Magnetic‐Field‐Dependent Electronic Relaxation of Gd³⁺ in Aqueous Solutions of the Complexes [Gd(H₂O)₈]³⁺, [Gd(propane‐1,3‐diamine‐N,N,N′,N′‐tetraacetate)(H₂O)₂]⁻, and [Gd(N,N′‐bis[(N‐methylcarbamoyl)methyl]‐3‐azapentane‐1,5‐diamine‐3,N,N′‐triacetate)(H₂O)] of interest in magnetic‐resonance imaging

EPR Spectra have been measured for aqueous solutions of a series of Gd³⁺ 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 zero‐field splitting produced by distortion of the complexes from perfect symmetry. The transverse and longitudinal relaxation matrices for an ⁸S ion with such an interaction have been calculated using Redfield theory with vector‐coupling methods, and diagonalised numerically to obtain relaxation rates and intensities for the degenerate transitions which contribute to the multiplet. The observed line width, which is inversely proportional to the magnetic field at low temperatures, is best 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, Δ², and a correlation time, τ_v, with activation energy, E_v. The physical significance of these parameters and the extent of validity of the theoretical approach are considered. The parameters are used to predict the magnetic‐field dependence of the longitudinal and transverse electronic relaxation times, which are discussed in the context of their relevance to ¹H‐NMR relaxivity.