Paper 5.5: "Interactions Between Nuclear Spins in Molecules," N. F. Ramsey and E. M. Purcell, Phys. Rev. 85, 143 (1952)

Most of my theoretical papers on nuclear interactions in molecules originated from our own experiments. However, some, like Paper 5.5, were to interpret puzzling observations by others. In 1951 Gutowsky, McCall, Slichter, Hahn and Maxwell discovered splittings of NMR resonance lines in molecules that are independent of the strength of the external magnetic field, in contrast to the usual NMR splittings that are due to magnetic shielding. They pointed out that their observations could be interpreted as the effect of an interaction between two nuclei with spins I_N and I_N', of the form h δ I_N · I_N', but no one could invent a theoretical mechanism for making the interaction constant d as large as observed. Purcell and I (Paper 5.5) proposed a new electron-coupled nuclear spin–spin interaction. The spin of one nucleus would interact magnetically with the spin of a nearby electron, which would affect the spin of another distant electron in a ¹Σ diatomic molecule because the total electron spin has to be zero. This distant electron would then interact magnetically with a nearby nuclear spin to give a net nuclear spin–spin interaction of the required form. This mechanism produces a much bigger effect at long distances than do direct magnetic interactions since the electron spins are exchange-coupled. We calculated this effect for HD and found a sufficiently large coupling constant to justify extending this interpretation to the reported experiments. We also suggested that the interaction be measured for HD to provide a better comparison of experiment to theory.

Measurements on HD were soon made, so I developed the theory further [Phys. Rev. 91, 303–307 (1953)] to show that, if the mean energy of the molecular excited states is given the reasonable value 1.4 Rydbergs, the electron-coupled spin interaction constant would be +43 Hz, in agreement with its measured magnitude [there is a minor misprint in my second theoretical paper and the sign of the last term in square brackets of Eq. (23) should be minus, with a corresponding change in Eq. (24)]. The theory has been confirmed by NMR experiments and by HD molecular beam resonance experiments [R. F. Code and N. F. Ramsey, Phys. Rev. A4, 1945–1959 (1971)]. As discussed in that paper, the latest theoretical calculation of δ is 42.57 Hz, in reasonable agreement with the latest NMR measurement of + 42.94 ± 0.10 Hz.

Subsequent to our original papers, electron-coupled spin interactions have been found to be important in many studies of both molecules and solids.

As a result of this work on electron-coupled nuclear spin–spin interactions, I wrote a report [Phys. Rev. 89, 527 (1953)] pointing out that such interactions would make dominant contributions to calculations of pseudoquadrupole effects for nuclei in molecules.