Nuclear magnetic shielding of hydrogen in the hydrides of the elements

Abstract

Values have been assembled for the molecular diamagnetic and paramagnetic components of the proton shielding in the simple binary hydrides of the elements, and are shown to increase in periodic fashion with the atomic number of the heavy atom. For molecules for which the diamagnetic term σ_d is unknown, this has been calculated by Flygare's method. An absolute value of the paramagnetic term is afforded by the spin-rotation interaction constants which are now known with reasonable accuracy for 13 of the hydrides (σ_p has been calculated where necessary from Ramsey and Flygare's equations). For the remaining hydrides, σ_p has been obtained by subtracting σ_d from the observed shielding referred to an absolute scale. The dependence of the terms on the size and shape of the molecule and the position of the hydrogen is illustrated by the values for diborane and tetra-hydroborate ion. Factors determining the periodicity are discussed; this is remarkably symmetrical for the diamagnetic and paramagnetic terms. Although proton shielding is often described as dominated by the diamagnetic term, the periodic correlation shows that variations in the resultant shielding may be determined by changes in the paramagnetic term, e.g. down the Group of the heavy atom and across the second Row (the lithium Row).