CALCULATION OF NUCLEAR SPIN-SPIN COUPLING-CONSTANTS WITH AB-INITIO MOLECULAR-ORBITAL WAVE-FUNCTIONS

We investigate the use of ab initio molecular wave functions to evaluate the Fermi contact contribution to nuclear spin-spin coupling constants (n)J(AB) (A, B = C, H) for a data set of 19 molecules having well-defined experimental geometry and coupling values. We critically examine the theoretical and numerical problems associated with the Dirac delta function operator and its perturbation-theoretic treatment. We show that the common approximation of neglecting basis orbitals not centered on the coupled nuclei is a major source of error in previous calculations. By using the Pople-Santry sum-over-states expression in conjunction with modest ab initio MO basis sets (i.e., 3-21G, 6-31G*), we find that calculated values of 3J(HH), 2J(HH), and 1J(CC) show high correlation with experiment (r2 congruent-to 0.96-0.99). Calculated (n)J(CH) values are less highly correlated with experiment, probably resulting from neglect of contributions other than the Fermi contact term. On the basis of these comparisons, we conclude that vicinal and geminal proton-proton coupling constants can be estimated with approximately 1-Hz absolute accuracy by applying modest levels of ab initio molecular orbital theory.