Determination of Nuclear Quadrupole Coupling by Simulation of EPR spectra of frozen solutions

Abstract

Nuclear electric quadrupole interactions can influence EPR spectra very strongly at certain magnetic field orientations through the positions and intensities of double-spin-flip transitions. Previously, we analyzed orientation-dependent EPR spectra of magnetically dilute single crystals having 63Cu in a variety of coordination sites to obtain principal components of the quadrupole coupling tensor. The values showed a striking correlation with site geometry and electronic structure. High-symmetry, large-coordination-number sites have the largest quadrupole coupling constants. This potentially useful structural tool has been limited by its restriction to dilute single crystals. Accordingly, we report now on the importance of the quadrupole-sensitive double-spin-flip transitions in EPR spectra of frozen solutions. The X-band and Q-band spectra of some frozen solutions of copper /f-ketoenolates (with the copper isotopes in natural abundance) in more-or-less coordinating solvents had rather broad, poorly resolved features. Nevertheless, inclusion of the double-spin-flip transitions proved essential to successful stimulation of the X-band spectra. By matching computed simulations with the experimental spectra we show that these extra transitions contribute sufficiently to permit one to extract nuclear quadrupole coupling information. The resulting quadrupole coupling constants follow the expected order of solvent ligand strength and lie in between those previously found for sites of 5- and 6-coordination. Specific axial solvent coordination is implied.