The extraction of a nuclear magnetic shielding function for the H2 molecule from spin-rotation and isotope shift data

The accurate spin-rotation interaction constants of Ramsey for the H2, HD and D2 molecules in low rotational states have been combined with the very recent deuterium N.M.R. measurements of Beckett and Carr for the variation of the isotope shift [σ(D2)-σ(HD)] with temperature to yield the first two coefficients in the nuclear magnetic shielding function σ=σsub(e)sup((0)) + σsub(e)sup((1)) xi + σsub(e)sup((2)) xi2 + ... where xi=(R-Rsub(e))/Rsub(e). The results are used to calculate the influence of rotational excitation on the shielding of H2, HD etc. and the variation of shielding with temperature in the low density gases. A more precise value of the shielding of gaseous hydrogen at room temperature is deduced than that existing in the literature. The calculated isotope shift [σ(HD)-σ(H2)] is in good agreement with earlier experimental measurements. The hitherto unknown chemical shift [σ(o-D2)-σ(p-D2)] is calculated to be 0.007 p.p.m. at low temperature. (author)