Nuclear quadrupole coupling constant measurements

Nuclear Quadrupole Coupling constants measure the,electric field gradient tensor at a nucleus in a molecule. The field gradients in free molecules are determined by the electronic structure, but, in the solid state, are affected by environmental factorsj hence their application to the study of inclusion complexes. 

S nuclear quadrupole coupling constants have been determined from line width values in some 3- and 4-substituted sodium benzenesulphonates33 63 and in 2-substituted sodium ethanesulphonates.35 Reasonably, in sulphonates R — SO3, (i) t] is near zero due to the tetrahedral symmetry of the electronic distribution at the 33S nucleus, and (ii) qzz is the component of the electric field gradient along the C-S axis. In the benzenesulphonate anion, the correlation time has been obtained from 13C spin-lattice relaxation time and NOE measurements. In substituted benzenesulphonates, it has been obtained by the Debye-Stokes-Einstein relationship, corrected by an empirically determined microviscosity factor. In 2-substituted ethanesulphonates, the molecular correlation time of the sphere having a volume equal to the molecular volume has been considered. 

NMR shieldings at the oxygen can also be obtained for mineral systems and are calculated to show the same angular trends as for silicon (Tossell and Lazzeretti, 1988a). However, O is a quadrupolar nuclide (i.e., with a nonzero quadrupole moment), and its nuclear quadrupole coupling constant is thus an easier to measure quantity and, probably, one of more interest. When few data on the O NMR of silicates existed, Janes and Oldfield (1986) noted that different bonding models for silicates predicted different dependence of q° upon Si-O-Si angle. In particular. 

Ab initio calculations of the nuclear quadrupole coupling constant suggest that the cyanate nitrogen atom is more ionic than that of the thiocyanate ion (118), but measurements show the latter to be more ionic than that in the selenocyanate ion (397). 

Relaxation and self-diffusion techniques in solution are widely used to study hydrogen-bonded systems. The nuclear quadrupole coupling constant (NQCC) presents a sensitive probe for the strength of hydrogen bonding. Unfortunately in the liquid phase this property cannot be measured in a direct way. Two new indirect methods are now presented for determining NQCC in H-bonded liquids. Ferris and Farrar showed that for the OD deuteron of ethanol the DQCC is related to the chemical shift of the hydroxy proton by a linear equation. Thus the straightforward measurement of the chemical shift... 

Nuclear quadrupole coupling constants have been measured for copper in six square-planar complexes, five with sulphur donors and the other with S,0 donors. ° The preparation of Cu(thiaz)X2 (X = Cl or Br), Cu(thiaz)2X2 (X = Cl or Br) and Co(thiaz)2X2 [X = Cl, Br, or I thiaz = 5-(2-hydroxyethyl)-4-methylthiazole] have been isolated and generally appear to contain MX " ions. An e.p.r. study of bis-(l,l-diethyl-3-benzoyl-selenoureato)copper(ii) (145) has been reported. ... 

The generalised polarisability describing the response to an applied field of the electric field gradient at a nucleus in Br2 is calculated ab initio. A value of 110.55 Oq 1 is found at the self-consistent -field level. This is about half the value derived by modelling the measured nuclear quadrupole coupling constants of the ammonia-bromine complex. 

Response properties have central roles in the theories describing intermolec-ular forces and the behaviour of molecules in external fields. It was recently demon8trated(l) that one such property which can be calculated ab initio is also accessible from the measured nuclear quadrupole coupling constants... 

The nuclear quadrupole interaction often produces measurable effects in electronic, rotational, atomic and molecular beam resonance, nuclear magnetic resonance, and MOssbauer spectra. In many cases it can be directly studied by nuclear quadrupole resonance (NQR). The magnitude of interaction is expressed by the nuclear quadrupole coupling constant, e2qQ/hy which is the product of Eq. (1) and (3) expressed as a frequency. 

However, because of the difficulty in measuring the correlation time, the quadrupole coupling constant determined from in the liquid state is not always reliable even when 17 is truly negligible. In special instances, r can be determined from Raman line shapes26 or the 7g of another quadrupole nucleus in the same molecule whose nuclear quadrupole coupling constant is accurately known.24... 

The 2 2-component of the EFG operator q is now to be determined by electronic structure theory either from relativistic or nonrelativis-tic wave functions. The expression e Qqzz in eq- (17) is the nuclear quadrupole coupling constant (NQCC) and can be obtained by experiment leading to the sought quadrupole moment eQ. Due to its dependence the EFG operator especially stresses the core region of the electronic wave function and relativity can be expected to play a major role. For the adequate treatment of heavy atoms and molecules containing heavy elements relativity is therefore indispensable and we will mention nonrelativistic results only for comparative purposes. Before we treat the subject of relativistic qzz calculations in detail a few common experimental techniques for accurate NQCC measurements are briefly discussed and the underlying physical principles mentioned. 

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