DIPOLAR BROADENING OF MAGNETIC RESONANCE LINES: A TOOL FOR STUDY OF DIMERIZATION AND CLUSTER FORMATION

Author Institution: Department of Chemistry, University of KentuckyThe ESR spectra of several copper (II) dithiocarbamates in a variety of solvents were investigated at room temperature in the concentration range of $10^{-3}$ to $10^{-1}$M. A very pronounced broadening of the magnetic resonance lines is observed at the higher concentrations. This broadening could not be interpreted by the Brownian motional model of Bloembergen, Purcell, and Pound, or similar theoretical formulations. The method of moments developed by Van Vleck for crystalline magnetic systems and extended to magnetically dilute crystalline cases by Kittel and Abrahams is adapted to solution cases involving molecules of size such that the motional correlation times become sufficiently long to allow a ``rigid liquid” treatment. The major weakness of the method of moments is the difficulty of characterization of the line shape, particularly in the near-Lorentzian case. A procedure is developed by which the line shapes are very closely approximated such that characterization of the line shape by the second moment alone is possible. The resulting first principle computations agree well with the data on the copper dithiocarbamates. In solutions containing dimers in addition to monomers, the dimer concentration (in the range of $10^{-4}$M) is obtained. The agreement between theory and experiment is also good for spectra of free radicals in solution