The interaction between the nuclear spin of hydrogen and a host metal like palladium is sensitive to “physics beyond the standard model.”
IOW – it is not well understood. However, it may be a good time to assemble the main features of spin coupling which leads to facilitation of fusion. In the article below, the authors present a variational approach and calculate the constant J in the hydrogen molecule with controlled numerical precision, using the adiabatic approximation. This study supposedly improves the reliability of the NMR theory for searching new physics in the spin-spin coupling. But it gets much harder to characterize spin coupling with heavy metal hydrides. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.083001 The constant J (and J-coupling) is not known to be relevant to a high energy reaction such as to facilitate nuclear fusion – but there must be more than electron chemistry involved to overcome the Coulomb barrier. Some isotopes however, have very high intrinsic nuclear spin and palladium has one such isotope. The standard model would need to be altered in order to find a way for spin coupling to overcome the Coulomb barrier – but that may happen easily, especially in the context of deuterium which is the only isotope with an overwhelming dipole bifurcation as it approaches a target nucleus. In addition to J-coupling we have Magic angles, a Nuclear Overhauser effect and Magnetic moment to deal with. https://en.wikipedia.org/wiki/J-coupling There is a distinct likelihood that the active isotope of cold fusion in a palladium lattice has been identified by the recent analysis by Biberian of a P&F cathode from the French experiments - which produced a large amount of thermal gain 20 years ago. This is the palladium-105 isotope which converts to silver-107. Aside from that identification – the exact mechanism of the reaction is not known, nor is it known if helium ash is produced in this transmutation, or if it is – how much energy it represents. Nor is it certain that there is only one type of fusion reaction in cold fusion. There could be another distinct reaction, but as of now - the hard proof of transmutation only exists for the high spin isotope – 105Pd. However, almost certainly this identification of the active isotope serves to eliminate the hypothesis that the amount of helium produced correlates exactly with an energy gain in the range of 24 MeV per fusion reaction. At best, the gain would be less per fusion and the helium derives from lithium-6 fusing with palladium 105… which seems unlikely to be the prime reaction. More likely - for those who favor the “two step” methodology of Mills/Holmlid etc. or the binuclear atom of Accomazi - the proposed route is for UDD (or the di-deuterino or the binuclear atom) to approach the 105Pd nucleus as a neutral species, from whence the spin coupling results in additional range of strong force attraction so that we end up with a transmuted nucleus - 107Ag as a result plus a free deuteron, which can thermalize without the high energy gamma via the intrinsic spin mechanism.