Interpair Nuclear Magnetic Relaxation in Hydrated Crystals

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Abstract
A set of pulsed rf spin‐lattice relaxation time measurements has been made on the proton system in CaSO4·2H2O. The measurements were performed at several crystal orientations, and over a temperature range from 160° to 365°K. A more limited set of measurements was performed on the proton and Li7 systems in Li2SO4·H2O powder. In both systems there is a strong relaxation mechanism arising from dipolar motion. It appears that this relaxation arises from 180° flips of the water molecules. The T1 data gives a motional activation energy of 6.2±0.5 kcal/mole in CaSO4·2H2O, a reasonable energy for the two hydrogen bonds which must be broken in the postulated motion. Steady‐power absorption line measurements show essentially no narrowing of the proton resonance, in spite of the fact that the characteristic motional frequency reaches the Larmor frequency, 9.0 Mc/sec, at about 230°K. Interpair, rather than intrapair, interactions are responsible for the relaxation. A simplified calculation using only two interacting pairs in CaSO4·2H2O indicates that the observed minimum value of T1 and the lack of appreciable motional narrowing is reasonably accounted for by the proposed mechanism. A similar calculation for the Li7 system in Li2SO4·H2O also yields satisfactory agreement with experiment. The results presented demonstrate the importance of an examination of details of internuclear interactions in a particular crystal in the interpretation of motional effects in magnetic resonance experiments.