Proton 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 GASPE spectrum of podophyllotoxin is shown. The signals at 8 56.0,108.6, and 152.0 each represent two carbons in identical magnetic environments, while the signal at 8 147.6 also represents two carbons that accidentally appear at the same chemical shift. Assign chemical shift values to various protonated and quaternary carbons in the structure. 

Hence, two signals will show-up in the NMR-spectrum corresponding to these two different chemical and magnetic environments. Furthermore, the areas under each signal are in the ratio of the number of protons in each part of the molecule, and thus actual measurement will reveal that the ratio of these areas is 5 3. 

Specificity NMR spectroscopy is a specific analytical method by its nature. Each type of proton located in a different chemical or magnetic environment in the same or different molecule can be differentiated through its characteristic resonance frequency (Figure 12.1A). Thus, every proton-containing chemical has a unique set of proton NMR signals that can be readily identified. Although the NMR signals may be dependent on the solvent conditions (for example, temperature, pH, hydrophobicity, etc.), each molecule still can be uniquely identified in an NMR spectrum. 

Here a is the shielding constant, a dimensionless quantity which is of the order of 10 for protons, since the shielding increases with the number of electrons. It should be noted that a-values are molecular constants which do not depend on the magnetic field. They are determined solely by the electronic and magnetic environment of the nuclei being observed. 

It is known that five-membered Cr(V) chelates are favored over six-membered ones.19,50,61 For Cr(V)-diolato complexes formed with linear diols, it was observed that all of the protons are equivalent in the isotropic EPR spectra,62 although the strain of a six-membered ring imparts inequivalence to the magnetic environment of the protons in the second coordination sphere.63 This observation at room temperature again points to rotational flexibility in the chelate ring (namely, puckering in the 8 or X configuration),20 similar to that observed for 1. 

Fig. 14. Plot of line shape change versus rate of exchange of protons between environments A and B. The intensities of the various line functions are not comparable. Reproduced by permission from "High-Resolution Nuclear Magnetic Resonance, by Pople, Schneider, and Bernstein. McGraw-Hill, New York, 1959.

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. 

The CH3 protons experience different magnetic environments, depending upon the orientation of the methylene protons (Box 4.7). 

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). 

Spin-Spin Splitting. The splitting of a signal into two, three, four, or more peaks which show a binomial distribution pattern is an indication of the number of hydrogen atoms on neighboring carbon atoms which change the effective magnetic environment of the proton under observation by small but predictable amounts. 

Clearly the protons are in closely similar magnetic environments in the two classes of compound, and there can be no doubt that the protonated forms of carboxylic acids and esters are now well-characterized organic compounds. 

The substitution of a different metal into an enzyme provides a very useful method for studying the immediate environment of the metal site. In addition to the use of Co2 for spectral studies, appropriate substitution allows the use of physical methods such as electron paramagnetic resonance (Co . Cu2 ). the Mdssbauer effect tFe2 ). proton magnetic resonance relaxation techniques (Mir ), or X-ray crystallography (with a heavy metal atom to aid in the structure solution). ... 

The 13C spectra discussed above consist of sets of single peaks each one due to a carbon atom in a particular magnetic environment. The majority of proton... 

The development of Fourier transform NMR spectroscopy made carbon NMR (13C NMR or CMR) possible, and high-field superconducting spectrometers allowed it to become nearly as convenient as proton NMR ( H NMR). Carbon NMR determines the magnetic environments of the carbon atoms themselves. Carbonyl carbon atoms, alkyne carbon atoms, and aromatic carbon atoms all have characteristic chemical shifts in the 13C NMR spectrum. 

Proton Magnetic Resonances of Vinylthio Fragments and Their Environment in Vinylthio Derivatives (153-159)"... 

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. 

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