The structural locations of hydrogen in minerals and their artificial analogues are not readily obtainable by x-ray diffraction. This important structural feature, however, is particularly well adapted to investigation by a method based upon the phenomenon of nuclear magnetic resonance (nmr). Such an investigation was carried out on Ca(OH)2, a representative trioctahedral layer hydroxide, for which the proton magnetic resonance absorption was observed in crystal powders at liquid nitrogen and at room temperatures.
The broadening of the nmr absorption line by interactions among the magnetic nuclei can be measured as the second moment, ΔH22. A particular structural model can be tested by calculating the second moment for it and comparing the result with the experimental value. Measurement of the second moment for a crystal powder enables one structural parameter to be determined. In our study, the Bernal-Megaw model is assumed for the position of hydrogen in trioctahedral layer hydroxides; the parameter dealt with is a separation distance p which determines the amount of pucker in the hexagonal nets of hydrogen nuclei.
The experimental values obtained for the second moment are 12.4±0.3 and 10.6±0.7 gauss2 at liquid nitrogen and room temperatures, respectively. The hydrogen positions obtained from the low-temperature data are given by p = 0.62±0.04 Å, which corresponds to zH = 0.437±0.004 and an O-H distance of 0.99±0.02 Å. The indicated errors are 95 per cent confidence limits. The smaller value of the second moment found at room temperature is due to the increased amplitudes of the proton motions and probably in part to a decrease in the effective O-H distance. In a brief discussion, these results are compared with related x-ray, neutron diffraction and infrared studies of Ca(OH)2 and an nmr study of brucite.