Proton Couplings

Proton-proton coupling constants of benzo rings of benzazoles can illuminate the bonding in such compounds. Thus, comparison of the J values for naphthalene with those for benzotriazoles of different types (Table 13) shows evidence of bond fixation, particularly in the 2-methyl derivative (98) (71PMH(4)l2l). 

Table 13 Proton-Proton Coupling Constants (Hz) in Benzotriazoles ...

The NMR spectra of heterocyclic compounds with seven or more ring members are as diverse as the shape, size and degree of unsaturation of the compounds. NMR is perhaps the most important physical method to ascertain the structure, especially the conformational statics and dynamics, of large heterocycles. Proton-proton coupling constants provide a wealth of data on the shape of the molecules, while chemical shift data, heteroatom-proton coupling constants and heteronuclear spectra give information of the electronic structure. Details are found in Chapters 5.16-5.22. Some data on seven-membered rings are included in Table 10. 

Useful compilations of NMRspectra are , . The H and NMR chemical shifts and the geminal and vicinal proton-proton coupling constants for oxirane and other heterocycles are given in a very readily compared manner,... 

H- H coupling constants, 4, 558 ring proton coupling to side chain atoms, 4, 559 C NMR, 4, 564-569, 940... 

Figure 1 The principal sources of structural data are the NOEs, which give information on the spatial proximity d of protons coupling constants, which give information on dihedral angles < i and residual dipolar couplings, which give information on the relative orientation 0 of a bond vector with respect to the molecule (to the magnetic anisotropy tensor or an alignment tensor). Protons are shown as spheres. The dashed line indicates a coordinate system rigidly attached to the molecule.

The connectivities found in the HH COSY diagram reveal two partial stuctures A and B according to Table 44.1. These are supported by the identical proton-proton coupling constants of the coupling protons. 

HMQC Heteronuclear multiple quantum coherence, e.g. inverse CH correlation via one-bond carbon proton-coupling, same format and information as described for ( C detected) CH COSY but much more sensitive (therefore less time-consuming) because of H detection... 

The n + 1 mle should be amended to read When a proton coupled to H , Hj, etc., and J b Jac Jad etc., the original signal for H, is split into n + 1 peaks by n H , protons, each of these lines is further split into n + 1 peaks by n protons, and each of these into n + 1 lines by n protons, and so on. Bear in mind that because of overlapping peaks, the number of lines actually observed can be less than that expected on the basis of the splitting rule. 

The substrates of catabolism—proteins, carbohydrates, and lipids—are good sources of chemical energy because the carbon atoms in these molecules are in a relatively reduced state (Figure 18.9). In the oxidative reactions of catabolism, reducing equivalents are released from these substrates, often in the form of hydride ions (a proton coupled with two electrons, H ). These hydride ions are transferred in enzymatic dehydrogenase reactions from the substrates... 

Rule 3 Two groups of protons coupled to each other have the same coupling constant, f. 

A long-range proton coupling, which was found to be transmitted by a sulfone group in thiolane dioxide systems292, is apparently facilitated by a nonbonding p-orbital on one of the sulfone oxygen atoms. This phenomenon is of interest for saturated cyclic systems. 

It is interesting to note that, despite drastic changes in the chemical frameworks of primary bisphosphines, there are minimal/no differences in the chemical shifts and coupling constants (Table 1). The proton coupled P NMR... 

Enterocytes in the proximal duodenum are responsible for absorption of iron. Incoming iron in the Fe " state is reduced to Fe " by a ferrireductase present on the surface of enterocytes. Vitamin C in food also favors reduction of ferric iron to ferrous iron. The transfer of iron from the apical surfaces of enterocytes into their interiors is performed by a proton-coupled divalent metal transporter (DMTl). This protein is not specific for iron, as it can transport a wide variety of divalent cations. 

Figure 5.5 shows the heteronuclear 2Dy-resolved spectrum of camphor. The broad-band decoupled C-NMR spectrum is plotted alongside it. This allows the multiplicity of each carbon to be read without difficulty, the F dimension containing only the coupling information and the dimension only the chemical shift information. If, however, proton broad-band decoupling is applied in the evolution period tx, then the 2D spectrum obtained again contains only the coupling information in the F domain, but the F domain now contains both the chemical shift and the coupling information (Fig. 5.6). Projection of the peaks onto the Fx axis therefore gives the Id-decoupled C spectrum projection onto the F axis produces the fully proton-coupled C spectrum.

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... 

How can we correlate protons that are coupled to each other (proton coupling network) ... 

Is it possible to obtain proton-proton coupling information from a ID H-NMR experiment ... 

Homonuclear shift-correlation spectroscopy (COSY) is a standard method for establishing proton coupling networks. Diagonal and off-diagonal peaks appear with respect to the two frequency dimensions. 

COSY (H-H correlation spectroscopy) An important 2D experiment that allows us to identify the protons coupled to one another. 

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