The principle of maximum hardness and structural effects of nonbonded interactions in chloronitromethanes

The three monochloronitromethanes encompass a variety of possible nonbonded intramolecular interactions, both attractive (Cl- - -O, N- - -O and H- - -O) and repulsive (O- - -O). These are responsible for the different NO2 orientations as well as the marked shortening of the C-Cl bond. We have investigated computationally the effectiveness of the principle of maximum hardness as a means of predicting the C-Cl bond lengths and Cl-C-N-O dihedral angles in the chloronitromethanes. The B3PW91/6-31G(d,p) procedure was used, and the hardness was determined from the energies of the highest and lowest occupied molecular orbitals. It was concluded that the maximum hardness principle is a reasonably good guide to the most stable conformations of these molecules, but less satisfactory with respect to the C-Cl distance.