Static quadrupole coupling constants

Bersohn 76) has calculated the crystal field created by the molecular dipoles in the lattice of CH3C1. The static dipole moment of the molecules induces through the polarizability of the molecules an additional dipole moment which increases the dipole moment of the free molecule by a factor of about 1.05. This in turn means that the C—Cl bond has increased in ionic character under the influence of the intermolecular electric fields and therefore (see Eq. (II.9 the quadrupole coupling constant will be lower relative to the gaseous state. Besides the dipole moment induced in the direction of the static dipole, a perpendicular partial moment should be induced, too. Therefore the axial symmetry of the C—Cl bond will be disturbed and the asymmetry parameter 77 may become unequal zero. A small asymmetry parameter 17 = 0.028 has been observed for the nuclear quadrupole interaction in solid CH3I. Bersohn also calculated from the known crystal structure of 1,3,5-trichlorobenzene the induced... 

The application of a strong static electric field modifies the quadrupole coupling constant as an effect of the sample polarization. The phenomenon has been reviewed by Lucken s) who mentions some results concerning halogens. 

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. 

Under anisotropic conditions, NMR lineshapes for a quadrupolar nucleus are dominated by chemical shielding and (first and second order) quadrupolar interactions. Dipolar interaction is usually a minor contribution only. First-order quadrupole interaction lifts the degeneracy of the allowed 21 (i.e. seven in the case of V / = V2) Zeeman transitions as shown in Figure 3.7, giving rise to seven equidistant lines, viz. a central line (mj = + V2 -V2. unaffected by quadrupole interaction) and six satellite lines. The overall breadth of the spectrum is determined by the size of the nuclear quadrupole coupling constant Cq the deviations from axial symmetry and hence the shape of the spectral envelope are governed by the asymmetry parameter. Static solid-state NMR thus provides additional parameters, in particular the quadrupole coupling constant, which correlates with the electronic situation in a vanadium compound. [ 1 The central component reflects the anisotropy of the chemical shift. 

Recently, Frydman et al. successfully analysed the Co static powder spectra of a series of cobalamins and cobaloximes measured at 4,7, 7.1 and 11.8 TP As pointed out by the authors, there were considerable changes in the Co quadrupole coupling constants depending on the crystallization conditions of the cobalamins, whereas the Co chemical shift tensors remained fairly constant. It was suggested that the different behaviour of the EFG and chemical shift tensors is due to a different dependence of these tensors on an uncharacterized structural rearrangement when the cobalamins were recrystallized. Furthermore, the close resemblance between the Co NMR data of cobalamins and cobaloximes illustrates that cobaloximes are much better model compounds of cobalamins than cobaltoporphyrins. In a study of polyammonium cobalticyanide supercomplexes, Zhou et al. combined the use... 

Because the alkali metal nuclei have quadrupole moments, the lines are considerably broader than those of nuclei with spin 1/2. The effect is small for Li and and only moderate for Na. With Rb, and Rb. quadrupole broadening makes solid-state NMR studies difficult, but even in these cases, the static line shapes can provide considerable information about the quadrupole coupling constants, asymmetry paramete s, and chemical shifts. 

The static deuterium quadrupole coupling constant is (e qQ/h) = 170 kHz for aliphatic carbon-deuterium (C—D) bonds. A change in the residual quadrupole splitting can be caused by two different mechanisms. First, the angle of the molecular fluctuations may increase or decrease, secondly, the molecule may undergo a conformational change which alters the orientation of the C—D bond vector with respect to the bilayer normal. 

---