Inverse detection methods

The NMR detection probe that is nsed for most heteronuclear experiments (such as the HETCOR experiment) is designed so that the receiver coils for the less-sensitive nncleus (the insensitive nn-cleus) are located closer to the sample than the receiver coils for the more sensitive (generally the H) nuclens. This design is aimed at maximizing the signal that is detected from the insensitive nncleus. As was described in Chapter 4, owing to a combination of low natural abundance and a low magnetogyric ratio, a c nnclens is abont 6000 times more difficult to detect than a H nucleus. A N nucleus is also similarly more difficult to detect than a H nucleus. 

The difficulty with this probe design is that the initial pnlse and the detection occur in the insensitive channel, while the evolntion period is detected in the H channel. The resolution that is possible, however, is mueh lower in the channel where the evolntion of spins is detected. In the case of a 

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The vibrational assignments and force constants have been determined from the IR and Raman spectra of S7NH, S415N4H4 and S415N4D4. The 1H and 15N NMR chemical shifts of all the cyclic sulfur imides have been determined by inverse detection methods.202... 

The sensitivity of 15N can be greatly improved by the use of inverse detection methods ( -detected 15N), by a theoretical maximum of 306 (I yH /1 yN )5/2 with respect to directly detected 15N (Fig. 3), such that signals can be detected in aqueous solutions at concentrations of physiological relevance (5 im). -Detected inverse methods are applicable to any... 

Detection of at natural abundance via Another report that dealt with the observation of at natural abundance via P inverse-detection methods was that of Carbajo and Lopez-Ortiz The authors report that enhanced sensitivity HSQC... 

The other common inverse-detection method, heteronuclear multiple quan-turn coherence (HMQC) relies on multiple-quantum coherence transitions during the pulse sequence. Due to the multiple-quantum coherence transitions it is more laborious to theoretically follow the course of magnetization, and the cross peak will be broader in the Fi dimension due to the /hh evolution. Unlike HSQC, HMQC can also be optimized for Jch couplings. This heteronuclear multiple bond correlation experiment, or HMBC, ° ° has lower sensitivity than HMQC/HSQC experiments, and the Jch correlations can appear as artefacts in the spectrum. However, the cross peak volume should follow the concentration of analyte, so with proper method validation HMQC and HMBC should also be applicable for quantification. 

In an interesting departure from the otherwise exclusive applications of the D-HMBC experiment in natural product structure elucidation that have been cited above, Carbajo etal. used the D-HMBC experiment in a study of alkenylcarbyne- and alkenylvinylidene-tungsten complexes. Using both H- - W and inverse detection methods, the authors demonstrated an approxi-... 

Heteronucleus-detected shift correlation experiments have now been largely supplanted by far more sensitive proton- or inverse -detected methods. The heteronu-cleus-detected experiments are now largely reserved, in laboratories with modem NMR spectrometers, for those occasions when very high digital resolution is needed in the carbon frequency domain because of high spectral congestion [109, 110]. The remainder of this section will focus on the now widely utihzed proton-detected heteronuclear shift correlation methods. 

The increase in sensitivity afforded by the proton-detected HMBC experiment revolutionized structure elucidation studies. The utilization of HMBC data in the characterization of alkaloid structures has been reviewed [70] and is also treated in a more general review of the application of inverse-detected methods in natural products structure elucidation [71]. Other applications of the experiment are quite... 

Undoubtedly, even more interesting applications of inverse-detected methods are possible and will be developed as structural problems drive the spectroscopist to find new solutions. Some of the applications that have been presented herein have yet to be applied to an alkaloid. It is, however, only a question of an alkaloid structural problem amenable to solution by a particular technique and an investigator who is aware that the technique exists before corresponding applications in alkaloid chemistry appear. Finally, the recent availability of microinverse and microdual probes (Crouch and Martin 1992a,b) also promises to lead to applications of inverse-detected techniques in the elucidation of the structures of minor alkaloidal constituents that have, until now, been available in quantities too small to allow their structures to be elucidated. 

The only platinum nucleus with magnetic properties is Pt, F=, (33.7% abundance). The resonance frequency in a magnetic field of 2.35 T is approximately 21.4 MHz. Satellite peaks from coupling with Pt were observed in H and P NMR spectra in the 1960s, and much of the early work on Pt detection used INDOR methods. Direct one-dimensional observation of Pt NMR spectra is now routine. Because Pt relaxation times are short for most compounds, there need be only a very short delay between pulses, allowing rapid accumulation. Two-dimensional inverse detection methods are also being increasingly used. 

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