Quadrupole isotropic shifts

The presence of the oxo bridge in oxyHr and metHrX is reflected in then-magnetic susceptibility, NMR, and MOssbauer properties. The coirq)lexes exhibit strong andferromagnetic coupling, small NMR isotropic shifts, and large Mdssbauer quadrupole splittings. J values obtained for metHr and oxyHr from fits to variable temperature susceptibiliy data are -134 and -77 cm-i, respectively (34). The J for metHr compares favorably with those observed for the synthetic... 

As if can be seen in fhese simulations, all MAS spectra are shifted towards low frequencies wifh respecf fo fhe true isotropic chemical shift—an effect known as the second-order quadrupole-induced shift. The centre of gravity ( g) of fhese lineshapes is given by ° ... 

Chemical shift relative to liquid CS2, some are peak positions not the isotropic shifts, see original references. Numbers in parentheses represent singularity positions of MAS powder patterns, t Estimated upper limit of the quadrupole effect parameter, except which is based on lineshape analysis. 

The isotropic shift and the quadrupole induced shift (QIS = Co(p)/Co(l), with Cs defined in Table 2.4) can readily be obtained from the data. Firstly, the 2D spectrum must be referenced correctly. To obtain the correct ppm scale in the FI dimension one must set the spectrometer frequency in the FI dimension to pVo. The shift reference is most readily set at the carrier frequency, i.e. the shift in ppm at the carrier frequency in the multiple quantum dimension Ft is the same as the shift in ppm at the carrier frequency in the single quantum dimension F2. This is valid for data obtained with or without delayed acquisition and processed with or without shearing. The isotropic chemical shift is the same for both the single quantum and the multiple quantum dimensions. The QIS is different in both dimensions, however, and is given by... 

Equation (16) is very important because it imcovers two central ideas. First, the second-order frequency splitting depends inversely on the Larmor frequency, thus the importance of this term diminishes with increasing external magnetic field strength. Second, the k=0 term has no orientation dependence (Do 0 = 1) or, in other words, it is an isotropic term. This means that the isotropic shift observed in the NMR spectriun of a quadrupolar nucleus has, in addition to the usual isotropic chemical shift, a contribution from the quadrupole cou-phng, which is given by... 

Table 6a. The contribution to the isotropic shift by the second-order quadrupole interaction of different spin quantum numbers...

Physical background. MAS will narrow the inhomogeneously broadened satellite transitions to give a series of sharp sidebands whose intensity envelopes closely follow the static powder pattern so that the quadrupole interaction can be deduced. The work of Samoson [25] gave real impetus to satellite transition spectroscopy by showing that both the second-order quadrupolar linewidths and isotropic shifts are functions of I and m. Some combinations of I and produce smaller second-order quadrupolar effects on the satellite lines than... 

The isotropic shift and the quadrupole-induced shift (QIS) can easily be obtained from the data. The QIS is different in both dimensions, however, and is given by... 

Figure 16 2D 3Q/MAS NMR spectrum of l anite measured by the three-pulse sequence with z-filter. No shearing transformation was applied. Labels "A," "CS," and "QiS" describe anisotropic axis, isotropic chemical shift axis and axis of quadrupole-induced isotropic shift, respectively. Three different AlOg structural units are clearly defined.

Subsequently, based on the principles of 2D NMR spectroscopy, distinct sites can be clearly resolved (Fig. 16), and, using mathematical analysis, the isotropic chemical shift and quadrupole-induced isotropic shift can be separated from the anisotropic second-order quadrupolar spHtting. The principle condition for obtaining the required results or separating the anisotropic component of the quadrupolar interaction from isotropic contributions is based on the correct setup of the ratio of t and <2 detection times, during which the 3Q generally p-quantum) and IQ coherences evolve. This ratio must fulfill the following condition [72,73] ... 

NMR spectra, where the v. axis nearly purely reflects the isotropic chemical shift, and the quadrupolar parameters (second-order quadrupolar broadening and quadrupole-induced shift) are separated along the V axis (Fig. 19D). 

The ZB and WZ forms of ZnS could be distinguished on the basis of their isotropic 67Zn and 33S static NMR chemical shifts [63] however, estimates of small CSAs obtained from static 33S NMR of the WZ forms of ZnS and CdS were obtained by ignoring any small second-order quadrupole broadening [65], which may account for discrepancies when NQCCs are assumed to be responsible for the broadenings [65]. 

Table 3 isotropic H-NMR Chemical Shifts (8iso) and Quadrupole Coupling Parameters (QCC, t ) Determined Experimentally for Selected Molecules... 

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