Double quantum filtered phase-sensitive

The data from H NMR studies of 63, which included double quantum filtered phase sensitive correlated spectroscopy (DQF-COSY) and rotating frame nuclear Overhauser effect spectroscopy (ROESY) experiments (Figure 12), are collected in Table 17. 

Dong and co-workers [61 ] derived the configurational assignments of polymethacrylonitrile (PMAN) from double-quantum-filtered phase-sensitive homonuclear shift[Pg.352]

Figure 5.37 (a) Conventional phase-sensitive COSY spectrum of basic pancreatic trypsin inhibitor, (b) Double-quantum filtered (DQF) phase-sensitive COSY spectrum of the same trypsin inhibitor, in which singlet resonances and solvent signal are largely suppressed. Notice how clean the spectrum is, especially in the region near the diagonal line. (Reprinted from Biochem. Biophys. Res. Comm. 117, M. Ranee, et al., 479, copyright (1983) with permission from Academic Press, Inc.)... 

The double quantum filter eliminates or at least suppresses the strong signals from protons that do not experience J-coupling, e.g. the solvent signal, which would otherwise dominate the spectrum and possibly be a source of troublesome tl noise. Compared to a phase-sensitive but non-DQ-filtered COSY with pure absorption lineshapes for the cross peaks but mixed lineshapes for the diagonal peaks, the phase-sensitive, DQ-filtered COSY has pure absoiption lineshapes throughout. 

By allowing multiple-quantum coherence to process during the evolution period of a two-dimensional experiment, Drobny et al. were able to detect its effects indirectly. This idea subsequently blossomed into the new technique of filtration through double-quantum coherence. Multiple-quantum coherence of order n possesses an n-fold sensitivity to radiofrequency phase shifts, which permits separation from the normal single-quantum coherence. This concept inspired the popular new techniques of double-quantum filtered correlation spectroscopy (DQ-COSY) and the carbon-carbon backbone experiment (INADEQUATE), both designed to extract useful connectivity information from undesirable interfering signals. 

Double Quantum Filtered COSY The double quantum filtered COSY experiment (DQF-COSY, Section 6-1 and Figure 6-16) is similar to COSY, with three 90° pulses in the sequence 90°-/i-90°-T-90°-f2 (acquire). The DQF-COSY experiment is performed in the phase-sensitive mode, but, unlike the situation in the phase-sensitive COSY experiment, in the DQF-COSY both diagonal and cross peaks can be phased as absorptive signals. This difference not... 

Finally, the very recently proposed double-quantum (DQ) and double-quantum filtered (DFQ) STMAS experiments [42] allow filtering out diagonal (and outer satellite transitions) peaks of STMAS spectra with no loss on sensitivity. The experiments efficiently convert inner ST coherences from single to doublequantum with a central-selective transition 71 pulse. The conversion allows the selection of double-quantum transfer pathways with phase cycling, filtering out the unwanted peaks. 

Previous sections have already made the case for acquiring COSY data such that it may be presented in the phase-sensitive mode. The pure-absorption lineshapes associated with this provide the highest possible resolution and allow one to extract information from the fine-structure within crosspeak multiplets. However, it was also pointed out that the basic COSY-90 sequence suffers from one serious drawback in that diagonal peaks possess dispersion-mode lineshapes when crosspeaks are phased into pure absorption-mode. The broad tails associated with these can mask crosspeaks that fall close to the diagonal, so there is potential for useful information to be lost. The presence of dispersive contributions to the diagonal may be (largely) overcome by the use of the double-quantum filtered variant of COSY [37], and for this reason DQF-COSY is the experiment of choice for recording phase-sensitive COSY data. 

Figure 5.41. The gradient-selected DQF-COSY experiment and coherence transfer pathway. No phase-cycling is needed as the required pathway is selected with gradient ratios of 1 2. Both gradient pulses are applied within spin-echoes for phase-sensitive presentations. Note only one pathway is retained from the double-quantum filter.

Figure 5>t2. The double-quantum filtered COSY spectrum (right) provides greater clarity close to the diagonal peaks than the basic phase-sensitive COSY (left) as it does not suffer from broad, disposive diagonal peaks. 

Check it 3.3.2.2 illustrates the phase correction of a phase sensitive IR COSY TPPI spectrum whilst in Check it 3.3.2.3 a phase sensitive COSY spectrum with double quantum filter is simulated. The double quantum filter effects the lineshape in the spectrum such that the diagonal and cross peaks have a narrower absorptive lineshape. 

The double quantum filter In homonuclear COSY the DQF plays an important role for phase sensitive spectra filtering out the dispersive coherence contribution to diagonal peaks. 

Proton NMR spectroscopy was used as a tool for profiling complex carbohydrate mixtures in non-fractionated beer by Meier et al Six samples of beer of different styles contained more than 20 small carbohydrates were tested. The authors used H1-H2 cross peaks in phase sensitive double quantum filtered COSY measurement as reporter signals to identify more than 50 structural motifs in beers. 

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