Carbon nuclei, polarization

Each 13C nucleus is magnetically coupled to the protons bonded to it. Under the right circumstances, this magnetic coupling allows the transfer of polarization from the protons to the carbon nucleus. The number of protons bonded to the 13C nucleus determines how this polarization transfer occurs. A DEPT experiment usually includes three spectral scans ... 

In general, the carbonyl carbon nucleus resonates at low field, in regions free from interference from other absorptions and is uncoupled. Measurement of the carbonyl group chemical shifts has been a particularly fruitful method for gaining information about the electronic structural features of this polar grouping. 

In general, the factors that affect the chemical shifts of carbons are the same as for protons (i.e. electron density around the nucleus in question, and anisotropy effects). Carbon chemical shifts can be readily calculated from tables of shift effects found in many texts. However, unlike protons attached to sp carbons, sp carbons attached to sp carbons exhibit only a small shift difference. There are also few good substituent parameters available for calculating the chemical shifts of alkene carbons bearing polar groups, unlike the calculation of NMR chemical shifts near polar groups. However, in systems where resonance is present, some predictions can be made of relative shift differences in the carbons (see Figure 1). 

The solvated phosphorane adds to the polarized carbonyl with the incipient C-21 methyl group pointing away from the bulk of the steroid nucleus. The newly formed carbon-carbon bond must then rotate in order for the tri-phenylphosphine group and oxygen atom to have the proper orientation for the elimination of triphenylphosphine oxide. This places the C-21 methyl in the CIS configuration. 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Nucleophilic substitutions of simple aromatic compounds which formally involve a hydride displacement are difficult to achieve because of the poor leaving group and the high electron density of the aromatic nucleus which repels approach of a nucleophile. However, rc-electron deficient aromatic compounds such as metal carbonyl complexes are susceptible to attack by certain carbon nucleophiles. Studies of this chemistry have shown [16] an opposite jegioselectivity to the corresponding electrophilic substitutions, in agreement with the polarity alternation rule. 

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