HYPERFINE LINE SHIFT IN THE EPR-SPECTRA OF RANDOMLY ORIENTED CU(II) CONTAINING SYSTEMS WITH AXIAL SYMMETRY

We revisit the problem of unequal spacing of the hyperfine lines in the g parallel to region of the experimental EPR spectra of some type 2 Cu(II) complexes displaying axial symmetry. This shift, already reproduced by our computer simulation model, cannot be justified fully in the framework of second-order perturbation theory. On this ground, we have performed an analysis, based on a computer simulation approach, of all the possible causes that can give rise to a shift of the hyperfine lines. The results show that in randomly oriented samples the overall high field lineshift is due to a superposition of Gaussian-like hyperfine lines and the background arising from the presence of different orientations of the paramagnetic ions with respect to the magnetic field. In addition, the effect induced on the EPR spectra by a superposition of adjacent hyperfine lines is discussed in connection with the distortion of the g parallel to pattern registered in the EPR spectra of some type 1 Cu(II) complexes. As a final remark, our analysis shows that a computer simulation approach is required to extract reliable values for the spin Hamiltonian parameters from the experimental EPR spectra of randomly oriented paramagnetic samples.