Trans protons

The couplings of vicinal protons in 1,2-disubstituted alkenes lie in the range 6-12 Hz for cis protons (dihedral angle 0°) and 12-17 Hz for trans protons (dihedral angle 180°), thus also following the Karplus-Conroy equation. Typical examples are the alkene proton AB systems of coumarin (16a, cis) and tra 5-cinnamic acid (16b), and of the cis-trans isomers 17a and b of ethyl isopente-nyl ether, in addition to those in problems 3, 4, 8, 11, 13 and 38. 

The position of the second CC double bond in the structural fragment E follows finally from the correlation of the C signals at 5c = 37.8 and 49.8 with the //signals at 3h = 4.47 and 4.65. Note that trans protons generate larger cross-sectional areas than cis protons as a result of larger scalar couplings. 

On the methylene bridge of 9 the proton cis to the peroxide linkage (HA) appears at 8 1.87 (solvent PhH) and shows long range W-plan coupling with the 5-, and 6-endo-protons (HD), while the corresponding trans-proton (HB) exhibits no 4-bond coupling and is at 8 1.35. Compound 9a shows a similar spectrum but offset to lower field by about 0.8 ppm 28). 

Determining whether an alkene is cis or trans in cases where the alkene is in the middle of a long alkyl chain is usually not possible by H NMR as both cis and trans protons have very similar shifts in such circumstances, as do the -CH2s attached to the alkene. Such a problem can however be dealt with using 13C NMR where the shifts of these CH2s are diagnostic. 

Cis - and trans isomers can be easily distinguished by NMR spectroscopy because cis and trans protons have different chemical shifts and coupling constant. 

Further evidence for this mechanism of spin-spin coupling in liquids is the fact that the coupling dies off as the number of chemical bonds separating the two nuclei increases. Moreover, the fact that couplings between trans protons in olefins are greater than between cis protons is incompatible with a direct interaction between the two nuclear magnetic dipoles. 

Electronegative substituents within the coupling path decrease while those at the coupling carbon atoms increase 3JCH with cis and trans protons, as shown for 2- and 3-bromopropene [129]. 

Later, Huisgen et al. 1 put forward a scheme which was similar to that of Dolfini. Their work indicated that both cis and trans adducts arose from the same intermediate, giving the cis adduct by internal proton shift, and both adducts, but mostly the trans compound, through the agency of external proton donors. Dolfini had shown that these adducts do not isomerize spontaneously below 80° and that the stability of the adducts to acid is high. In the NMR spectrometer the larger coupling constant for trans protons over cis protons provides a simple method for structure and product ratio determination. 

Fig. 3. The chemical shift of the trans proton vs. the mole ratio (CH3)/[(CH3) + (C2H3)] at various temperatures A, — 50° B, —30° C, —4° andD, +36°C. Each curve is offset by 2 cps (one unit on the graph) on the chemical shift scale (in cps, 60 MHz).

Hb has two nearby nonequivalent protons that split its signal, the getninal proton He and the trans proton Hj. Hj splits the Hb signal into a doublet, and the He proton splits the doublet into two doublets. This pattern of four peaks is called a doublet of doublets. 

In phenyloxiranes, the protons adjacent to the aromatic ring absorb at lower field than the cis and trans protons. This low-field shift can be interpreted in terms of the ring current of the phenyl group. In 1,2-diphenyloxiranes there is resonance at lower field for the cis isomers. The effect of this has been calculated on... 

Daniewski and coworkers noticed that the signal of the proton H-3 cis to the C-8 acetoxy group was shifted to lower field than that of the corresponding trans proton (87). 

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