PRESSURE-INDUCED ENHANCEMENT OF THE FERROMAGNETIC INTERMOLECULAR INTERACTION OF AN ALPHA-NITRONYL NITROXIDE ORGANIC RADICAL

The effect of high pressure on the ferromagnetic properties of a crystalline sample of an organic radical, 2-(4-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy 3-oxide (I), has been studied with the use of a combination of a Faraday susceptometer and a Be-Cu high-pressure clamp cell. The magnetic susceptibility of I increases markedly with increasing pressure at low temperatures, and furthermore its increment increases monotonically with decreasing temperature down to about 5 K. The Weiss constant under a pressure of 9 kbar corresponds to a 40% increase in the ferromagnetic intermolecular interaction, and the magnetization at 9 kbar also comes to saturation more rapidly compared with the behavior under an ambient pressure. A simple calculation of the intermolecular overlap integrals between the frontier orbitals can semiquantitatively interpret the pressure-induced enhancement of the ferromagnetic coupling. © 1990, American Chemical Society. All rights reserved.