D NMR spectra

Markham, K.R., Mues, R., Stoll, M. and Zinsmeister, H.D. NMR Spectra of Flavone Di-C-glycosides from Apometzgeria pubescens and the Detection of Rotational Isomerism in 8-C-Hexosylflavones. Z. Naturforsch. 42c, 1987, pp 1039-42... 

In this protocol, 20 mg cyanidin 3-(2 - glucosyl-6" - rhamnosylglucoside) dissolved in 0.5 ml CDfDD/CFfCOOD (95 5 v/v) represents a typical sample. Using a relatively concentrated sample ( 50 mM) facilitates shorter experiment time and better quality of the 1-D 13C and 2-D NMR spectra. 

Two-dimensional NMR spectra are mainly produced as contour maps. These maps may be best imagined as looking down on a forest where all the trees (representing peaks in the spectrum) have been chopped off at the same fixed height. 2-D NMR spectra are produced by homonuclear (1H-1H) and heteronuclear ( H- C) experiments. 

NMR spectroscopists collect this type of classical or one-dimensional (1-D) NMR spectra on modem FT-NMR instruments by applying a 90° pulse and collecting the FID signal that is induced in the receiving coil, To make... 

At this point, it is possible to solve the overall structure by using the wealth of information in the 1 -D NMR spectra. This (traditional) approach will be explored first, followed by modern use of 2-D NMR spectra. Let us note at this time that the stereochemistry of this molecule cannot be proved using simple H and I3C NMR spectra. 

The remarkable development of NMR now demands four chapters. Identification of difficult compounds now depends heavily on 2-D NMR spectra, as demonstrated in Chapters 5,6,7, and 8. 

The specttal data required to support an identification by NMR are the sample specnum, the reference specnum, and the specnum of a blank sample (if available). All these should be recorded under comparable conditions. The reference specnum may be from a library or database, it may be recorded from the authentic reference chemical, or it may be a specnum of a closely related chemical in this last case, specnal interpretation must be enclosed (43). Other 1-D and 2-D NMR spectra (see Sections 5.2 and 5.3) may have been recorded from the test sample to support the interpretation and identification. 

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. 

The characteristic H(4)-H(5) H NMR coupling constants of the 3-oxabicyclo[3.2.0]heptane derivatives anti-% (V//,//=1.3 Hz) and ry -51 Jh,h = 5.6 Hz) have been used to establish the structural assignments for these isomers <20060L491>. The structure of the rearranged cembrane derivative ciereszkolide 52 has been elucidated by onedimensional (TD) and 2-D NMR spectra, high-resolution fast atom bombardment mass spectrometry (HRFAB-MS), infrared (IR), and ultraviolet (UV) as well as by single crystal X-ray analysis <2004EJ03909>. 

Figure 2.24, Determination of the enantiomeric excess of 1-phenylethanol [30, 0.1 mmol in 0.3 ml CDCI3, 25 °C] by addition of the chiral praseodymium chelate 29b (0.1 mmol), (a, b) NMR spectra (400 MHz), (a) without and (b) with the shift reagent 29b. (c, d) NMR spectra (100 MHz), (c) without and (d) with the shift reagent 29b. In the NMR spectrum (d) only the C-a atoms of enantiomers 30R and 30S are resolved. The and C signals of the phenyl residues are not shifted these are not shown for reasons of space. The evaluation of the integrals gives 73 % R and 27 % S, i.e. an enantiomeric excess (ee) of 46 %...

The COSY spectra are DQF-COSY spectra. The labeled frequency for all 2-D NMR spectra is that of the acquired signal (F2). 

Unless otherwise labeled, the NMR spectra were obtained at 300 MHz for protons, 75.5 MHz for 13C CDC13 was the solvent unless otherwise labeled. The IR spectra were obtained neat (i.e., no solvent) unless otherwise labeled. The mass spectra were obtained by GC/MS. CH4 was used to obtain the chemical ionization mass spectra. The COSY spectra are DQF-COSY spectra. The labeled frequency axis for all 2-D NMR spectra is that of the acquired signal (F2). The FI axis may also be labeled. 

The NMR and C NMR spectra discussed in the preceding sections have one frequency axis and one intensity axis 2-D NMR spectra have two frequency axes and one intensity axis. The most common 2-D spectra involve H- H shift correlations they identify protons that are coupled (i.e., that split each other s signal). This is called H- H shift-correlated spectroscopy, which is known by the acronym COSY. 

D NMR spectra that show shift correlations are called HETCOR (from... 

RB=NR and alkynes <20040M5048>. The structure and stereochemistry of the novel bora derivatives of T[(2-hydroxy-l-naphthyl)methyl]proline were elucidated by means of two-dimensional nuclear magnetic resonance (2-D NMR) spectra as well as AMI semi-empirical calculations <2005TA2019>. 

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