractive interaction energy of ca -6 kJ mol(-1) for Cl ... O at the 6-31G* basis set level. Calculations for Br ... O at the (only available) [6s4p1d] basis set level indicate that this interaction is more attractive than Cl ... O.