Proton-detected correlation methods

Indirect detection Method for the observation of an insensitive nucleus (e.g., 13C) by the transfer of magnetisation from an abundant nucleus (e.g., 1H). This method of detection offers great improvements in the sensitivity of proton-carbon correlated techniques. 

Hyphenated heteronuclear shift correlation methods Many of the experiments described thus far can be used as building blocks to create more sophisticated pulse sequences. The earliest variants of hyphenated NMR experiments, such as HC-RELAY, were heteronuclear detected [56]. More recent hyphenated heteronuclear shift correlation methods are based on proton detection and include... 

The experiments based on proton detection of rare spin nuclei are usually the most sensitive methods of determining NMR parameters of magnetically diluted spin systems. Unfortunately, recording of 2D correlation maps is usually also a time consuming experiment, especially if wide spectral bands have to be covered in the indirectly detected dimension. The most frequently encountered situation is that only one or a few peaks are expected within a narrow spectral band. However, the position of this band is not known. Several attempts have been made to reduce the experimental time needed to perform such experiments. One approach would be to record a highly truncated data set and use the linear prediction [86,87] to reduce the effect of the data truncation on the appearance of the spectrum. This technique is now available with most... 

Naturally, the above methods require proton-bearing carbons to be present, and when applicable should prove to be far more useful in structural analysis than their C detected counterparts. However, even these are likely to be employed only after more sensitive methods, such as those based on proton-proton and proton-carbon correlations, have failed to solve the problem at hand. 

Figure 6.8. A comparison of signal suppression methods used in proton-detected heteronuclear correlation experiments (see descriptions in text). Spectrum (a) is taken from a conventional ID proton spectrum without suppression of the parent resonance and displays the required satellites. Other spectra are recorded with (b) phase-cycling, (c) optimised BIRD presaturation, and (d) pulsed field gradients to remove the parent line. All spectra were recorded under otherwise identical acquisition conditions and result from two transients. Complete suppression can be achieved with gradient selection, but at some cost in sensitivity in this case (see text).

It should be remembered that the 2D methods presented here are suitable for the correlation of a variety of different nuclides [17], and do not necessarily require proton detection, so long as the observed nucleus exists at high abundance. As an illustration. Fig. 6.15 shows a phosphorus detected P- C correlation of 6.4, The basic HMQC sequence was used with additional broadband proton decoupling applied throughout but without carbon-13 decoupling... 

Traditional methods of heteronuclear shift correlation follow the scheme of Fig. 6.1b above, relying on the direct observation of the X-spin and with the source nucleus (typically protons) detected indirectly. As discussed at the beginning of this chapter, this general approach is less sensitive than... 

Following a very brief review of heteronuclide-detected methods, the discussion of new proton-detected direct and long-range shift correlation experiments... 

Beginning with the report of the HMQC experiment in 1986 by Bax and Subramanian- the utilization of proton- or inverse-detected heteronuclear shift correlation experiment was essentially ushered in. Despite reports that have demonstrated the superior resolution of single quantum-based heteronuclear shift correlation methods,-" the HMQC experiment still remains the most widely employed, proton-detected heteronuclear shift correlation method. Gradients were incorporated into the HMQC experiment in 1991 by Hurd and John- and are discussed in the excellent contribution of Ruiz-Cabello and co-workers- and... 

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