NQR Studies of Inclusion Compounds

Abstract

Nuclear Quadrupole Resonance (NQR) spectroscopy is a branch of radiofrequency spectroscopy. It is, by its very nature, restricted to the study of solids, and is thus welladapted to the study of inclusion complexes. In NQR, transitions are observed between energy levels arising from the interaction between a nuclear electrostatic quadrupole moment and the electric field-gradients produced by the various charged particles, electrons and nuclei, that surround this quadrupolar nucleus. Both the nuclear quadrupole moment and the electric field-gradients are tensor quantities, the interaction between them is also a tensor, the nuclear quadrupole coupling tensor. The frequencies of NQR transitions thus depend on a nuclear property, the nuclear quadrupole moment, on the one hand and, on the other, on a molecular property, the electric field-gradient. They cover a wide range of the radiofrequency spectrum; from around one hundred kilohertz for nuclei with small nuclear quadrupole moments surrounded by very few electrons, for example the deuteron, to as much as one gigahertz for nuclei with large quadrupole moments, such as 127I, immersed in an electron-rich environment. Many different types of spectrometer, using different detection techniques, are therefore necessary to cover the complete range of the NQR spectrum.