Quadrupole frequency

ESEEM of FeMo-co does not show the nitrogen quadrupole frequencies observed for the M center in the protein. This shows that the observed splitting is likely due to an l N atom on the protein. Further, this nitrogen is probably not of the deprotonated amide type, which is the mode in which NMF is thought to bind to FeMo-co. 

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. 

The first 27A1 MAS NMR study of zeolites was carried out by Freude and Behrens (151). They measured, first, chemical shifts and half-widths of signals from stationary samples of zeolites Na-A, Tl-A, Na-Y, and Tl-Y at 16 MHz. For MAS frequencies of vR such that vR > Vq/vl the central line of the 27A1 resonance is reduced to about j of its original value. Freude and Behrens next calculated the quadrupole frequencies vQ and shifts of the center of gravity of each line due to the quadrupole interaction, i.e., vQ = (v - vL)/vL at 70 MHz. Then, apparent line positions and line widths, S and <5v 2MAS, were measured experimentally using MAS at 70 MHz. The corrected chemical shift value at 70 MHz was then calculated from the relationship <5, = 5ex — <5q. They were several ppm different from the apparent values (see Table XII). 

The NMR lineshape of solids is determined by the quadrupolar interaction, which can be described by two parameters the quadrupole frequency, o Q, and the asymmetry parameter, t/ (19,20). The parameter q)q is determined by the electric quadrupole moment of the deuteron and the zz component of the electric field gradient at the deuteron site. For deuterons bonded to carbon atoms, the asymmetry parameter is approximately zero and the z axis is along the C—D bond. In this case, the dependence of the resonance frequency, m, from the orientation of the molecule with respect to the magnetic field applied is given by a relation similar to Eq. (18) (19). 

Figure 2.16. Effect of a changing distribution in the main component of the electric field gradientVzz comparing the effect on A. the satellite transition and B. the centreband with a mean quadrupole frequency of 1 MHz and then a distribution of quadrupole frequencies as shown. Only one half of the satellite transition is shown. The Larmor frequency is 104.26 MHz and a spinning speed of 11.5 kHz has been taken. From Jager et al. 1993 with permission of the copyright owner.

At this point an echo forms but the coefficients of the isotropic terms are different and so are not refocused at this point. This has important consequences for the experiment. The isotropic part of the quadrupole frequency is... 

Figure 2.27. The overall efficiency of a 2-pulse MQ pulse sequence for different excitation (3Q, 5Q, 7Q, 9Q) for different spins with the quadrupole frequency scaled by the spin factor to allow direct comparison of the different spins with the optimum pulse angle as given in Table 2.9, after Amoureux and Fernandez (1998).

Figure 3.18. The dependence of the intensity of the centreband of an MAS spectrum for the A. central and B. satellite transition as a function of the spinning speed v, the quadrupole frequency vq and the Larmor frequency Vo along with C. the Al MAS NMR spectrum of Y3AI5O12 at 7.05 T at a spinning speed of 7 kHz. From Massiot et al. (1990) with permission of copyright owner.

In Section 2.2.6, the nutation frequency of a quadrupolar nucleus was seen to vary from (I + /2)v] when vi << vq to V when V >> vq. The nutation spectra display distinct features in the region where vi vq. Simulation of nutation behaviour has shown that the useful range of rf field-strengths lies with vq/vi in the range 0.05-1. The distinct features of nutation spectra allow the quadrupole parameters to be obtained. This means that only quadrupole frequencies up to a certain limit are accessible with currently available rf-fields of 500 kHz in specially dedicated probeheads, and in fields of around 100 kHz in most commercially available systems. This constraint especially limits the investigation of spin I = 3/2 nuclei such as Na. 

The SCAN-STMAS pulse sequence is shown in Fig. 24b. The extra pulse in the middle of the fj period transfers the coherence between the two satellite transitions. The two satellite transitions, ST and ST , have first-order quadrupole frequencies that... 

The resolution enhancement stems from the fact that the quadrupole frequencies for both transitions are correlated. At specific times the anisotropic parts of the quadrupole interaction are refocused and an echo forms. The frequency of an m, -m) transition is given by... 

The nuclear quadrupole frequency parameter P contains second-order corrections to the first-order part. 

The feasibility of ENDOR-like experiments in zf using optieal detection was suggested by Schmidt and van der Waals (1969). The observation of spectra of this type was reported nearly simultaneously by Harris et al (1969) and Chan et al (1969). The focus in these studies was on the nuclear spin sublevels of phosphorescent quinoxalines. The mechanisms responsible for the intensity of the ENDOR transitions in zf have been discussed (Harris and Buckley, 1976). The technique was extended to both chlorine isotopes by Buckley and Harris (1970). Chan and van der Waals (1973) used the ENDOR technique to determine the N quadrupole frequencies in perdeuteroquinoline. The analysis given rests on the assumption of the coincidence of principal axes, information not normally available from the zf experiment. C and H ENDOR transitions were observed in the low-field experiments of Hochstrasser et al (1973,1978) on (nn ) benzophenone. It has also been shown that certain components of the proton hyperfine tensors can be obtained from a careful analysis of the zf ENDOR spectra of azanaphthalenes (Dennis and Tinti, 1975). Chan and Walton (1977) have observed proton ENDOR signals in zf. More recently, Dinse and Winscom... 

Fig. 17. Cross-relaxation model, which explains satellites observed in the level anticrossing spectra of triplet benzophenones in 4,4 -dibromodiphenylether. drawn approximately to scale for C-benzophenone with H z. The solid (dashed) arrows indicate field positions at which the triplet sublevel separations are equal to the Br ( Br) nuclear quadrupole frequencies of the host (Mucha and Pratt, 1977b).

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