Crystal field effects in nuclear quadrupole resonance

Abstract

Listen

Translate article

A quantitative investigation of the crystal field effect is quite difficult. From the large number of NQR measurements available, the following qualitative conclusions can be drawn. a) The crystal field effect is of the order of δv (35Cl) = ±500 kHz in Cl atoms bound to carbon. b) With increasing ionic character of a bond Cl-X, the crystal field effects become more pronounced, because the multipole moments of the molecules in the lattice and the quadrupole polarizability of the core electrons of the 35Cl nucleus increase. This second point is equivalent to an increasing antishielding factor γ∞ for Cl. c) Quantitative calculations of the charge transfer in molecular compounds or of the ionic character induced within a molecule through the formation of the complex are only possible if the crystal structure is known and single-crystal NQR data (including η) are available. From NQR powder data, only qualitative conclusions can be drawn. d) The temperature dependence of NQR frequencies should be investigated carefully to correct for the crystal field effect as accurately as possible. e) For other nuclei besides 35Cl, and 79,81Br, such as 127I, 14N,..., more experimental evidence is needed to clarify the connection between NQR and the specific interactions between molecules in the solid state and in molecular addition compounds.