Cross heteronuclear

CH COSY Correlation via one-bond CH coupling, also referred to as HETCOR (heteronuclear shift correlation), provides carbon-13- and proton shifts of nuclei in C//bonds as cross signals in a 5c versus 8h diagram, assigns all C//bonds of the sample... 

A more useful type of 2D NMR spectroscopy is shift-correlated spectroscopy (COSY), in which both axes describe the chemical shifts of the coupled nuclei, and the cross-peaks obtained tell us which nuclei are coupled to which other nuclei. The coupled nuclei may be of the same type—e.g., protons coupled to protons, as in homonuclear 2D shift-correlated experiments—or of different types—e.g., protons coupled to C nuclei, as in heteronuclear 2D shift-correlated spectroscopy. Thus, in contrast to /-resolved spectroscopy, in which the nuclei were being modulated (i.e., undergoing... 

The HETCOR spectrum of a naturally occurring isoprenylcoumarin is shown in Fig. 5.41. The spectrum displays one-bond heteronuclear correlations of all protonated carbons. These correlations can easily be determined by drawing vertical and horizontal lines starting from each peak. For example, peak A represents the correlation between a proton resonating at 8 1.9 and the carbon at 8 18.0. Similarly, cross-peaks E and F show that the protons at 8 4.9 and 5.1 are coupled to the same carbon, which resonates at 8 114.4 i.e., these are the nonequivalent protons of an exomethylenic... 

The one-bond HETCOR spectrum and C-NMR data of podophyllo-toxin are shown. The one-bond heteronuclear shift correlations can readily be made from the HETCOR spectrum by locating the posidons of the cross-peaks and the corresponding 5h and 8c chemical shift values. The H-NMR chemical shifts are labeled on the structure. Assign the C-NMR resonances to the various protonated carbons based on the heteronuclear correlations in the HETCOR spectrum. 

The HMBC spectrum of podophyllotoxin is shown. The cross-peaks in the HMBC spectrum represent long-range heteronuclear H/ C interactions within the same substructure or between different substructures. Interpretation should start with a readily assignable carbon (or proton), and then you identify the proton/s (or carbon/s) with which it has coupling interactions. Then proceed from these protons, and look for the carbon two, three, or, occasionally, four bonds away. One-bond heteronuclear interactions may also appear in HMBC spectrum. 

The one-bond hetero-COSYspectrum of 7-hydroxyfrullanoIide exhibits interactions for all nine protonated carbons. The most downfield crosspeaks, K and L, represent one-bond heteronuclear correlations of the two vinylic exomethylenic protons resonating at 8 5.71 and 6.06 with the C-13 carbon (8 120.5). The C-6a proton, which resonates downtield at 8 4.97 due to the directly bonded oxygen atom, displays correlation with the carbon resonating at 8 80.9 (cross-peak D). Cross-peaks G and M represent h interactions of the C-1 methylene protons (8 1.33 and 1.31, respectively) with C-1 (8 38.1). Similarly, cross-peaks E and F display heteronuclear interactions of the C-8 methylenic protons (8 1.48 and 1.72) with C-8 (8 30.7), while cross-peak C couplings of C-3 methylene protons at 8 1.97 and 1.99 with C-3 (8 32.5). Couplings between the C-1 methylene protons and C-1 (8 38.1) can be inferred from cross-peak A, though in this case both the C-1 a and protons resonate very close to each other (i.e., 8 1.31 and 1.33). Cross-peak C is due to C-9 methylene, while cross-peak I represents the C-15 methyl. The heteronuclear interactions between the most upheld C-2 methy-... 

The most downfield cross-peaks, V-Y, are due to heteronuclear couplings of the aromadc or vinylic protons and carbons. For instance, cross-peak Y represents heteronuclear interaction between the C-1 vinylic proton (8 5.56) and a carbon resonating at 8 134.0 (C-1). The downfield cross-peaks, V and W, are due to the heteronuclear correlations of the ortho and meta protons (8 7.34 and 7.71) in the aromatic moiety with the carbons resonating at 8 128.3 and 126.9, respectively. The remaining cross-peak X is due to the one-bond correlation of the C-4 aromatic proton (8 7.42) with the C-4 carbon appearing at 8 131.4. The cross-peak U displays direct H/ C connectivity between the carbon at 8 77.9 (C-6) and C-6 methine proton (8 4.70). The crosspeak T is due to the one-bond heteronuclear correlation of carbon... 

The HMQC spectrum of podophyllotoxin shows heteronuclear crosspeaks for all 13 protonated carbons. Each cross-peak represents a one-bond correlation between the C nucleus and the attached proton. It also allows us to identify the pairs of geminally coupled protons, since both protons display cross-peaks with the same carbon. For instance, peaks A and B represent the one-bond correlations between protons at 8 4.10 and 4.50 with the carbon at 8 71.0 and thus represent a methylene group (C-15). Cross-peak D is due to the heteronuclear correlation between the C-4 proton at 8 4.70 and the carbon at 8 72.0, assignable to the oxygen-bearing benzylic C-4. Heteronuclear shift correlations between the aromatic protons and carbons are easily distinguishable as cross-peaks J-L, while I represents C/H interactions between the methylenedioxy protons (8 5.90) and the carbon at 8 101.5. The C-NMR and H-NMR chemical shift assignments based on the HMQC cross-peaks are summarized on the structure. 

The HMQC spectrum of vasicinone shows nine cross-peaks representing seven protonated carbons, since two of them (i.e., A and B, and C and D) represent two methylene groups. The C-4a and )8 methylene protons (8 2.70 and 2.20) show one-bond heteronuclear correlations with the carbon resonating at 8 29.4 (cross-peaks A and B), while the C-So and )3 methylene protons (8 4.21 and 4.05) exhibit cross-peaks... 

Multidimensional and heteronuclear NMR techniques have revolutionised the use of NMR spectroscopy for the structure determination of organic molecules from small to complex. Multidimensional NMR also allows observation of forbidden multiple-quantum transitions and probing of slow dynamic processes, such as chemical exchange, cross-relaxation, transient Over-hauser effects, and spin-diffusion in solids. 

Dipolar recoupling may also be accomplished using continuous rf irradiation as demonstrated in the heteronuclear and homonuclear case by the CP at MAS conditions (or for low-y heteronuclear spins called double-cross-polarization, DCP [103]) and HORROR (homonuclear rotary resonance) [26] experiments, respectively. These experiments may easily be described by transforming the description into the interaction frame of the rf irradiation using (14a) exploiting... 

Recently a new type of proton assisted recoupling experiments has been developed for coherence transfer where rf irradiation is taking place on all involved rf channels. This embraces the homonuclear proton assisted recoupling (PAR) [45, 140, 141] and the later resonant second-order transfer (RESORT) [142] experiments, as well as the heteronuclear proton assisted insensitive nuclei (PAIN) cross polarization [44] experiments. In PAR and PAIN, spin-lock CW irradiation is applied on both passive ( H) and active spins (13C, 15N) without matching rotary resonance conditions. In RESORT a phase alternation irradiation scheme for the passive spins is used. 

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