JOURNAL OF MAGNETIC RESONANCE 60, 54-65 (1984) Determination of TI and T2 by Simulation of EPR Power Saturation Curves and Saturated Spectra. Application to Spin-Labeled Iron Porphyrins KLJNDALIKAM.MORE,GARETH R. EATON, ANDSANDRA S.EATON Departments of Chemistry, University of Demer, Denver, Colorado 80208, and University oJColorado, Denver, Colorado 80202 Received March 20, 1984 Values of T, and Tz have been obtained by computer simulation of fluid-solution EPR power saturation curves, unsaturated spectra, and saturated spectra. Unresolved proton hyperfme splitting was included explicitly. Ixlter- and intramolecular spin-spin interactions between two nitroxyl spin-labeled porphyrins and high-spin iron(III) were examined. In these systems the spin-spin interaction intluences Tz more than it influences T, . 8 1984 Academic p, Inc. INTRODUCIION Considerable work has been done on methods to obtain values of Tr and T2 from continuous wave EPR data (1-8). These studie:s dealt with compounds in which there was no unresolved proton hyperfine coupling or the unresolved coupling could be approximated by a Gaussian distribution of spin packets. We are interested in studying the changes in T, and T2 of nitroxyl radicals due to inter- and intramolecular interactions with paramagnetic transition metals. In fluid solution at low concentra- tions the EPR spectra of many nitroxyl radicals exhibit partially resolved proton hyperfine splitting, so the spectral envelope cannot be approximated as a Gaussian distribution of lines. Since values of proton hyperfme splittings for a variety of nitroxyl radicals have been obtained by NMR and ENDOR measurements (9-13) it is possible to treat the proton hype&e splitting explicitly. In previous studies certain features of the power saturation curve were analyzed. Inherently a simulation of the full curve should be more informative. Furthermore simulation of EPR spectra as a function of microwave power should provide more information than the analysis of the power saturation curve alone. We have applied this approach to the analysis of the intra- and intermolecular interactions between spin-labeled porphyrins and high-spin iron(II1) in complexes III, IV, VII, VIII, and X. EXPERIMENTAL Physical measurements. Infrared spectra were obtained in nujol mulls on a Perkin-Elmer 283B spectrometer. Visible spectra were obtained in chloroform solution on a Beckman Acta V spectrometer. Spectra are reported below with wavelengths in nanometers and log e in parenthesis following the wavelength. OQ22-2364184 $3.00 Copyright 0 1984 by Academic FWss, Inc. All rights of reproduction in any form reserved. 54