Precision determination of the difference in shielding by protons in water and hydrogen and an estimate of the absolute shielding by protons in water

摘要

In order to determine the difference in the screening of protons in water and protons in hydrogen, the authors used cylindrical samples and recordied NMR signals in the field of an electromagnet (B = 2.142 T) and a superconducting magnet (B = 11.747 T). The use of two magnets allows the influence of the volume magnetic susceptibility of the samples to be eliminated. The difference in the chemical shift for protons in water and hydrogen is found to be σ(H_2) - σ (H_2O) = 608.0(±1.5) × 10~(-9) at 25.0℃. The value of proton screening in hydrogen σ(H_2) is known from the literature: σ(H_2) = 26 288(±2) × 10~(-9). With this in mind, for the screening of protons in water we have σ (H_2O) = 25 680(±2.5) × 10~(-9) at 25.0℃. The new result should be compared with the result for the screening of protons in water σ(H_2O) = 25 694(±14) × 10~(-9) at 25.0℃, which is shown in the list of fundamental physical constants (CODATA). This comparison shows that, although the two results were obtained by completely different methods, they agree within the limits of uncertainty.