Inverse Proton Detected Correlation Spectroscopy

For smaller quantities of compounds more sensitive inverse detected techniques are available, such as HMQC ( IH-I C one bond correlation via heteronuclear multiple quantum coherence, analogous to HETCOR) and HMBC (proton detected heteronuclear multiple bond correlation spectroscopy) (15). The last provide, in addition to the intraresidue multiple bond correlations, interresidue correlations between the anomeric carbon and the aglycone protons.We follow this general strategy for the structural determination of tri terpenoid saponins of Bupleurum fruticosum (16) andArdisia japonica (9). 

With regard to the wealth of pulse schemes for X, "Y correlation, some consideration has to be put in the selection of the best-suited method for a certain application. In H,"X correlation spectroscopy, the high receptivity of protons makes inverse detection schemes generally the most effective protocol from the point of sensitivity. 2D experiments with detection are... 

In the structure elucidation of unknown compounds, NMR is routinely used in conjunction with NMR spectroscopy. An important two-dimensional technique is inverse H, C-correlation to assign direct correlations between adjacent protons and carbon atoms. Due to the low natural abundance of the isotope of 1.1%, several hours of accumulation and injected sample amounts in the low pg range in the 1.5 pL detection volume are required for successful 2D NMR experiments. 

In the case of an unknown chemical, or where resonance overlap occurs, it may be necessary to call upon the full arsenal of NMR methods. To confirm a heteronuclear coupling, the normal H NMR spectrum is compared with 1H 19F and/or XH 31 P NMR spectra. After this, and, in particular, where a strong background is present, the various 2-D NMR spectra are recorded. Homonuclear chemical shift correlation experiments such as COSY and TOCSY (or some of their variants) provide information on coupled protons, even networks of protons (1), while the inverse detected heteronuclear correlation experiments such as HMQC and HMQC/TOCSY provide similar information but only for protons coupling to heteronuclei, for example, the pairs 1H-31P and - C. Although interpretation of these data provides abundant information on the molecular structure, the results obtained with other analytical or spectrometric techniques must be taken into account as well. The various methods of MS and gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy supply complementary information to fully resolve or confirm the structure. Unambiguous identification of an unknown chemical requires consistent results from all spectrometric techniques employed. 

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