Electric field gradient geometry

Electric Field Gradient, fully optimized geometry. 

Electric Field Gradient, frozen monomer geometry. 

Electric Field Gradient, frozen monomer geometry. Electric Field Gradient, fully optimized geometry. 

Table 3.21 Changes of electric field gradient and dipole moment caused by H-bonding (atomic units) calculated at experimental geometries with a [642/531/31] basis set for [C1,P/N/H]. ...

Most separated local field experiments concentrate on the CSA. In speetra of halfinteger nuclei with / > 1/2, the CSA is often much less important than the second-order quadrupole interaction. Static or MAS spectra display a typical quadrupole powder pattern from which the quadrupole coupling constant and the asymmetry parameter can be determined. These quadrupole parameters can then be used to determine the principal components of the electric field gradient tensor, which are related to the local environment of the quadrupole nucleus. Knowledge of the local geometry is extremely valuable, and correlation of the quadrupole and dipole information of OH groups provides useful insights into structure. 

Fig. 3.1.1 Anisotropic spin interactions, (a) Quadrupole coupling of nucleus to the electric field gradient of a C- H bond, (b) Dipole-dipole coupling between C and H. (c) Anisotropic magnetic shielding of C nuclei. Left Geometry of the interaction and principal axes of the coupling tensor. Middle NMR spectrum for a single molecular orientation. Right The average over all orientations is the powder spectrum. The parameters A>, denote the anisotropy of the interaction k. S is the chemical shift. Adapted from [Blii3] with permission from Wiley-VCH.

The dependence of the dipolar couplings on the orientation of the internuclear vector in the principle axis system of the electric field gradient has Inherent information about the molecular geometry of the relevant and sites. For example, in a peptide bond, where two carbons are bound to the same nitrogen, the dipolar coupling of each depends on the orientation of its respective internuclear vector with respect to the common coordinate system of the nitrogen electric field gradient principle axis system. Thus, the NMR spectrum is analyzed in terms of the relative orientation of the internuclear vectors, In principle, this can lead to an independent determination of the conformation of the peptide bond. 

Ab initio SCF-MO calculations have been performed on BFg, and alterations in predicted geometries with different gaussian-orbital basis sets noted. Further non-empirical SCF calculations on this molecule have been reported by Rothenberg and Schaeffer. Values of the dipole moment, quadrupole moment, octupole moment, second and third moments of the electronic charge distribution, diamagnetic susceptibility, and electric field gradient were calculated. 

In AC and DC dielectrophoresis, the dielectrophoretic force acting on the polarized particle causes it to move either up or down the induced electric field gradient, which is created by (a) nonuniform electrode geometry in AC fields or by (b) a nonuniform insulator geometry of obstacles in an otherwise uniform DC field or DC-offset AC field. This spatially nonuniform field or gradient dotted with the polarized particle dipole yields a net dielectric force ... 

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