Methyl protons, magnetic environment

The physical basis for peak splitting mil dichloroethane can be explained with the aid of Figure 13 13 which examines how the chemical shift of the methyl protons IS affected by the spin of the methme proton There are two magnetic environments for... 

The same nucleus (say methyl protons) in different chemical environments A and B will generally have nuclear magnetic resonances at different frequencies. If the exchange of pro-... 

It has also been possible to confirm the presence of the reduction product of a Schiff base on the polymer by proton magnetic resonance. For this purpose we have used unmodified poly(ethylenimine), since it too catalyzes the decarboxylation of oxalacetate to its product, pyruvate. Unmodified polyethylenimine was mixed with oxalacetate-4-ethyl ester. One-half of this solution was treated with NaBH4 the second half was exposed to a similar environment but no NaBH4 was added. The borohydride-treated polymer exhibited a strong triplet in the nmr spectrum centered at 3.4 ppm upfield from the HOD resonance. This new feature would be expected from the terminal methyl protons of the oxalacetate ester attached to the polymer. Only a very weak triplet was found in the control sample not treated with borohydride. These observations are strong evidence for the formation of Schiff bases with the polymer primary amine groups. Evidently the mechanistic pathway for decarboxylation by the polymer catalyst is similar to that used enzymatically. 

So, the methylene (CH2) protons can exist in four magnetic environments (due to the different orientations of the three methyl protons) and so will give four lines (a quartet) in the H NMR spectrum, of relative intensities 1 3 3 1 (Figure 4.13b). 

If all protons were shielded by the same amount, they would all be in resonance at the same combination of frequency and magnetic field. Fortunately, protons in different chemical environments are shielded by different amounts. In methanol, for example, the electronegative oxygen atom withdraws some electron density from around the hydroxyl proton. The hydroxyl proton is not shielded as much as the methyl protons, so the hydroxyl proton absorbs at a lower field than the methyl protons (but still at a higher field than a naked proton). We say that the hydroxyl proton is deshielded somewhat by the presence of the electronegative oxygen atom. 

The most stable conformation of 1,2-dichloropropane (in the margin) shows that the diastereotopic protons on Cl exist in different chemical environments. They experience different magnetic fields and are nonequivalent by NMR. The NMR spectrum of 1,2-dichloropropane is shown in Figure 13-35. The methyl protons appear as a doublet at 51.6, and the single proton on C2 appears as a complex multiplet at 54.15. The two protons on Cl appear as distinct absorptions at 53.60 and 53.77. They are split by the proton on C2, and they also split each other. 

For closely related compounds, absolute configuration has been determined from NMR spectra in the presence of chiral shift reagents [55]. For compounds with greater structural differences there are many difficulties and uncertainties to overcome before concrete results can be obtained. One of the problems is the manner of variation of induced differential shift, AA5 of resonances of protons as a function of reagent-substrate molar ratio as shown for 2-phenyl-2-butanol in Fig. 10.23. It is clear from the figure that AA<5 for a-methyl resonances increase steadily, while for -methyl protons reaches a maximum, then declines and reaches plateau. For the ortho protons, a reversal in the sense of nonequivalence occurs at a molar ratio of approximately unity. Some of the implications of such a behaviour are both theoretical and practical. If the complex formation constants for enantiomers are different, then the sense of non-equivalence should be the same for all the proton resonances, and AA<5 should increase to a maximum and level off. Since this is not the case, different magnetic environments or stoichiometries of shift reagent-substrate adducts may be the factors for the observed anomalous variation of AA5. 

F nuclear magnetic resonance data there are two fluorine environments, a low-field doublet and a high-field triplet, each component being further split into a septuplet because of coupling between F and the methyl protons. The doublet is due to apical F atoms /p.f = 772 c.p.s. 5 = — 74p.p.m. (from an external trifluoroacetic acid reference). (Checkers find 776 and —74.3, respectively.) The high-field triplet is due to the equatorial F atoms Jp.f = 960 c.p.s. 5 = 4-9.8 p.p.m. (from an external trifluoroacetic acid reference). (The checkers find 960 and 4-9.8, respectively.)... 

Nuclear magnetic resonance spectroscopy is such a powerful tool for stracture determination because protons in different environments experience different degrees of shielding and have different chemical shifts. In compounds of the type CH3X, for example, the shielding of the methyl protons increases as X becomes less electronegative. Inas-... 

Table V displays the XH-NMR data of [Cp2Yb(ketim)]2 at various temperatures. The rather different isotropic shifts(opposite signs ) of the two methyl groups suggest their location in fairly different "magnetic environments". It is, however, not possible to decide if there are N- or O-bridging links. As only one Cp ring proton resonance appears, some fluxional behaviour of the bridging ketim ligand is not unlikely.

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