Inverse H detection

A variety of sequences exist, which differ with respect to the detected interaction ( J, or Jx ) and the mode of detection ( C or H detected, magnitude or phased mode, phase cycling or gradients for coherence selection). In view of the reduced sensitivity of heteronudear experiments with respect to homonuclear COSY experiments and the steadily decreasing sample amounts submitted for NMR experiments, there is no doubt that the inverse ( H) detected, gradient enhanced experiments are currently the best methods to apply. However on older type spectrometers, not equipped for inverse detection the old-fashioned direct C detected experiments are still in use. 

The experiments are used to correlate H- and C chemical shifts. The C detected "C/ H-COSY experiment (Fig. 3.29a) is the most popular heteronuclear 2D experiment available with older type spectrometers, not yet equipped with inverse" H detection mode. Due to the surviving geminal couplings in FI the signals of methylene groups with non-equivalent protons are immediately recognized. 

COLOC experiment but takes advantage of the higher sensitivity of inverse H detection. 

Fig. 3. The theoretical increase in receptivity (abundance X sensitivity) obtainable by isotope enrichment and inverse ]H detection of13C, 15N and 195Pt. In practice, inverse 1H- 195Pt detection is limited by the broad linewidths of the 195Pt satellites.

Invaluable structural information was obtained employing 2D homo-and heteronuclear correlations performed under F-MAS. The inverse H-detected heteronuclear experiments under F-MAS conditions with spinning frequency exceeding 50 kHz are shown in Fig. 2.30. 

HMBC Heteronuclear multiple bond correlation, inverse CH correlation via long-range CH coupling, same format and information as described for ( C detected) CH COLOC but much more sensitive (therefore less time-consuming) because of H detection... 

Kessler, H. Detection of inttamolecular mobility by NMR specttoscopy. 13. Detection of hindered rotation and inversion by NMR specttoscopy. Angew. Chem. Int. Ed. 1970, 9, 219-235. 

In contrast to the isotope effects observed in d° olefin polymerization catalysts, Brookhart and coworkers [134] have detected an inverse H/ H isotope effect of 0.59 in the Co(iii)-catalyzed polymerization of ethylene. This inverse effect was ascribed to a P-agostic CH bond which is stronger in the ground state than the free C-H bond during or just prior to the insertion step (see Scheme 1.6). 

Homonuclear as well as heteronuclear 2D shift correlation experiments ( H/ H-COSY, H/ C-COSY, H/C COSY- H/ H-TOCSY), involving the perturbation of either one or two types of nuclei respectively and in the heteronuclear case including both the conventional, direct C detection, as well as the more sensitive, indirect ( inverse or reverse ) H-detection. 

CHI 2D H/ C shift correlation spectrum with H detection ( inverse ... 

ID ll spectrum, and crosspeaks are arranged symmetrically around the diagonal. There is only one radio frequency channel in a homonuclear experiment, the H channel, so the center of the spectral window (set by the exact frequency of pulses and of the reference frequency in the receiver) is the same in If and F (Varian tof, Bruker ol). The spectral widths should be set to the same value in both dimensions, leading to a square data matrix. Heteronuclear experiments have no diagonal, and two separate radio frequency channels are used (transmitter for F2, decoupler for F ) with two independently set spectral windows (Varian tof and dof, sw, and swl, Bruker ol and o2, sw(If), and sw(I )). Heteronuclear experiments can be further subdivided into direct (HETCOR) and inverse (HSQC, HMQC, HMBC) experiments. Direct experiments detect the X nucleus (e.g., 13C) in the directly detected dimension (Ff) using a direct probe (13C coil on the inside, closest to the sample, H coil on the outside), and inverse experiments detect XH in the To dimension using an inverse probe (XH coil on the inside, 13C coil outside). 

The CH COSY technique with carbon-13 detection and proton decoupling, and more sen-sitive and thus time saving methods with H detection and 13C decoupling denoted as inverse CH COSY, correlate l3C shifts in one dimension with the lH shifts in the other via one-bond CH coupling JCH-... 

Glass et al., were the first to mention the enormous sensitivity gain achievable by H-detected Se correlation spectroscopy " They showed for a number of representative organoselenium compound that the theoretical SIN ratio enhancement factor of 72, as compared to direct Se detection, can nearly be achieved so that this technique should be applied for recording biomolecules in low concentration, e.g., in selenoproteins. If inverse detection is combined with isotopic enrichment of Se, the factor may be increased up to 800. Selenomethionine, selenocysteine, and related compounds have been recorded using Se, H HETCOR experiments. ... 

When modern NMR was in its infanq, few if any scientists realized that they would ever require a two-coil NMR probe in which the coil lies inside the broadband coU. It was not until the demonstration that experiment time can be drastically reduced for the acquisition of 2-D heteronuclear correlation NMR data sets by employing H detected methods and a two-coil NMR probe with the coil configurations inverted (relative to the normal coil configuration described above) that the inverse probe really caught on. Now the inverse probe (a probe with the coil closer to the sample than the broadband coil) is a staple in nearly every modern NMR laboratory. 

Heteronuclear correlation experiments can be useful in terms of identification and quantification of traces in complex mixtures such as soil or water. The detection of trace amounts of organophosphorus compounds related to the chemical weapons convention impressively demonstrates how the advantages of each nucleus were combined, namely the sensitivity of the H NMR spectra and the freedom from background noise for P NMR spectra [26]. This was achieved by measuring inverse H- P NMR spectra utilizing the HSQC experiments. These techniques will be introduced to drug analysis in body fluids. 

The classical approach to generating multiplicity-edited proton spectra was to use the INEPT or DEPT sequences in reverse to transfer initial carbon magnetisation onto the proton for detection. These suffered from low sensitivity and poor suppression of H— resonances, so they were never popular. Greater sensitivity can be achieved by using tbe inverse H approach combined with multiple-quantum filtering [24, 30] as for... 

Live DH, Armitage IM, Dalgarno DC, Cowburn D (1985) Two-dimensional H- Cd chemical-shift correlation maps by H-detected multiple-quantum NMR in metal complexes and metalloproteins. J Am Chem Soc 107 1775-1777 Martin GE, Crouch RC (1991) Inverse-detected two-dimensional NMR methods applications in natural products chemistry. J Nat Prod 54 1-70 Martin GE, Crouch RC (1994) Two-dimensional NMR experiments in natural and unnatural products chemistry. In Croasmun WR, Carlson RMK (eds) Two-dimensional NMR spectroscopy - applications for chemists and biochemists, chap 11, 2nd edn. VCH, New York, pp 873-914... 

Figure 4 The NMR pulse sequence for the H-detected (inverse detection), correlation 2D NMR spectrum...

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