Shifts of Hydrogen Atoms

Partial Structural Chemical Shift Partial Structural Chemical Shift 

Explain why the chemical shift of the hydrogen atoms of (CH3)4Sn appears at higher field thanTMS. 

3-Dichlorobutane has resonances 1.60, 2.15, 3.72, and 4.27 ppm downfield from TMS. Assign each resonance to the individual hydrogen atoms. 

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Transannular shifts of hydrogen atoms have also been observed. 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

The comments made about the diradical hypothesis with respect to the photochemistry of cyclopentanone are equally applicable to cyclohexanone. Since the formation of none of the products listed in reactions 15-18, and (15,29) is quenched by even 10-20 mm. of oxygen the existence of diradical intermediates in this system is subject to question. The alternative mechanism would be one that causes a concerted split of the ketone molecule in the excited state into two (in the cases of reactions 15 and 16) or three (reaction 17) molecular fragments. Both 16 and 17 are analogous to reactions 3 and 2 in the photochemistry of cyclopentanone and do not involve a shift of hydrogen atom from one... 

Proton NMR spectra of many criss-cross adducts formed from hexafluoroacetoneazine have been reported <75CB1460, 79LA133, 82LA845, 82LA853). -Shifts of hydrogen atoms of some typical bicyclic compounds of this type, including one partially saturated one, are summarized in Scheme 4. 

Table 9.1 H NMR chemical shifts of hydrogen atoms attached to carbon atoms in different environments...

Table 8.3 Chemical Shifts of Hydrogen Atoms Attached to Various Functional Groups...

Polymers with branched alkanes as recurrent units, as in polypropylene, react with peroxides by an intra-molecular zip reaction. Fig. 4.44. This intra-molecular shifting of hydrogen atoms encourages the formation of hydrogen peroxides. 

Indirectly, chemical shifts can be used in two ways for the determination of protein structures. An old idea is based on computations approximating the ring currents in aromatic residues and their relatively large contribution to the chemical shifts of hydrogen atoms. This approach is used in a qualitative way to explain large deviations of chemical shifts observed in folded proteins from the values they would have in unfolded polypeptides. This use of chemical shifts is primarily a test applied to a final structure rather than a tool to determine this structure. 

The characteristic high-field shift of hydrogen atoms bound to transition metal complexes has been suggested already... 

A final prosthetic group that can be studied is the phosphate moiety of vitamin 6,2 (S-deoxy adenosyl cobalamin). This group is instrumental for the shifting of hydrogen atoms as part of the mechanism of several enzymes. 

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