The determination of hydrogen coordinates in lanthanum nicotinate dihydrate crystals by Gd+3–proton double resonance

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Abstract
We have given a description of the EPR spectrum of Gd+3 located at a crystal site with no spatial symmetry by means of a spin Hamiltonian that fits our 833 measurements of ‖B0‖ required for resonance, with B0 in various directions in three mutually perpendicular crystal planes, with a root mean square deviation 11.4 G. We have shown that by use of the EPR absorption by Gd+3 at this site ENDOR spectra that provide values of proton coordinates for the Gd+3 water ligands can be obtained. By adjustment of coordinate values in a simplified model of the physics of the Gd+3–proton interaction, to give a fit to a simple six‐parameter spin Hamiltonian that we used to describe our ENDOR data, we obtained the proton coordinates in a crystal fixed axis system with origin at the Gd+3 site. These coordinates had standard deviations in the range 0.051–0.099 Å and gave values of the Gd+3–proton separation distances with standard deviations in the range from 0.044 to 0.060 Å. We have made comparisons of values of Ln+3–proton and proton–proton separation distances obtained by this method for the present Gd+3 case with corresponding values for the previously studied Nd+3 case, and with x‐ray data for the rare earth nicotinates. The comparisons show agreements within the uncertainties of the methods and demonstrate the reliability of the values that have been obtained in the present study. We have also employed a very simple experimental procedure for measuring values of coordinates of protons in the vicinity of Gd+3. In this procedure the ENDOR frequency is measured for only one direction of B0, namely that direction for which the shift of the frequency from the free proton frequency is maximum. This measured frequency, together with the values of the direction cosines of B0 in a crystal fixed axis system with Gd+3 at its origin, gives the values of: (a) the three coordinates of the proton; and of (b) its distance from Gd+3 with estimated standard deviations, 0.10 and 0.01 Å, respectively. The values obtained by this very simple technique for proton coordinates, Gd+3–proton separation distances, and interproton distances in the H2O ligands, are in agreement with their values obtained by the first described method, within the somewhat larger uncertainties associated with the second method. The peculiar merits of the Gd+3 ion for use in magnetic resonance crystallography have been reviewed in this article. The demonstrations that reliable structural information can be obtained in spite of the very considerable complications introduced in the spectra by (a) the fine structure, and by (b) the occurrence of the Gd+3 at a site in the molecule with no spatial symmetry that would simplify interpretation of the spectra, i.e., the type of site at which in general Gd+3 would be bound to a protein, are thus of special significance.