Coupling constants compounds

Compound Solvent H-1 H-2 Chemical shift (S-values) H-3 H-4 H-5 H-6 H-7 H-8 Coupling constants (Hz) Ref... 

As might be anticipated from the behaviour of the parent heterocycles, C-2 of indole, benzo[i]furan and benzo[i]thiophene (Table 13) is shifted to lower field than C-3. However, the shifts for C-2 (O, 144.8 Se, 128.8 S, 126.1 NH, 124.7 Te, 120.8) and C-7a (O, 155.0 Se, 141.3 S, 139.6 NH, 135.7 Te, 133.0) in the benzo[i] heterocycles vary irregularly (80OMR(l3)3l9), and the sequence is different to that observed for C-2 in the parent heterocycles, namely 0>Se>Te>S>NH. Also noteworthy is the upheld position of C-7, especially in indole and benzofuran, relative to the other benzenoid carbons at positions 4, 5 and 6. A similar situation pertains in the dibenzo heterocycles (Table 14), where not only are C-1 and C-8 shifted upheld in carbazole and dibenzofuran relative to the corresponding carbons in dibenzothiophene and fluorene, but similar, though smaller, shifts can be discerned for C-3 and C-6 in the former compounds. These carbon atoms are of course ortho and para to the heteroatom and the shifts reflect its mesomeric properties. Little variation in the carbon-hydrogen coupling constants is observed for these dibenzo compounds with V(qh) = 158-165 and V(c,h) = 6-8 Hz. 

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). 

The possibility offered by new instruments to obtain N NMR spectra using natural abundance samples has made " N NMR spectroscopy a method which holds no interest for the organic chemist, since the chemical shifts are identical and the signal resolution incomparably better with the N nucleus (/ = ) than with " N (/ = 1). H- N coupling constants could be obtained from natural abundance samples by N NMR and more accurately from N-labelled compounds by H NMR. Labelled compounds are necessary to measure the and N- N coupling constants. 

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. 

Structure elucidation does not necessarily require the complete analysis of all multiplets in complicated spectra. If the coupling constants are known, the characteristic fine structure of the single multiplet almost always leads to identification of a molecular fragment and, in the case of alkenes and aromatic or heteroaromatic compounds, it may even lead to the elucidation of the complete substitution pattern. 

Table 2.5. Typical HH coupling constants (Hz) of aromatic and heteroaromatic compounds...

Hence the compound is nona-2,6-dienal. The relative configuration of both CC double bonds follows from the HH coupling constants of the alkene protons in the H NMR spectrum. The protons of the polarised 2,3-double bond are in trans positions Jhh 5.5 Hz) and those on the 6,7-double bond are in cis positions Jhh = 10.5 Hz). The structure is therefore nom.-2-tmns-6-cis-dienal, D. 

Rather large HH coupling constants in the aliphatics range (72.5 and 15.0 Hz) indicate geminal methyl protons in rings. In order to establish clearly the relevant AB systems, it makes sense first to interpret the CH COSY diagram (Table 52.1). From this, the compound contains two methylene groups, A and B. 

Fluoro- carbon Number Compound F-NMR Signal Coupling Constant JfF JpF JhF Jfff trans cis trans CIS... 

Compound No. N- Position 1 K Chemical shift (t v alue) Coupling constant (cps) ... 

H N coupling constants, often measured in H NMR using labeled compounds, are discussed together with N NMR (Section VI,D). Taking into account that H NMR data are now present in every paper dealing explicitly or implicitly with tautomerism, only a few articles are documented in this section. 

Other authors have used coupling constants instead of (or simultaneously with) chemical shifts. In some cases they have been determined by NMR spectroscopy, in other cases, labeled compounds and or NMR spectroscopies have provided these couplings. These couplings have been used for determining tautomeric composition (see the discussion by Begtrup in 87MI371). Most examples involved and... 

Z-Configuration of the initial compounds does not change in the reaction course, as indicated by the coupling constants of the methoxyethenyl group vinyl protons of the triazoles 166 and 167. Adducts of -configuration are present only as admixtures (83DIS). 

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

Consequently, structures 85b and 85c must be considered resonance structures rather than valence isomers. Hyperfine coupling constants were computed for a series of dithiazolyl radicals and related compounds [96MRC913]. An absolute mean deviation of 0.12 mT with respect to experimental data is reported for 10 sulfur hyperfine coupling constants obtained from UB3-LYP/TZVP calculations. 

The ]H NMR spectrum shown is that of a compound isomeric with the one in Problem 19.65. This isomer has an IR absorption at 1730 cm-1. Propose a structure. [Note-. Aldehyde protons (CHO) often show low coupling constants to adjacent hydrogens, so the splitting of aldehyde signals is not always apparent.]... 

From the invariance of the coupling constant 2J(SiP) between HMPA-P and Si over a wide temperature range (compounds 4-7, 9-11) a rigid coordination of the HMPA to silicon can be deduced. Only in the case of the methyl complex 6 above 25 °C is the beginning of exchange of HMPA observed. However, a fast exchange of the coordinated acetonitrile at room temperature has been found for 12. 

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