ELECTRON-PARAMAGNETIC RESONANCE STUDY OF INTER-MOLECULAR EXCHANGE COUPLING IN AN UNDILUTED VANADYL COMPLEX

Pure crystals of cis-bis(1-phenyl-l, 3-butanedionato)oxovanadium(IV) display anomalous fine structure in their EPR spectra; 13 or fewer distinct features appear depending upon orientation. The features originate in a combination of 51V hyperfine structure with fine structure caused by dipolar and exchange coupling of nearest-neighbor molecules related by inversion through the unit cell origin. The spectra can be simulated to reasonable accuracy with coaxial g and A matrices, a spin-spin coupling tensor D based on point dipoles at the metal nuclei, a small ferromagnetic exchange coupling, and an orientation-dependent line width predicted by the Van Vleck dipolar broadening formula. The best-fit parameters are [Formula Omitted] and[Formula Omitted] and J in units of MHz). Both direct and indirect paths are available for the intersite exchange process, which corresponds to an exchange rate of ~109/s. © 1978, American Chemical Society. All rights reserved.