Study of water dynamics and distances in paramagnetic solids by variable-temperature two-dimensional ~2H NMR spectroscopy

摘要

A recently proposed two-dimensional ~2H NMR experiment is used to measure the H(spin 7=1)quadrupolar and paramagnetic shift anisotropy interactions in powdered CuCl_2 centre dot 2D_2O as a function of temperature.The principal components of the quadrupolar and paramagnetic shift anisotropy,tensors and the Euler angles describing the orientations of the tensors in the molecular frame are determined at each temperature.For this purpose an analytical approach is introduced to extract desired parameters from motionally averaged two-dimensional line shapes where the averaging is introduced by rapid 180° flips around C_2 axes of D_2O molecules.This approach can be readily applied to study various materials containing water of crystallization.It is also clearly shown that the rapid continuous rotation of D_2O molecules around their C_2 axes is not taking place in the studied solid in the range of temperatures between 209 and 344 K.Once the paramagnetic shift anisotropy of a deuterium atom is measured accurately it is used to estimate the distance between deuterium and the nearest copper atom bearing an unpaired electron.Excellent agreement is found between structural parameters obtained in this study and those provided by neutron and x-ray diffraction,showing that the paramagnetic shift anisotropy is a sensitive probe of distances in paramagnetic solids.