ESR Study of OH Radicals Originating from Water of Crystallization of Carboxylic Acids. I. Sodium Hydrogen Maleate Trihydrate

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Abstract
The OH radicals originating from water of crystallization have been found in an irradiated single crystal of sodium hydrogen maleate trihydrate. This may be the first example of OH radical formation in hydrated organic crystals. We have found two types of OH radicals having different orientations and g tensors with nearly the same hyperfine tensors. The principal values of the g tensors were determined to be (2.0034, 2.0081, 2.0618) for OH(I), and (1.9947, 2.0065, 2.1198) for OH(II). The hyperfine tensors were found to be (− 27.0, − 44.7, + 3.5)G and (− 29.4, − 44.8, + 4.8)G, respectively, the relative sign being determined by the forbidden transitions. It was also found from the principal directions of the hyperfine and g tensors that the OH radicals are trapped in such a way that the O–H bonds in OH(I) and OH(II) are approximately parallel and perpendicular to the molecular plane of the hydrogen maleate ion, respectively, making an angle of 100°–105° with each other. The larger g anisotropy in OH(II) is considered to come from such a difference in the trapping site that the radicals interact with the surrounding molecules more weakly. The energy separations of the two oxygen Pπ orbitals were estimated to be 0.58 and 0.29 eV for OH(I) and OH(II), respectively.