ESR Study of Radical Structure and Symmetrical Hydrogen Bond in a Single Crystal of Potassium Hydrogen Maleate Irradiated at 77°K

Abstract

ESR study has been carried out on the structure of radicals produced in an irradiated single crystal of potassium hydrogen maleate which is known as a typical compound having a symmetrical intramolecular hydrogen bond. It was found that in the radical formed by removal of the acidic proton the unpaired electron occupies the σ antibonding orbital between the in-plane p orbitals of the two ring oxygen atoms, when the crystal is irradiated at 77°K. The principal values of the g tensor for this radical were determined to be gxx=2.0043, gyy=2.0039, and gzz=2.0079. The hyperfine couplings for the two vinylene protons are equivalent and have the nearly isotropic value of 5±1 G. On the other hand, if the crystal is warmed up to room temperature after irradiation at 77°K, the same species of radical is formed but the unpaired electron occupies the delocalized π orbital. This radical is the same as that found in room-temperature irradiation which was previously reported. Both the radicals possess the mirror symmetry perpendicular to the C=C bond as does the mother molecule. The formations of such symmetrical radicals are closely related to the symmetrical structure and the extremely short O···O distance of the parent molecule, which are due to the symmetrical hydrogen bond. The cause of the formations of the σ and π electron radicals from the same mother molecule is also discussed.