NMR absolute shielding scale and nuclear magnetic dipole moment of 207 Pb

Abstract

An absolute shielding scale is proposed for ²⁰⁷Pb nuclear magnetic resonance (NMR) spectroscopy. It is based on ab initio calculations performed on an isolated tetramethyllead Pb(CH₃)₄ molecule and the assignment of the experimental resonance frequency from the gas-phase NMR spectra of Pb(CH₃)₄, extrapolated to zero density of the buffer gas to obtain the result for an isolated molecule. The computed²⁰⁷Pb shielding constant is 10 790 ppm for the isolated molecule, leading to a shielding of 10799.7 ppm for liquid Pb(CH₃)₄ which is the accepted reference standard for ²⁰⁷Pb NMR spectra. The new experimental and theoretical data are used to determine μ(²⁰⁷Pb), the nuclear magnetic dipole moment of ²⁰⁷Pb, by applying the standard relationship between NMR frequencies, shielding constants and nuclear moments of two nuclei in the same external magnetic field. Using the gas-phase ²⁰⁷Pb and (reference) proton results and the theoretical value of the Pb shielding in Pb(CH₃)₄, we find μ(²⁰⁷Pb) = 0.59064 μ_N. The analysis of new experimental and theoretical data obtained for the Pb²⁺ ion in water solutions provides similar values of μ(²⁰⁷Pb), in the range of 0.59000–0.59131 μ_N.