Shielding and Deshielding

Thus, deshieldings and shieldings alternate along the polyene chain, as demonstrated by the carbon-13 shifts of N,N-dimethylaminoacrolein, N,N,N, N -tetramethyltri-methinecyanine and their vinyl analogs [343],... 

The antimony(-1-5) chemical shift range spans approximately 3500 ppm. Rather more information is available for Sb than for As NMR chemical shifts. To date, (SbS4) and (SbBre) occupy the deshielded and shielded ends of the range, respectively. 

The reverse mesomeric effect (p -p conjugation) is believed to be very favourable in the A -phospholen system (5). Compared with the corresponding A -phospholens (6), the conjugated system (5) shows both Y and the phosphorus atom to be deshielded, and the vinyl proton is shielded. The cyclic nature of the molecule is important because the analogous... 

Oxygen functionalities, such as alcohols, ethers, and acetate groups, and halogens at the allylic position, deshield when the fluorine is at the terminal position and shield when it is at the internal 2-position (Scheme 3.43). 

In the case of pyridine, large differences in chemical shift are observed for fluorines at the 2-, 3-, and 4-positions, with fluorines at the 2-position of pyridines and quinolines being the most deshielded, and those at the 3-position being the most shielded. (Scheme 3.60). 

Most of the multifluoro-substituted pyridines were prepared more than 30 years ago in the Birmingham fluorine group using CoF3 technology. Scheme 6.34 provides the multitude of fluorine and proton chemical shift data that were accumulated at that time. It will be seen that, all other things being equal, fluorines at the 2-position are most deshielded and fluorines at the 3-position are the most shielded. Scheme 6.35 provides a few examples of substituted tetrafluoropyridines. 

Several formylfuranboronic acids and similar compounds have been prepared by regular methods and studied with respect to their 13C and H NMR spectra. As in the benzene series, the protons ortho to the boron are deshielded and the meta protons are shielded. Good conjugation exists between the furan ring (donor) and both the boron atom and the carbonyl group (acceptors). A Hammett-Jaffe analysis yielded ap+ + 0.256 for the B(OH)2 group.234... 

A different pH-triggered deshielding concept with hydrophilic polymers is based on reversing noncovalent electrostatic bonds [78, 195, 197]. For example, a pH-responsive sulfonamide/PEl system was developed for tumor-specific pDNA delivery [195]. At pH 7.4, the pH-sensitive diblock copolymer, poly(methacryloyl sulfadimethoxine) (PSD)-hZocA -PEG (PSD-b-PEG), binds to DNA/PEI polyplexes and shields against cell interaction. At pH 6.6 (such as in a hypoxic extracellular tumor environment or in endosomes), PSD-b-PEG becomes uncharged due to sulfonamide protonation and detaches from the nanoparticles, permitting PEI to interact with cells. In this fashion PSD-b-PEG is able to discern the small difference in pH between normal and tumor tissues. 

This is one of the quirks in the numbering system. Physically and psychologically, a minus means less (less shielding), and DOWNfield is further left on the paper yet the value of <5 goes UP. Another system uses the Greek tau (t) — that s 10.0-<5. So <50.0 (ppm) is lO.Ot. Don t confuse these two systems. And don t ever confuse deshielding (or shielding) with the proper direction of the chemical shift. 

In the light of the appreciable puckering found in the seven-membered ring of [16], Childs (1984) recalculated the expected chemical shifts for the exo and endo H(8) protons of [12]. He calculated the difference in chemical shift (A6) to be 6.9 ppm which is in good agreement with the observed AS - 5.86 ppm. However, his calculations revealed that both the exo and endo protons are shielded. This surprising result is opposed to the accepted intuitive view that in an aromatic/homoaromatic system protons with the H(8)(exo) orientation should be deshielded and those with the H(8)(cndo) orientation shielded. This result closely parallels the analogous calculated data for the homocyclopropenyl cation [2] (Schindler, 1987). 

Carbon-13 shift of common non-aromatic heterocycles with endo- and exocyclic double bonds are reviewed in Table 4.66 [416-432], - Deshieldings of / -carbons induced by carbonyl groups in heterocyclic a, /1-enones due to (—)-M electron withdrawal (e.g. 2-pyrones, coumarins) and shieldings of [ carbons in cyclic enol ethers arising from (+ )-M electron release (e.g. 2,3-dihydrofuran and oxepine derivatives in Table 4.66) fully correspond to the effects described for the open-chain analogs. Outstandingly large shift values are observed for the lithiated carbon in cyclic a-lithium enol ethers (Table 4.66). In terms of its a and / carbon-13 shifts, 2,7-dimethyloxepine is also a typical enol ether [420], Further, 2,6-dimethyl-4-pyrone [421] and flavone [422] display similiar shift values for the a, /1-enone substructure. 

The equilibrium favors the alkylmagnesium halide, however, upon addition of hexa-methylphosphoramide, as monitored by a deshielding of the benzylic carbon and shield-ings of benzenoid C-l and C-4 in benzylmagnesium chloride [479],... 

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