Deuteron Magnetic Resonance Study of Cupric Sulfate Pentahydrate

Abstract

A deuteron magnetic resonance study is made of single crystals of CuSO4·5D2O between 130° and 360°K. The electric quadrupole coupling constant, the asymmetry parameter, and the directions of the principal axes of the field-gradient tensor are determined for each deuteron at 133.2° and 294.8°K. At low temperatures ten pairs of lines are observed, corresponding to five nonequivalent stationary water molecules. At temperatures above room temperature, five pairs of lines are observed; these are shown to result from fast molecular reorientation about the bisecting axis of each water molecule. An activation energy for the 180° reorientational motion is derived for each water molecule and is discussed in relation to the lonepair coordination of the oxygen atom of the water molecule and the hydrogen-bond strength. The field-gradient tensor for each stationary water molecule is discussed in relation to the structure of the hydrogen-bond system. For relatively weak asymmetric hydrogen bonds of O–H···O, an empirical equation, eQq/h = 310.0 − 3.0 × 190.6 / R(O···H)3 (kHz), is found to represent the observed values well, where R(O···H) is the O···H distance in angstrom units. For strong hydrogen bonds, there is a significant deviation from the above relation, indicating that the covalent effect is of importance in these cases. Coordination to a paramagnetic ion has no marked effect on the quadrupole coupling constant, but it appears to cause a rotation of the directions of principal x and y axes about the O–H bond. Two possible cases of the principal y-axes directions for the field-gradient tensor of the reorienting water molecule are analyzed in detail, and the available experimental data are classified into two groups. Theoretical formulas for the DMR spectra in the paramagnetic crystals are derived and compared with that in diamagnetic crystals. A local field due to the cupric ions is derived from the observed paramagnetic shift at 133.2°K for each deuteron and is found to be due to the electron nuclear spin dipole coupling. The spectra are observed at temperatures up to the dehydration point, and it is found that the free rotation of the water molecule does not occur in this temperature range.