Electronic g-factor measurement from ENDOR-induced EPR patterns: malonic acid and guanine hydrochloride dihydrate

Measurement of electronic g-factors (g) from radicals in irradiated organic crystals is generally difficult because the overall EPR pattern is usually the composite of several components, e.g., from multiple radicals and from multiple magnetic sites. However, when an ENDOR line is fully resolved, the method of ENDOR-induced EPR (EI-EPR, or EIE) in principle permits identification of the EPR pattern from the individual component yielding the line. To examine this method as an approach useful for measuring g, we used it to measure those of known radicals in two different crystal systems. First, to verify correspondence of the EIE and EPR sufficient for using EIE patterns to extract g, we used both EIE and EPR to measure g of (*CH(COOH)(2) from irradiated crystals of malonic acid. Then, to illustrate the procedure applied to a system giving a more complex EPR pattern, we used EIE to measure g of the O6-protonated anion radical of guanine in irradiated guanine.HCl.2H(2)O crystals. EPR results from the malonic acid radical are g(max)=2.00374(2), g(mid)=2.00331(2), and g(min)=2.00234(3); EIE results from the same radical are g(max)=2.00375(2), g(mid)=2.00334(2), and g(min)=2.00238(2), where numbers in parentheses indicate statistical uncertainties in the respective least significant digits. In addition, eigenvectors from the two sets of measurements agree to approximately 1 degrees. Results from the guanine radical are g(max)=2.00490(2), g(mid)=2.00318(4), and g(min)=2.00218(4). (The uncertainties should reliably indicate relative accuracy, while absolute accuracy is within +/-0.0002 as indicated by simultaneous measurement of Cr(3+) in MgO.)