Quadrupole coupling constant nuclei

Dechter J J, Henriksson U, Kowalewski J and Nilsson A-C 1982 Metal nucleus quadrupole coupling constants in aluminum, gallium and indium acetylacetonates J. Magn. Reson. 48 503-11... 

The experimentally observed quadrupole splitting AEq for Fe in inorganic compounds, metals, and solids reaches from 0 to more than 6 mm s [30, 32]. The range of AEq for other Mossbauer isotopes may be completely different because of the different nuclear quadrupole moment Q of the respective Mossbauer nucleus, and also because the EFG values may be intrinsically different due to markedly different radial distributions of the atomic orbitals (vide infra). As Q is constant for a given isotope, variations in the quadrupole coupling constants eQV can only arise from... 

One method of determining nuclear quadrupole moment Q is by measuring the quadrupole coupling constant, given by eqQ/h, where e is the charge of the electron and q the electric field gradient due to the electrons at the atomic nucleus. The extraction of Q depends on an accurately calculated q. As a test of our finite-field relativistic coupled cluster approach, preliminary results for Cl, Br, and I are presented. 

Numerous X-ray investigations have unravelled the solid state structure of contact and solvent-separated ion pairs. It was therefore considered to be of interest to evaluate also the potential of solid state NMR as a tool for the investigation of this structural problem. In addition to the study of chemical shifts discussed above (Section II.B), the quadrupole coupling constant of the nuclide Li, x( Li), was expected to be an ideal sensor for the bonding situation around the lithium cation because, due to its dependence on the electric field gradient, the quadrupolar interaction for this spin-3/2 nucleus is strongly influenced by local symmetry, as exemplified in Section II.C.3. This is also shown with some model calculations in Section ILF. 

While the nuclei 3H and 13C relax predominantly by the DD mechanism, relaxation of a quadrupole nucleus such as deuterium essentially involves fluctuating fields arising from interaction between the quadrupole moment and the electrical field gradient at the quadrupole nucleus [16]. If the molecular motion is sufficiently fast (decreasing branch of the correlation function, Fig. 3.20), the 2H spin-lattice relaxation time is inversely proportional to the square of the quadrupole coupling constant e2q Q/H of deuterium and the effective correlation time [16] ... 

While S relaxation behaviour can provide interesting information about interactions in solution, nuclear quadrupole coupling constants are a very sensitive probe in the study of the electronic distribution around the sulphur nucleus. 

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. 

In these studies, the parameters that could provide the most interesting information are likely to be the electric field gradient (nuclear quadrupole coupling constant) at the 33S nucleus and its asymmetry parameter. Indeed, modifications of the lattice structure in different cement matrixes are expected to influence the symmetry of the electronic distribution around the sulphur nucleus more than the chemical environment of sulphur. 

Quantum mechanical calculations of 33S nuclear quadrupole coupling constants are not an easy matter (not only for the 33S nucleus, but for all quadrupolar nuclei). Indeed, the electric field gradient is a typical core property, and it is difficult to find wave functions correctly describing the electronic distribution in close proximity to the nucleus. Moreover, in the case of 33S, the real importance of the Sternheimer shielding contribution has not been completely assessed, and in any case the Sternheimer effect is difficult to calculate. 

Whereas the majority of the studies of NQR in the first decades after its discovery were devoted to what we may call the molecular applications of quadrupole coupling constants, i.e. the systematics of the variation in the coupling constant of a particular nucleus as a... 

The quadrupole coupling constant is proportional to the electric-field gradient, eq, which measures the asymmetry of the electron density surrounding the nucleus. Since the core electrons and the valence-shell s electrons are spherically distributed, they contribute... 

Table 1-11. The dipole moment (in D) and nuclear quadrupole coupling constants (in MHz) for the chlorine nucleus of the (H20)2HC1 cluster...

The main part of the electric field gradient is determined by the chemical bond. For 35C1 NQR in organic molecules, this would be the C—Cl bond. No temperature influence on this bond is assumed. The main influence of temperature on the quadrupole coupling constant, therefore, is due to the fast motion of the molecule considered, and to the motions of certain groups of the molecule. A certain part of the temperature dependence of the NQR frequencies is due the motion of the nucleus within the electric field gradient created at the site of the nucleus. 

The strong vibrational dependence of the quadrupole coupling constant eqo Q is consistent with this very simple picture. The electric field gradient qo must be zero at the nucleus for the isolated N atom. The approach of the N2 molecule towards the dissociation limit, with its increasing intemuclear distance as v increases, must therefore result in decreasing values of the electric field gradient. 

Absolute value of the electronic charge Electric quadrupole moment of a nucleus Atomic nuclear quadrupole coupling constant Nuclear quadrupole coupling constant Ionic character of a bond Nuclear spin... 

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