Spectrum-edited methods

INEPT sequence. The delays t are set to 1/2Jhx required for the INEPT transfer and the delay set as necessary for the INEPT refocusing stage to enable the use of decoupling. The diffusion decoding gradient is applied as a bipolar pair during the first part of the INEPT transfer step. 

TOCSY mixing DPFGSE seiection with zero-quantum suppression 

Whilst the COSY- and TOCSY-derived methods described above offer the potential for crosspeak dispersion, they become less effective when analysing mixtures of similar compounds since crosspeak overlap may still be a problem. In such cases, an alternative approach may be to use the homonuclear 7-resolved technique in an attempt to eliminate multiplet overlap by placing /-coupling fine structure orthogonal to the shift axis (Chapter 7). In this approach, the intensity decay of the 2D peaks may be fitted to extract diffusion coefficients or alternatively the projections of the tilted 2D data set may be treated as a homonuclear decoupled ID spectrum and the peak intensities fitted as for conventional diffusion experiments [67]. 

One example of this approach is the constant-time HSQC-IDOSY [72] experiment (Fig. 9.37) that combines the diffusion and constant-time periods and incorporates the encoding-decoding bipolar gradient pairs within the INEPT transfer steps. Additional gradients are utilised to provide coherence selection and/i quadrature detection according 

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The most straightforward isotope-editing method for selecting protons bound to a heteronucleus and suppressing all others is the simple acquisition of a spectrum with an indirect heteronuclear dimension (in the literature the term isotope editing is often used as a synonym for these techniques). This can be accomplished by a simple 2D HMQC or HSQC shift correlation, or a more elaborate 3D technique including an additional NOESY or TOCSY step (3D X-edited NOESY/TOCSY etc.), or even 4D experiments with a second heteronuclear shift dimension [13, 14]. 

It is common practise to compare the solid state NMR spectrum with that obtained in the solution state (if available), as the techniques available to assign the latter are well established and reasonably robust (see Section 4.2.4.1). In addition to the basic ID CP-MAS experiment, there are a number of editing methods available which yield information about the multiplicity of the... 

A comparison of DEPT-135 with both the original and enhanced DEPTQ is made in Pig. 4.36 demonstrating the retention of quaternary centres and the improved performance achievable with the enhanced version. This method would now appear to be the optimal approach to spectrum editing with retention of all carbon centres and is attractive as a routine carbon-13 screening tool for the organic laboratory. The use of frequency-swept (adiabatic) 180° pulses is equally valid for the standard DEPT sequence, or indeed any using 180° pulses, and should also provide enhanced performance when large carbon bandwidths become problematic see Chapter 10 for more information on these. 

In Section 9.3.1, the problem of assigning the many resolved resonances in a ID MAS spectrum was mentioned, and ID spectral editing methods were introduced. In this section, we describe homonuclear 2D correlation experi-... 

Figure 16 Comparison of the F projections of the multiplicity-edited GHSQC, 60 Hz 1,1-ADEQUATE, the UIC calculated HSQC-1,1-ADEQUATE, and GIC calculated HSQC-1,1-ADEQUATE spectra (power 0.5) of strychnine (1). While Snyder and Bruschweiler have noted52 that signal-to-noise (s/n) measurements of covariance spectra may not be the most viable means of making comparisons, in the present example, the differences are so dramatic that they still allow valid conclusions to be drawn. The C12 (77.6 ppm) resonance provides a convenient s/n comparator. The region from 80 to 100 ppm, which is devoid of responses in the spectrum of strychnine, was used to define the "noise" region for the measurement. There is certainly no question that multiplicity-edited GHSQC is a high-sensitivity experiment as attested by a s/n for the C12/H12 correlation response of 290 1. In comparison, the C12 resonance of the 60 Hz 1,1-ADEQUATE spectrum is a much more modest 22 1. In contrast, the s/n for the C12 resonance in the HSQC-1,1-ADEQUATE spectra calculated using UIC and GIC methods was 293 1 and 257 1, respectively.

Figure 18A shows the overlaid multiplicity-edited GHSQC and 60 Hz 1,1-ADEQUATE spectra of posaconazole (47). As will be noted from an inspection of the overlaid spectra, there is an overlap of the C46 and C47 resonances of the aliphatic side chain attached to the triazolone ring that can be seen more clearly in the expansion shown in Figure 18B. In contrast, when the data are subjected to GIC processing with power = 0.5, the overlap between the C46 and C47 resonances is clearly resolved (Figure 18C). In addition, the weak correlation between the C3 and C4 resonances of the tetrahydrofuryl moiety in the structure is also observed despite the fact that this correlation was not visible in the overlaid spectrum shown in A. This feature of the spectrum can be attributed to the sensitivity enhancement inherent to the covariance processing method.50...

Sends the plot as a vector-oriented metafile to the MS-WINDOWS clipboard from where it can be transferred into any other MS-WINDOWS program. The plot area of the output is taken from the currently displayed area in the Spectrum window. Parameter lists and the title cannot be transferred using this method. They can be copied by transferring the output, including the title and lists, first into the Preview window, from where it may be copied to the MS-WINDOWS clipboard using the Copy option in the Edit pull-down menu in the Preview window menu bar. 

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