ESR Studies of Nitrobenzene,o-Dinitrobenzene, andm-Dinitrobenzene Alkali-Metal Salts

Abstract

The hyperfine splittings for the lithium, sodium, potassium, and cesium salts of nitrobenzene and the sodium, potassium, and cesium salts of o-dinitrobenzene over a range of temperature in DME are reported, as well as the splittings for the nitrobenzene and o-dinitrobenzene radicals produced by electrolytic reduction in DME. The sodium salt of nitrobenzene was also studied in a mixture of DME and DMSO. Results for nitrobenzene and o-dinitrobenzene are correlated with those previously reported for m-dinitrobenzene salts. For all three anions it was found that the perturbation of the anion on ion-pair formation increases along the series Cs+ to Li+. Line-broadening effects associated with cation motion from one nitro group to the other in the o- and m-dinitrobenzene salts correlate well with the extent of the perturbation of the anion by the metal cation. On the basis of (1) similar magnitudes and similar variation with temperature of the alkali-metal hyperfine splittings for all three anions, (2) the absence of line-broadening depending on alkali-metal quantum number, and (3) the observations in the mixed solvent system, a model for the ion pairs in DME is proposed in which the cation interacts with a single nitro group at a given instant of time, the cation is tightly bound to the nitro group with a fairly strongly bound cage of DME molecules solvating the nitro group cation complex, and temperature variation of the alkali-metal splitting is attributed to motion of the cation in the vicinity of the nitro group. Simple molecular-orbital calculations were performed in which the electrostatic interaction of the cation with the nitro group was simulated by increasing both the oxygen and nitrogen Coulomb integral parameters and good agreement between calculated and experimental changes of anion hyperfine splittings on going from the nitrobenzene free ion in DME to the sodium salt was obtained.