Weakly Coupled Proton Interactions in the Malonic Acid Radical: Single Crystal ENDOR Analysis and EPR Simulation at Microwave Saturation

Abstract

The α-proton hyperfine coupling observed by electron paramagnetic resonance (EPR) spectroscopy on the radical •CH(COOH)2 in irradiated crystals of malonic acid, CH2(COOH)2, has served as a standard against which hundreds of observations of similar couplings have been held and scaled. The major doublet of the malonic acid radical is accompanied by less intense “forbidden” (f) α-proton transitions and “spin-flip” (s) transitions due to weakly interacting protons. Both s and f transition lines exhibit microwave power saturation behaviors different from that of the major doublet. At high microwave power, the prominence of these s and f lines may be misinterpreted as originating from different radical species. Computer simulations could help distinguish between the different cases, but no computer simulation programs taking into account the microwave power saturation case are commonly available. On the basis of classical line-shape theory, an algorithm describing the microwave power dependence of an EPR line shape has been developed and implemented in an existing simulation program. To test this new program, malonic acid was selected because of the simplicity of its EPR spectra. However, sufficiently detailed information about the hyperfine coupling parameters for a satisfactory simulation of the room-temperature data (including s and f lines) was not available in the literature. Therefore, a detailed room-temperature EPR/ENDOR study on a single crystal of malonic acid was performed. In addition to the major α-proton coupling, seven weaker proton interactions have been characterized and partly identified. Simulations under nonsaturating conditions reproduce very well all features of the experimental EPR spectra. Simulations under saturating conditions similarly reproduce the power-dependent EPR spectra and yield information about the relaxation behavior of the radical system, which is amenable to verification using other spin-resonance methods.