Fine structure has been observed in the nuclear paramagnetic resonance absorption line for protons in crystalline hydrates. The magnetic field of 6820 gauss was provided by a permanent magnet, the inherent stability of which facilitated detailed study of line shape. Measurements on a single crystal of CaSO4·2H2O show a splitting into four component lines with maximum separation varying from zero to 22 gauss, depending upon the direction of the externally applied magnetic field in the crystal. Both the number of component lines and the dependence of their spacing on field direction are calculated by treating the magnetic dipole-dipole interaction as a perturbation of the proton two-spin system within the water molecule; the effect of the more distant protons, neglected in this calculation, gives a finite width to the component lines. Variation of the splitting with field direction determines the orientation of the line joining protons in the water molecule, which is found to be consistent with positions ascribed to hydrogen nuclei in the lattice through simple considerations of chemical bonding. The distance between protons in the water molecule is measured by the splitting to be 1.58A for CaSO4·2H2O; if one assumes an H–O–H bond angle of 108°, the O–H distance is 0.98A. Powdered hydrates show a characteristic fine structure arising from isotropic distribution in solid angle of single crystal granules. This type of fine structure determines the proton-proton distance somewhat less accurately than does the single crystal experiment.

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The experimental absorption line widths, for nuclei with spin 1/2, at nuclear magnetic resonance are given as a function of temperature for a number of molecular crystals. Temperatures ranged from 90°K to the melting points of the compounds. In some cases it has been possible to relate observed line structure and transitions in the line width to the existence and frequency of certain types of hindered rotational motion in the solid state. These deductions are based on mathematical considerations of the quantitative effect of such motions on the structure and second moment of an absorption line. It is emphasized that relatively low frequency motion of the order 105 cycles/second suffices to narrow the width of an absorption line from its value in the absence of that motion. 1,2-dichloroethane, 1,1,1-trichloroethane, and perfluoroethane were found to have line-width transitions coinciding with changes in crystal form and anomalies in the heat capacity. In 1,2-dichloroethane and perfluoroethane these transitions correspond to rotational motion about the long axis of the molecule. 1,1,1-trichloroethane has a line-width transition corresponding to this type of motion, but the heat capacity anomaly and change in crystal form coincides with a further small decrease in the line width to that characteristic of the liquid. A number of molecules including acetonitrile, methyl iodide, nitromethane, dimethyl mercury, and ammonia have absorption lines at 90°K corresponding to molecular rotation about the C3 figure axis. Various other data and interpretations are presented for 2,2-dimethyl propane, methanol, ethanol, acetone, methylamine, and the ethyl halides. The possibility of estimating the potential barriers hindering rotation in solids from the line-width transitions is discussed.

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The proton magnetic resonance absorption has been measured in NH4Cl, NH4Br, and NH4I crystal powders from —195°C to room temperature, and for NH4Cl to 200°C. Line width transitions were found at —144, —171, and about —198°C in the chloride, bromide, and iodide, respectively. The N–H distance in the ammonium ion was determined to be 1.035±0.01A from the second moments of the broad, low-temperature absorption lines observed in NH4Cl and NH4Br, with allowance for broadening by modulation effects and narrowing by zero-point torsional oscillations. The relatively narrow room temperature line shapes are consistent with the broadening estimated for inter-NH4+ interactions, the intra-NH4+ interactions averaging to zero over the hindered rotational motions of the NH4+ ions. No appreciable line shape changes were found at the λ temperatures. An electrostatic calculation is made of the potential barriers to rotation of the NH4+ ion giving values comparing favorably with experimental inferences. The results clarify the motions which narrow the absorption line at low temperatures and emphasize the dynamic aspects of the order-disorder process.