Nuclear Quadrupole Resonance (NQR) Spectroscopy

Abstract

AbstractThe article concerns the usefulness of nuclear quadrupole resonance (NQR) spectroscopy in inorganic and organometallic chemistry. NQR‐active nuclei have spinI ≥ 1; those of high enough abundance to be commonly useful are found in the odd‐numbered groups of the periodic table. These are tabulated and examples given for each. The number of NQR frequencies observed in a solid‐state compound reflects the number of chemically different quadrupolar atoms in the compound and also the number of crystallographically inequivalent atoms; changes in the latter may indicate phase transitions or different polymorphs of a given compound. More commonly, one uses average NQR frequencies for chemically different atoms, and from these average frequencies, derives atomic parameters such as populations of valence orbitals, partial ionic charges, π‐bond orders, and covalent bond orders of the quadrupolar atom. The article mainly reviews selected applications of halogen (35Cl,79, 81Br,127I) NQR to specific bonding questions in inorganic chemistry, and divides these according to whether the halogen or other atoms are (a) single bonded, (b) single bonded but with secondary bonds with other atoms, or (c) bonded to several atoms. For singly bonded atoms, the applications reviewed include (a1) relativistic effects, (a2) effects of nearby ionic charges on haloacetate ions, (a3) the contrasting effects of nearby ionic charges on halometallate ions, (a4) effects of intramolecular partial charges on chlorines in organosilicon compounds, (a5) cis‐and trans‐ ligand effects in substituted halometallate complexes, and (a6) interpretations of multicentered bonding in trihalide ions. Secondary bonding of single‐bonded halogen atoms are reviewed particularly in the cases of (b1) halocarbons as ligands, (b2) chloroacetates and chlorophenolates as chlorine‐donor ligands, and (b3) in salts of weakly coordinated halogenated anions. Finally, some illustrations are given of NQR studies of central multicoordinated atoms (for example, transition‐metal atoms in complexes).