MAGNETIC-FIELD-DEPENDENT ELECTRONIC RELAXATION OF GD3-SOLUTIONS OF THE COMPLEXES [GD(H2O)8]3+, OPANE-1,3-DIAMINE-N,N,N',N'-TETRAACETATE)(H2O)2]-, AND -3-AZAPENTANE-1,5-DIAMINE-3,N,N'-TRIACETATE)(H2O)] OF INTEREST IN MAGNETIC-RESONANCE-IMAGING( IN AQUEOUS)

EPR Spectra have been measured for aqueous solutions of a series of Gd 3+ complexes at variable temperature and a range of magnetic fields; S -band (0.14 T), X-band (0.34 T), Q-band (1.2 T), and 2-mm-band (5.0 T) . The major contribution to the observed line widths is magnetic-field -dependent and is interpreted as being due to the modulation of the ze ro-field splitting produced by distortion of the complexes from perfec t symmetry. The transverse and longitudinal relaxation matrices for an 8S ion with such an interaction have been calculated using Redfield t heory with vector-coupling methods, and diagonalised numerically to ob tain relaxation rates and intensities for the degenerate transitions w hich contribute to the multiplet. The observed line width, which is in versely proportional to the magnetic field at low temperatures, is bes t described by the intensity-weighted mean transverse relaxation time for the four transitions with non-zero intensity. A least-squares fit of the data yields the square of the zero-field splitting tensor, DELT A2, and a correlation time, tau(v), with activation energy, E(v). The physical significance of these parameters and the extent of validity o f the theoretical approach are considered. The parameters are used to predict the magnetic-field dependence of the longitudinal and transver se electronic relaxation times, which are discussed in the context of their relevance to H-1-NMR relaxivity.