Interpair dipolar interactions in solids containing spin-½ pairs

Recent experimental work in powdered crystalline hydrates has shown that the interpair second moment of the proton N.M.R. absorption spectrum is ⅚ of the value given by the van Vleck expression. Using a model of two planar spin-½ pairs, this reduction factor of 5/6 is shown to originate in the quenching of non-secular terms in the interpair dipolar hamiltonian which give rise to weak, unobservable resonances far into the wings of the spectrum. This effect is analogous in origin to the well-known reduction factor of 4/9 in the second moment due to the interaction between resonant and non-resonant spins.