Quadrupole coupling constant experiments

W(VI) compounds with 5cP electronic configuration, the EFG is reasonably weU described by the lattice contribution only. The regular octahedral WOs structure yields zero EFG in agreement with experiment, while the deformed tetrahedral W04 strucmre results in significant quadrupole coupling constants of either sign, e Q > 0 and also <0, depending on the distortion of the O4 tetrahedron. 

Other key properties of these complexes relevant for comparison with experiment are the nuclear quadrupole coupling constants. Theoretical NQCC values, as calculated via electric field gradients, are discussed exhaustively in several recent theoretical investigations [26,28,30,33,34] and in experimental work [4-11]. BH HLYP/aug-cc-pVTZ calculated NQCCs [30,34], as evaluated for XY- -NH3 complexes, are listed in Table 10. 

The temperature dependent T data are shown in Fig. 9. 7j values decrease from 28 ms at 21°C with increasing temperature, and show a minimum of 6.4 ms at 80° C. These results indicate the presence of the motion with a Larmor frequency of 30 MHz at this temperature. This minimum was found to be attributed to the flipping motion of a phenyl ring from the result of our other experiments discussed in later section.13 The jump rates of the flipping motion were estimated with a two-site jump model that a C-2H bond jumps between two equivalent sites separated by 180°, and that the angle made by the C-2H bond and the rotational axis is 60°. The quadrupole coupling constant of 180 kHz and the asymmetry parameter approximated to zero were used in the calculation. The calculated values for fitting with the... 

Grey and Vega (161) have demonstrated the use of the TRAPDOOR experiment for measuring the 27Al quadrupole coupling constant in zeolites and applied it to studies of trimethylamine in zeolites. In other spectroscopic work, Fripiat and co-workers (162) used REDOR and various other NMR methods to characterize acid sites in zeolites treated with ammonia. Ernst and Pfeiffer (163) have reported a 13C MAS NMR study of the reactions of methanol and ammonia to make methylamines in zeolite HZSM-5. We reported NMR evidence of the synthesis of cyclopentylamine from cyclo-pentanol and ammonia on zeolite CsX (102). 

It is important to note that for 0 the nuclear quadrupole coupling constant is also an important NMR parameter, which can be used to characterize both the identity of the tetrahedrally coordinated atoms, T, in a T-O-T linkage, and the value of the T-O-T angle. Earlier calculations (24) showed a decrease in the O NQCC as the Si-O-Si angle decreases, a trend verified by experiment (34, 35). Later calculations(36) indicating a progessive decrease in the O NQCC for Si-O-Si, Si-O-Al and Al-O-Al angles were also verified. 

The one-electron properties such as the deuteron quadrupole coupling constant have been less studied until recently, but Dixon et al33 have recently reported the results for this property and also values of the diamagnetic shielding and susceptibility computed with a medium-size basis set. Moderate agreement with experiment was obtained. 

Let us assume that we have found from NQR experiment the quadrupole coupling constant and the asymmetry parameter 17. 

The axial component of the total magnetic hyperfine interaction, A 3/2, in the 2n3/2 component is equal to a+ (b + c)/2, where a, b and c are the Frosch and Foley [192] constants. In the 2n1/2 component the axial hyperfine constant, h /2, is equal to a — (b + c)/2. Since both fine-structure components can usually be studied by pure microwave experiments, a partial separation of the magnetic hyperfine constants can be achieved. The electric quadrupole coupling constant, eq0 Q, is obtained for both isotopes. 

Recent 170 NMR experiments on phthalate species has confirmed that both the 17C> isotropic chemical shift and 170 quadrupole coupling constant decrease as hy-... 

The optimal proton position can be slightly out of the Si-O-Al plane. On the basis of the quadrupole coupling constant measurements and asymmetry values obtained from deuterium NMR experiments, Kobe et al. suggested that the... 

In bis(acetylacetonato)beryllium, Be(acac)2, the Be quadrupole coupling constant can be determined from a combined and Be relaxation experiment, which is favoured in this molecule of D2J symmetry, by the fact that the C-H bond vector for the olefinic carbon is coincident with the z principal axis of the e.f g. tensor. Hence it is the same diffusion constant which modulates and For the quadrupole coupling constant a value of 350kHz was derived. (Ill)... 

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. 

The MQMAS experiment is inefficient for studying resonances with large quadrupole coupling constants, even when using the largest radio-frequency (Bi) fields available (ca. 250 kHz). In extreme cases (low and large quadrupole couplings), the peak intensity may be so low that the resonance remains buried in the noise and is not detected. 

Nuclear magnetic resonance (NMR) is perhaps the simplest technique for obtaining deuterium quadrupole coupling constants in solids or in liquid crystalline solutions. In ordinary NMR experiments with a magnetic field Hq > 104 gauss, the nuclear quadrupole interaction [Eq. (6)1 for deuterium is much smaller than the Zeeman interaction and can be treated as a perturbation to the Hamiltonian... 

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