AB-INITIO CALCULATIONS OF NMR SPIN-SPIN COUPLING CONSTANTS IN SMALL DIATOMIC MOLECULES

Abstract

The Fermi contact interaction accounts for nearly all of the nuclear spin-spin coupling in small diatomic molecules AB, and the coupling constant is a cross-term in the second-order energy, i.e., ${J}_{AB}\sim<\Phi^{1}_{A}{H}^{1}_{a}\Phi^{0}$ where $\Phi^{0}$ is the unperturbed wavefunction, $\Phi^{1}_{A}$ is the first-order wavefunction due to the perturbation $H^{1}_{A}$ and $H^{1}_{A}$ and $H^{1}_{B}$ are the contact operators on centers A and B respectively. Consideration of the hydrogen atom perturbed by the contact term leads us to include the basis functions $Os\sim\frac{1}{\tau}{e}^{-{r}}$ and In S $\sim$ (In r) $e^{-{r}}$ along with the usual Slater orbitals in the basis set for the first-order orbitals. We obtain an approximate $\Phi^{1}_{A}$ by minimizing the self-coupling energy, ${J}_{AA}$. The form of the contact interaction taken from the Dirac equation is used in place of a delta function to keep ${J}_{AA}$ finite. Results for HD and (optimistically) LiH will be reported.