ELECTRON-SPIN ECHO ENVELOPE MODULATION ANGLE SELECTION STUDIES OF AXIAL PYRIDINE COORDINATION TO COPPER(II) BENZOYLACETONATE

Magnetic-field-dependent electron spin echo envelope modulation (ESEEM) spectroscopy is used to characterize weak ligand hyperfine interactions for 14N- and 15N-labeled pyridine axially coordinated to bis(benzoylacetonato)copper(II) in frozen solution Because of the g and hyperfine anisotropies for Cu(II), at any magnetic field within the EPR absorption envelope one obtains overlapping single-crystal-like ESEEM spectra. These spectra reflect the angular dependence of the hyperfine energies of the ligand nuclei. Their analysis yields both magnetic coupling constants and orientations of the principal axes of the pertinent coupling matrices with respect to the Cu(II) g matrix. Spectra at different microwave frequencies are best simulated within the point-dipole model by locating a nitrogen in the axial position, effectively 2.6 Å from the plane defined by the metal and ligand atoms of the molecule. Corrections for defects of the point-dipole model are expected to be small; they would reduce the nitrogen-copper distance to about 2.3 Å, in good agreement with known axial pyridine-to-copper(II) bond lengths. For 15N, the diagonal elements of the A matrix are 1.45, 1.45, and 0.09 MHz; for 14N they are -1.03, -1.03, and -0.06 MHz. The 14N quadrupole parameters are e2qQ = 4.4 MHz and η = 0.35, with the principal axis of the electric field gradient oriented coincident with the nitrogen-to-copper bond. The simulations obtained demonstrate the ability to characterize a weakly coupled nitrogen nucleus from the field dependence of the ESEEM spectrum. © 1990 American Chemical Society.