The interpretation of NQR-perturbed NMR rotation patterns with a general rotation axis using crystal symmetry considerations

Abstract

A method is developed to determine quadrupole coupling tensors from a single rotation pattern of satellite transitions if there are chemically equivalent but physically inequivalent sites in the structure. For tensors in a general orientation with respect to the crystal axes the problem consists of the determination of five parameters for the tensor and of two parameters for the orientation of the rotation axis in crystal coordinates. In the rotation pattern each physically inequivalent site is found with its individual orientational dependence of the NMR-NQR transitions. From each site three coefficients are obtained. With an n-fold axis relating the different sites ( n = 3, 4, 6) a single rotation pattern yields enough information to determine the seven wanted parameters. With a two-fold axis there is not sufficient information contained in a single rotation pattern unless the rotation axis is known in crystal coordinates or the tensor is not in a general orientation with respect to the crystal axes due to its location on a symmetry element of the structure. In such a case three parameters are sufficient to describe the tensor in crystal coordinates and if there are still physically inequivalent sites present then a single rotation pattern contains all the information even with a two-fold axis. The method is applied to a phase transition study of [(CH 3) 4N] 2CUCl 4, by means of 14N NMR-NQR rotation patterns.