Anisotropy of the nuclear spin–spin coupling tensor in water, ammonia, and methane molecules

The anisotropy of the nuclear spin–spin coupling tensor relative to H–H and X–H (X = O, N, C) interactions is investigated through extended ab initio calculations on the molecules H2O, NH3, and CH4. Spin–orbit, spin–dipolar, and Fermi contact-spin–dipolar cross terms are accounted for in the coupled Hartree–Fock perturbative scheme. The major role played by the Fermi contact-spin–dipolar cross term is ascertained. A large discrepancy emerges between theoretical and experimental anisotropy for the CH coupling, which evidences the need for further careful analyses of the measured data.