Antiphase signals

Short mixing times (e.g., 30 ms) lead to INEPT-type spectra (or COSY-type 2D spectra), where transfer is mostly limited to a single jump over one J coupling. Unlike INEPT and COSY, however, the transfer results in an in-phase rather than antiphase signal. This is a significant advantage as the peaks have the same shape and pattern as they do in a ID spectrum. 

A product operator analysis of the observable coherences for the standard DEPT sequence [5.59] shows that in addition to the wanted coherence Sx (where Sx is the x-component of the S spin product operator and S is the non-abundant 3c spin) several other single quantum coherences are generated. These terms such as I Sx lead to phase distortions in the H coupled spectra because signals which could be assigned to either the Sx or Sy state are overlapped by antiphase signals from coherences like I Sy. Consequently the signal intensity of the related decoupled spectrum is reduced by these coherences. In the DEPT++ sequence only the required single quantum coherences Sx (or... 

The delays Aj and A2 are chosen to be of the order of l/2JrH and l/3JcH respectively, in order to prevent the mutual cancellation of antiphase signals. If a phase-sensitive display of the entire spectrin is desired, then it is useful to insert 180q,180h pulse pairs into the midpoints of the Aj and A2 delays, but such a display mode is rarely helpful. 

Figure 7.7 H NMR spectra detected during the hydrogenation of propylene using the IRMOF-3-SI-Au catalyst and (a) normal Hj or (b) pHj. The enhanced antiphase signals...

Since many of the signals in COSY spectra are in antiphase, they may not show up as cross-peaks due to the intrinsic nature of the polarization transfer experiment. The intensities of cross-peaks in COSY spectra may be represented by an antiphase triangle (Fig. 5.33B), in contrast to multiplet... 

Thus, the phase of the X pulse ( x) directly influences the phase of the term generated from the H,X antiphase term (Eq. (5)), but does not affect the phase of the in-phase term at all (Eq. (6)) Simple additive combination of two scans with a 90° x (X) pulse will therefore lead to a selective cancellation of the signals from 1H-X pairs, while a subtractive combination will suppress all signals from protons not bound to X. 

As shown in Fig. 2.48 (e), the vectors of C13 — C13 doublet magnetizations are aligned in opposite directions when irradiated by the 90 pulse. Thus, in the INADEQUATE spectrum the C13 —C13 doublet signals will appear with the corresponding antiphase relationship, as shown in Fig. 2.49, which also demonstrates the effective suppression of the strong 13C — 12C signals of piperidine. Analysis of carbon-carbon coupling constants can be performed easily in this simple case. 

The antiphase relationship of the C13 —C13 doublet signals in the INADEQUATE spectrum can be eliminated by an additional spin-echo sequence (— 1/4Jcc — 180° — 1 /4 Jcc —) before the 90 monitor pulse [58]. The sensitivity of the experiment may be improved by the application of stronger magnetic fields or by using proton polarization transfer techniques [59]. 

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