What determines the sign of the Fermi-contact contribution to the NMR spin–spin coupling constant?

The Dirac vector model for the sign of the spin–spin coupling constant (J) between a pair of atoms states that one-bond coupling constants are positive, two-bond negative, three-bond positive, etc. However, this rule is often violated. In an attempt to understand the sign of J, we propose a new model, the Nuclear magnetic resonance triplet wavefunction model (NMRTWM) which is based on the phases of excited triplet state wavefunctions, and the response of magnetic nuclei to these phases. The model demonstrates that, (1) the excited states which contribute to the coupling constant do so in a regular manner; (2) the sign of an individual contribution is determined by the nodal character of the excited-state wavefunction and the response of the nuclei to the phases of this function; and (3) the sign of the total coupling constant is the result of competing positive and negative contributions from various states. NMRTWM provides a fundamental explanation for both the successes and failures of the Dirac vector model, and can be used to gain insight into some puzzling results for the orientation dependence of F–F coupling between two HF molecules.