The unique role of the nitro group in intramolecular interactions: chloronitromethanes

We have extended an earlier crystallographic/computational study which revealed an exceptionally short C-Cl bond in chlorotrinitromethane, Cl-C(NO2)(3). We show that the C-Cl bond length progressively decreases when NO2 groups are introduced into chloromethane. This is attributed to intramolecular attractive interactions between the chlorine and the closest NO2 oxygens. Computed electrostatic potentials on molecular surfaces support this interpretation, as well as the N-O interactions between neighboring NO2 groups that help to determine the molecular conformations. The calculated C-Cl bond energies decrease as the NO2 groups are added, which is expected, but means that the usual inverse relationship between bond energy and bond length is not being obeyed. For purposes of comparison, the computational analyses, which were primarily at the B3PW91/6-311G(3d,2p) level, were also carried out for the corresponding chlorocyanomethanes and chlorofluoromethanes. These do not show anomalously short C-Cl bond distances.