Halogen...O(Nitro) Supramolecular Synthon in Crystal Engineering: A Combined Crystallographic Database and <i>Ab Initio</i> Molecular Orbital Study

Abstract

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 Å 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 &lt; Br &lt; 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—NO 2 plane is preferentially less than 45° for Cl, with an increasing tendency to in-plane X approach in the order Cl &lt; Br &lt; 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 &lt; Br &lt; 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) [6 s 4 p 1 d ] basis set level indicate that this interaction is more attractive than Cl...O.