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A detailed consideration of the cw saturation technique for measuring spin-lattice relaxation times (T1) in liquid solutions is given. An improved method for more accurate measurement of cavity Q is described. The saturation technique was used to measure the T1 of the benzene anion radical over the temperature range 150–250°K. The measurements were made at five different concentrations (0.01), 0.03, 0.1, 0.3, and 1.0M). For benzene concentrations ≦0.1 M, the linewidths and hence T2 are independent of concentration. However, the linewidth displays a minimum and T2 a maximum at 173°K for all benzene concentrations. Our measurements of T1 show that although there is no discernable dependence upon benzene concentration, a maximum in T1 is observed at approximately 188°K. Below 188°K, T1 decreases to the solvent freezing point. In addition, analysis of the line shape of the benzene anion radical spectrum yields strong evidence that the benzene anion radical prepared from Na–K alloy in 2:1 tetrahydrofuran-1,2-dimethoxyethane is ion paired.

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