Coupling constant quadrupolar

Champmartin D and Rubini P 1996 Determination of the 0-17 quadrupolar coupling constant and of the C-13 chemical shielding tensor anisotropy of the CO groups of pentane-2,4-dione and beta-diketonate complexes in solution. NMR relaxation study/norg. Chem. 35 179-83... 

The moment of inertia 1 determines the rotational constant 0 = h /IT, which is the parameter that controls the strength of quantum effects. The other parameter of the model, which is the quadrupolar coupling constant K, can be conveniently taken as the energy and temperature scale. We can thus reduce all quantities related to energies by K, and define, e.g., the dimensionless temperature = k T/K, energy = E/K, and rotational... 

The theoretical background for the quadrupole coupling constant was developed already in the 1950s . Direct calculation of quadrupolar couphng constants is stiU not straightforward and depends heavily on the basis set used The quadrupolar coupling constant is thus mainly used as an empirical parameter. Nevertheless, experimental trends can be reproduced quite well with available theoretical methods as shown in Section n.F. 

In solution, the quadrupolar coupling constant is not measured directly. Instead, the quadrupolar splitting constant is obtained from relaxation time measurements. The QSC is related to x according to equation 10 ... 

From these investigations it is clear that the Li chemical shift gives a clear indication of the lithium cation position when there are direct effects from ring currents in delocalized anions. However, as shown for the quinuclidine CIP and THF SSIP fluorenyllithium complexes, the correct assignment cannot be reached solely based on the chemical shifts. Furthermore, there is no clear-cut information about solvation to be gained from the chemical shifts. As we discuss in the following Section, the quadrupolar interaction is much more sensitive to these effects. In order to obtain maximal structural information from Li NMR spectroscopy, the chemical shift should be determined and used in combination with the quadrupolar coupling constant. 

This aspect is further supported by observations made for the dimers of phenyllithium systems 10,17 and 31 (Scheme The quadrupolar coupling constants in this series... 

CIP vs. SSIP The quadrupolar coupling constant as a new Indicator of structure... 

In many cases the temperature dependence of the quadrupolar coupling constant is an indicator of dynamic processes, because the symmetry around the lithium cation is affected by motions which are fast on the NMR time scale. If the rate of these processes exceeds 1/x, the effective symmetry around the lithium cation increases and a decrease in x( Li) results. In Li MAS spectra, a broadening of the satellite transitions can be observed which eventually disappear completely if the rate of the dynamic process comes in the order of the quadrupole frequency. This behaviour was observed for the THF solvated dimer of bis(trimethylsilylamido)lithium, where the Li MAS spectrum at 353 K shows only the central transition and the sidebands caused by CSA and homonuclear Li- Li dipole coupling (Figure 27) . The simulation of the high-temperature spectrum yielded —20 ppm and 1300 Hz for these quantities, respectively. The dipole coupling agrees closely with the theoretical value of 1319 Hz calculated from the Li-Li distance of 2.4 A, which was determined by an X-ray study. 

Dual spin probe experiments quadrupolar coupling constants Dual spin probe (DSP) techniques [20] are based on the correlation of relaxation data of two different nuclei, allowing for the determination of quadrupolar coupling constants. In cases where these coupling constants exist, the nuclei under investigation are within the same molecule or aggregate. 

In Fig. 5b, which was obtained at 30°C, the powder pattern displays a severely distorted, intermediate rate line shape. This line shape is characteristic of both fast methyl group rotation and 2 fold molecular re-orientation about the carbonyl bond at a rate comparable to the reciprocal of the quadrupolar coupling constant ( 105 Hz). At room temperature, therefore, the acetone-d6 molecules in the microporous channels of sepiolite are able to undergo restricted re-orientations. 

A spectrum of perdeuterated potassium laurate in oriented soap-water multilayers (5) is presented in Figure 2. The measured quadrupolar coupling constants are much smaller than those of a static C-D bond, which are about 167 kHz (6) the residual quadrupolar splittings... 

In addition to the glycine-containing polypeptides and peptides, we show here new results on the 170 NMR chemical shift and quadrupolar coupling constant of poly(L-alanine)s in both a-helix and P-sheet forms, in the solid state. There results help clarify the relationship between hydrogen-bond length and e2qQ/h and chemical shift parameters. 

Solid State 170 NMR Spectral Analysis. The 170 nucleus has a 5/2 spin and so has a quadrupolar moment. This means that static and MAS 170 NMR spectrum contains information about quadrupolar interactions and the chemical shift. In order to obtain the quadrupolar coupling constant and chemical shift separately, we have carried out spectral analysis with theoretical calculation taking into account quadrupolar interactions and chemical shift interactions. 

We can successfully measure 170 MAS NMR spectra of poly(L-Ala)s using high-field and high-speed magic-angle spinning technique. From the 170 MAS spectra three kinds of NMR parameters such as chemical shift, quadrupolar coupling constant and asymmetric parameter were obtained exactly and understood the relationship... 

---