Nuclear Magnetic Interactions in the HF Molecule

TLDR

Magnetic relaxation times of hydrogen and fluorine in anhydrous hydrofluoric acid cannot be explained by a pure dipole‑dipole interaction between the nuclei. The authors applied this approach to study the structure of the HF molecule in solution. They introduced a scalar AI·S term in the Hamiltonian to account for indirect electron‑coupled interaction and used the nuclear Overhauser effect to separate dipole‑dipole and scalar contributions, enabling calculation of scalar splitting and proton exchange rates, while water exchange renders hydrogen motion fast relative to its Larmor frequency. The scalar splitting was measured as 615 cps; in solution HF exhibits long hydrogen relaxation times (T1≈T2) and short fluorine.

Abstract

Magnetic relaxation times of hydrogen and fluorine in anhydrous hydrofluoric acid cannot be accounted for on the assumption of a pure dipole-dipole interaction between the hydrogen and the fluorine nuclei in the same molecule. However, the introduction, in the Hamiltonian, of a scalar term AI·S resulting from the indirect electron-coupled interaction between the two nuclei removes all the discrepancies between the calculated and observed decay times. Although the splitting due to this scalar term is smeared out by the rapid chemical exchange of the protons with the traces of water present in the acid, the nuclear Overhauser effect provides the extra parameter required to separate the dipole-dipole interaction from the scalar interaction and to calculate separately the scalar splitting and the exchange rate of the protons. The value obtained for the splitting is A/h=615 cps. The same method has been applied to investigate the structure of the HF molecule in solutions. The experimental results can be explained qualitatively by the following picture: (a) The fluorine nuclei form long chains that move more slowly than the Larmor frequency of fluorine. (b) The chemical exchange with the relatively large quantity of water present makes the motion of the hydrogen nuclei fast with respect to the hydrogen Larmor frequency. The results are: Long relaxation times for hydrogen with T1 and T2 equal, relatively short relaxation times for fluorine resonance, with T2 shorter than T1. Solutions of salts of hydrofluoric acid (KF) show the same character as solutions of the acid.