RELATION BETWEEN NUCLEAR INDUCTION MULTIPLETS AND MOLECULAR ELECTRONIC STRUCTURE

Abstract

Electron-spin coupled internuclear interactions which give rise to nuclear induction ""spin-spin"" multiplets can be expressed in terms of a second-order magnetic interaction between two electron spins and two nuclear $spins.^{1,2}$ It can be shown that the extent to which magnetic nucleus K interacts with magnetic nucleus L in molecule M (to give nuclear induction multiplets) depends, in part, on the extent to which there is a nonrandom correlation of the electron-spin polarizations on atoms K and L. This correlation of the electron spins associated with nuclei K and L is readily understood when nuclei K and L are directly bonded to one another by a single homopolar electron pair bond: when the electron spin on K is ""up,"" the electron spin on L is ""down"" and vice versa. Nuclear spin-spin interactions have now been observed for nuclei separated from one another by as many as four bond lengths---this implies an effective long-range correlation of electron-spin polarizations. Calculations of such long-range spin polarizations have been made using the molecular-orbital and valence-bond approximations. It has been found that both of these approximations can account qualitatively for these long-range electron-spin correlations; it can be shown however, that quantitative expressions for spin-spin interactions are quite sensitive to the inclusion of interactions frequently neglected in ""classical"" descriptions of molecular electronic structure, e.g., spin-orbit interactions.