Field‐dependent relaxation and absolute nuclear shielding of 207Pb in liquid PbCl4

Abstract

Relaxation times T1 and T2 of 207Pb in liquid PbCl4 have been studied for the purpose of establishing an absolute nuclear shielding scale for lead. T1 has been decomposed into scalar and spin‐rotation contributions using variable temperature measurements at the two field strengths 6.59 and 16.90 kG. T2 is much shorter than T1 and is strongly dominated by scalar coupling to chlorine. Knowledge of the scalar contributions to T1 and T2 determines the halogen relaxation times (τ35 = 7.15 μsec at 25°C), the lead‐chlorine coupling constant [(J(207Pb−35Cl) = 705 Hz)], and the reorientational correlation time (τθ = 1.72 × 10−11 sec at 25°C). A correlation time τJ for the angular momentum vector has been computed using J diffusion theory, but the value obtained appears to be too short (0.8–2.9 × 10−14 sec) to be meaningful in terms of classical diffusion of rigid molecules. Nevertheless, the known range of 207Pb chemical shifts places an absolute upper limit on τJ of 6 × 10−14 sec at 263°K. The physical significance of such short correlation times is discussed, and it is concluded that τJ probably describes collision‐induced distortions in the molecular structure. Experimental values of the spin‐rotation contribution to (T1)−1 are used in conjunction with estimated values of τJ to compute limiting values for the spin‐rotation constant that place limits on the paramagnetic part of the magnetic shielding constant. A shielding scale previously deduced from optical pumping data is discussed, and a source of possibly substantial error in this scale is pointed out.