App!. Magn. Reson. 20, 151-157 (2001) Applied Magnetic Resonance © Springer-Verlag 2001 Printed in Austria Frequency Dependence of Electron Spin Relaxation for Three S = 1/2 Species Doped into Diamagnetic Solid Hosts S. S. Eaton', J. Harbridge', G. A. Rinard', G. R. Eaton', and R. T. Weber 2 'Departments of Chemistry and Biochemistry and Engineering, University of Denver, Denver, Colorado, USA 2 EPR Division, Bruker Instruments, Inc., Billerica, Massachusetts, USA Received January 15, 2001 Abstract. Electron spin lattice relaxation rates (1/T1 ) were measured as a function of temperature at two or three microwave frequencies for three S = 1/2 species in temperature ranges with different dominant relaxation processes. Between 10 and 50 K the contribution from the direct process to the relaxation rate was substantially greater at 94 than at 9.5 GHz for a vanadyl porphyrin doped into zinc tetratolylporphyrin. For bis(diethyldithiocarbamato)copper(TI) doped into the diamagnetic Ni(II) analog the relaxation rate between 25 and 100 K is dominated by the Raman process and exhibits little frequency dependence between 9.2 and 94 GHz. For 4-hydroxy-2,2,6,6-tetramethylpiperidinol- oxy (tempol) doped into a diamagnetic host the relaxation rate between about 40 and 100 K is domi- nated by the Raman process. In this temperature range, relaxation rates at 3.2, 9.2, and 94 GHz exhibit little frequency dependence. Above about 130 K, the relaxation rate for tempol decreases in the or- der S-band > X-band > W-band. The relaxation rates in this temperature range fit a model in which ]IT, is dominated by a thermally activated process that is assigned as rotation of the methyl groups on the nitroxyl ring. 1 Introduction Multifrequency electron paramagnetic resonance (EPR) studies were important contributions to testing the applicability of the direct, Raman, and Aminov-Orbach relaxation processes to various paramagnetic centers (mostly metals) at tempera- tures in the liquid He range [1, 2]. We recently examined the temperature de- pendence of electron spin relaxation rates for a range of S = 1/2 species in di- lute solids and glassy matrices between 10 and 300 K [3]. To test the resulting models of the relaxation mechanisms, three of those species were selected for examination at W-band with emphasis on temperature regimes where particular relaxation processes were predicted to dominate: a vanadyl complex, a Cu(II) complex, and a nitroxyl radical. To facilitate handling of the samples at W-band, doped solids were selected. Increasingly, nitroxyl radicals are being studied by EPR at multiple frequencies to increase resolution of overlapping species and study molecular dynamics, and some relaxation times have been measured at more