Weak intermolecular interactions between nitrogen and oxygen atoms

A recent study of organic crystalline compounds containing nitro groups, which concentrated on the intermolecular approach of oxygen to the N of the NO2 group, has revealed typical N…O distances much shorter than would be expected from the van der Waals radii. Ab initio dimer calculations reported here substantiate this finding, and indicate that these interactions are comparable in strength with weak hydrogen bonds. Dimer binding energies of 10–13 kJ mol−1 are obtained at the MP2/6–31++G∗∗ level, and the optimal N…O distance is ≈ 2.85 Å. The difference between one-electron (Hartree-Fock) and correlated treatments, combined with an atoms-in-molecules decomposition of the charge distribution, suggests that this interaction is dominated by dispersive forces.