Chemical-shift difference filter

Fig. 7.2.5 Chemical-shift difference filters, (a) Spin-echo method [Dixl, Wehl]. Two data sets are measured. For one set the Hahn and gradient echoes coincide, for the other both echoes are separated by 2ti f 2 Aco). Here Aco denotes the frequency difference between two lines.

As an alternative to selective pulses, chemical-shift-selective filters (CSSF) were successfully used in ID COSY, ID RELAY and ID NOESY experiments when signals partially overlapping, but different in their chemical... 

In a ID COSY-RELAY experiment [38] (fig. 13(a)) a multistep relay transfer is applied after the filtration. If the filtration is performed on the H-2 proton, the CSSF is incorporated into the first spin-echo. If there is not sufficient chemical shift separation between H-2 protons, the filter is shifted to the second spin-echo. The method is illustrated for the separation of the spin systems of two terminal /3-glucopyranose residues of a modified LPS (5) containing a total of nine saccharide units [76]. The anomeric proton resonances of the two /3-glycopyranoses overlapped almost completely, with a chemical shift difference of only 1.9 Hz, while the corresponding H-2 resonances were separated by 55.0 Hz. The length of the filtration interval, Ti, was adjusted to yield a maximum antiphase magnetization of H-2 pro-... 

Partial plots extracted from HMBC spectra recorded with the five different low-pass J filters are shown in Figure 7. Clearly, ambiguities may arise, particularly if accidental degeneracy in chemical shifts causes true long-range correlations and residual 1Jch correlations to overlap. 

Zheng et al. [1] postulated that the driving force for placing Zr and B on the same carbon might stem from interactions between the zirconium and oxygen or boron and chlorine atoms. However, an X-ray analysis of 22 revealed that there are no intra- or intermo-lecular interactions between any of these atoms [35]. Compound 22 was also unambiguously characterized by 1H-1H double quantum filtered COSY [36] and 13C-1H heteronuc-lear chemical shift correlation NMR spectroscopy [37,38]. Considerable differences in the chemical shifts of the diastereotopic Cp groups were found in both the XH and 13C NMR spectra. The NMR study unequivocally showed that the methine proton was at-... 

X-ray crystallography, docking modes can be validated by various NMR techniques NOEs may be observed between the ligand and the receptor protein by heteronuclear-filtered NOE spectroscopy [51], chemical shift changes of protein resonances upon binding can be analyzed by simulation of shifts caused by ring currents and electrostatic effects [52], and saturation transfer difference measurements indicate which part of the ligand is in direct contact with the protein [52]. 

Peak three corresponds to the equivalent P atoms of the P2O7 unit, the other four peaks correspond to the two inequivalent PO4 units, each split by its proximity to two configurations of the P2O7 unit which differ in the position of the bridging oxygen. B. 2D double-quantum P spectrum demonstrating the connectivity between two equivalent coupled P nuclei (P2O7 unit) with the same chemical shift (— 18.6 ppm). The fl2 dimension is the 2D spectrum filtered by the double quantum, the fl] dimension is the double quantum spectrum. From Pichot et al. (2001) by permission of Elsevier Science. 

One of the most important features of DOSY is its ability to separate signals of compounds within a mixture, based on their diffusion coefficients which, in fact, reflect their size and shape, thus providing a means for virtual separation . Here, the information is spread over the entire plan thus also simplifying peaks assignment. However, it was commented that for separating peaks of different compounds, which happen to have the same chemical shift, a large difference in the diffusion coefficients (a ratio of about 3) is needed for DOSY to provide the accurate numbers. An important fact to remember is that diffusion sequences, in fact, act as filters and can therefore be imbedded or coupled to nearly any NMR sequence, including most of the multidimensional NMR sequences. 

The selectivity of multi-quantum filters is demonstrated in Fig. 7.2.28 by spectra recorded for poly(isoprene) (PI) with the multi-quantum filter from Fig. 7.2.26(d) [Schl 1, Schl2, Schl3]. The conventional single-quantum H NMR spectrum of PI shows a broad line with little to no chemical-shift resolution. But the double- and triple-quantum filtered spectrum of the material exhibit peaks at different chemical shifts which can be assigned primarily to the signals from CH2 and CH3 groups, respectively. At longer... 

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