ON THE CALCULATION OF NUCLEAR SPIN-SPIN COUPLING-CONSTANTS - THE BOND-LENGTH DEPENDENCE OF THE FERMI CONTACT TERM IN H-2 AND HD

A theoretical study of the Fermi contact contribution to the H-H and H -D spin-spin coupling constant is reported, with special emphasis on i ts calculation using quantum chemical techniques over a wide range of internuclear distances, that has necessitated an extension of the exis ting methodology so the effects of near-degeneracy are properly treate d. A detailed configuration interaction calculation on H-2 shows that as the molecule is stretched the coupling constant displays a sharp in crease before decaying to zero as the molecule dissociates. Such dista nce dependence is reflected in the calculated vibrational averages of the coupling constant for HD that show a rapid increase with vibration al excitation.