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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 < 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° 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.
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