JOURNAL OF MAGNETIC RESONANCE 52,435-449 (1983) Metal-Nitroxyl Interactions. 29. EPR Studies of Spin-Labeled Copper Complexes in Frozen Solution SANDRA S.EATON Department of Chemistry, University of Colorado at Denver, Denver, Colorado 80202 AND KUNDALIKA M.MORE,BHIMRAO M. SAWANT,PAULM.BOYMEL, ANDGARETH R. EATON Department of Chemistry, University qf Denver, Denver, Colorado 80208 Received September 7, 1982 Frozensolution EPR spectra have been obtained for 12 spin-labeled copper complexes. A perturbation calculation including both exchange and dipolar interaction was used to simulate the spectra. The distances between the two spins ranged from 7.5 to 13.5 A. The absolute value of the electron-electron exchange coupling constant ranged from 0 to about 2000 X 1O-4 cm-‘. INTRODUCTION .Understanding electron-electron spin-spin interaction is important in areas as diverse as metal-metal dimers, organic diradicals, and biological systems containing two unpaired electrons. In fluid solution in the rapid tumbling limit, the dipolar interaction contribution to the EPR lineshape is averaged to zero by the tumbling of the molecule but the exchange information is retained (I). Values of the electron- electron coupling constant J have been obtained from fluid solution spectra for a wide variety of spin-labeled transition metal complexes containing Cu(II), Ag(II), and VO(IV) (1-3). Single crystal EPR spectra permit a complete analysis of both the exchange and dipolar contributions to the spin-spin interaction (4). However it is often difficult to obtain single crystals of compounds of interest. It is therefore im- portant to determine the extent to which information can be obtained from frozen solution or powder EPR spectra. To illustrate the approaches that have been taken to obtain information concerning spin-spin interaction from frozen solution EPR spectra some typical examples are mentioned here. When the electron-electron exchange coupling constant J is of the order of magnitude of thermal energies, the value of J can be obtained from variable temperature magnetic susceptibility measurements. Values of J as small as 0.5 cm-’ 435 0022-2364183 $3.00 Copyright 0 1983 by Academic Press. Inc. All rights of reproduction in any form reserved.