Electric field gradient correlation

Eckart, criteria, 264, 298 procedure, 267 Effective charge, 274, 276 Effective Hamiltonian, 226 Elastic model, excess entropy calculation from, 141 of a solid solution, 140 Electric correlation, 248 Electric field gradient, 188, 189 Electron (s), 200... 

Solid state 2H NMR parameters are almost exclusively governed by the quadrupole interaction with the electric field gradient (EFG) tensor at the deuteron site.1 8 The EFG is entirely intramolecular in nature. Thus molecular order and mobility are monitored through the orientation of individual C-2H bond directions. Therefore, 2H NMR is a powerful technique for studying local molecular motions. It enables us to discriminate different types of motions and their correlation times over a wide frequency range. Dynamics of numerous polymers has been examined by solid state 2H NMR.1 3,7,9 Dynamic information on polypeptides by NMR is however limited,10 26 although the main-chain secondary structures of polypeptides in the solid have been extensively evaluated by 13C and 15N CP/MAS NMR.27,28... 

The relativistic coupled cluster method starts from the four-component solutions of the Drrac-Fock or Dirac-Fock-Breit equations, and correlates them by the coupled-cluster approach. The Fock-space coupled-cluster method yields atomic transition energies in good agreement (usually better than 0.1 eV) with known experimental values. This is demonstrated here by the electron affinities of group-13 atoms. Properties of superheavy atoms which are not known experimentally can be predicted. Here we show that the rare gas eka-radon (element 118) will have a positive electron affinity. One-, two-, and four-components methods are described and applied to several states of CdH and its ions. Methods for calculating properties other than energy are discussed, and the electric field gradients of Cl, Br, and I, required to extract nuclear quadrupoles from experimental data, are calculated. 

Quadrupole splitting (A q) correlates to electric field gradient and, based on model compounds, can identify some ligand types Can observe changes in ligand field induced by sample perturbation Can only detect iron sites Magnetic circular dichroism (MCD) spectra Require low temperature to observe (—2-70 K)... 

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] ... 

Although 1 is one of the best investigated molecules, there is, apart from data concerning its electron density distribution, very little information available on its one-electron properties. In principle, accurate data could be obtained by correlation-corrected ab initio methods, but almost nothing has been done in this direction, which of course has to do with the fact that experimental data on one-electron properties of 1 are also rare, and therefore, it is difficult to assess the accuracy and usefulness of calculated one-electron properties such as higher multipole moments, electric field gradients, etc. 

Yi and Ys - gyromagnetic ratio of spin 1 and spin S nuclear spin, rJS = intemuclear distance, tr= rotational correlation time, x< = reorientation correlation time, xj = angular momentum correlation time, Cs = concentration of spin S, Cq = e2qzzQ/h = quadrupole coupling constant, qzz = the electric field gradient, Q = nuclear electric quadrupole moment in 10 24 cm2, Ceff = effective spin-rotational coupling constant, a = closest distance of appropriate of spin 1 and spin S, D = (DA+DB)/2 = mutual translational self diffusion coefficient of the molecules containing I and S, Ij = moment of inertia of the molecule, Ao = a// - ol-... 

The determination of correlation times requires particular attention and is not a problem that can be easily resolved. It is necessary to consider not only the overall molecular tumbling, but also internal rotation motions, which can modulate the interaction between the nuclear quadrupole moment and the electric field gradient. In any case, it must be considered that % depends on the square root of and, even if in most cases it is not possible to measure or calculate precise values of t , the uncertainties introduced in the values of x may often be accepted. 

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. 

Hexamethylenetetramine has been extensively studied in this respect30 , and it has been shown that for temperatures ranging above room temperature, reorientational motions are responsible for the strong relaxation. More exactly, hindered rotations interchange the tetrahedron apexes and the nuclei are thus subjected to a sudden reorientation of the electric field gradient. The correlation time of such motions can be derived from the observed signals and experimental results may be depicted by the relation... 

O. Methods of various degrees of sophistication, for the V electron density at the nitrogen atom. Taking the ft population as constant, there is a good correlation between calculated and observed a values 70>. To extend 71> 72> the calculation to asymmetric molecules, like pyrimidine or pyridazine, the new orientation of the electric field gradient tensor must be considered. In pyridazine the two nitrogen Oz axes are rotated in a way that brings them nearer parallel and, from microwave measurements, this rotation was estimated at 9° 73 ... 

CW = continuous-wave CW-NQR = continuous wave NQR DFT = density fiuictional theory EFG = electric field gradient IR = infrared NMR = nuclear magnetic resonance NQR = nuclear quadmpole resonance OSSE = octahedral site stabilization energy PAC = perturbed angular correlation pulse-FT = pulse-fourier transform TMED = tetramethylethylenediamine. 

Hyperfine interactions, that is the interaction of a nuclear magnetic moment with extranuclear magnetic fields and the interaction of a nuclear quadrupole moment with electric field gradients from extranuclear charge distributions, are measured via time differential perturbed angular correlation (TDPAC) of y-rays emitted from radioisotopes. 

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