Carbon chemical shifts electron deficiency

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

Electron deficiency at a carbon causes drastic deshielding. This is observed for the sp2 carbons typical of carbocations [79], In such systems, the sp2 13C chemical shift range may approach 400 ppm relative to TMS. If the positive charge is dispersed in a carboca-tion, e.g. by resonance, the electron deficient carbon will be more shielded. The following comparison of f-butyl-, dimethylhydroxy- and dimethylphenyl-carbenium ion illustrates this ... 

The 13C chemical shifts of benzenoid carbons largely depend on the mesomeric interaction between substituent und benzene ring. Electron releasing substituents (e.g. — NH2, — OH) will increase the electron density at the o and p carbons relative to benzene (128.5 ppm), while slight electron deficiencies will be induced by electron withdrawing groups (e.g. — N02, —CN). 

The extreme electron deficiency of the metal-bonded carbon atom is shown by the very low field, 3C chemical shifts (ca. 300-450 ppm), and the short... 

We note upon inspection of the data in Table XIV that the chemical shifts in these carbonium ions (Via, b, and c) follow the normal order observed when substituents on a fully substituted carbon atom are varied from R = H (most shielded) to CH3 to C6H5 (least shielded), rather than for substituents on an electron-deficient trivalent carbon atom, where the CH3-substituted carbon atom is less shielded than the analogous phenyl-substituted carbon atom (58). This is a rather clear-cut indication that the carbinyl carbon atoms in complexes Via, b, and c are nearly fully bonded and only slightly electron-deficient. The slight increase in shielding of the CH3 carbon when VIb is formed from the alcohol and of the C-l carbon of the phenyl group of Vic provides further confirmation of this. 

When a molecule takes part in a reaction, it is properties at the molecular level which determine its chemical behaviour. Such intrinsic properties cannot be measured directly, however. What can be measured are macroscopic molecular properties which are likely to be manifestations of the intrinsic properties. It is therefore reasonable to assume that we can use macroscopic properties as probes on intrinsic properties. Through physical chemical models it is sometimes possible to relate macroscopic properties to intrinsic properties. For instance 13C NMR shifts can be used to estimate electron densities on different carbon atoms in a molecule. It is reasonable to expect that macroscopic observable properties which depend on the same intrinsic property will be more or less correlated to each other. It is also likely that observed properties which depend on different intrinsic properties will not be strongly correlated. A few examples illustrate this In a homologous series of compounds, the melting points and the boiling points are correlated. They depend on the strengths of intermolecular forces. To some extent such forces are due to van der Waals interactions, and hence, it is reasonable to assume a correlation also to the molar mass. Another example is furnished by the rather fuzzy concept nucleophilicity . What is usually meant by this term is the ability to donate electron density to an electron-deficient site. A number of measurable properties are related to this intrinsic property, e.g. refractive index, basicity as measured by pK, ionization potential, HOMO-LUMO energies, n — n ... 

Data for both hydrogen and carbon monoxide chemisorption for the series of reduced [Rhe Ir,]/NaY (x = 0,2,4, and 6) are presented in Table XXV. It was found that, in particular, the CO/M values on Rh-rich cluster-derived catalysts were higher than those for Ir-rich clusters. The trend of CO chemisorption amounts is in good agreement with the decreasing order of electron deficiency on the clusters inside zeolites which has been estimated from XE NMR chemical shifts (SS). 

To a good approximation, three terms dominate the C-NMR chemical shifts diamagnetic, paramagnetic, and anisotropy shielding terms [57]. Lamb developed a theoretical expression for the diamagnetic term that focuses on the electron density at a specific nucleus [58]. Therefore, one would expect the carbon s hybridization to have a large chemical shift effect. Obviously, electron deficiency in a carbocation will profoundly effect the chemical shift [59-62]. 

The chemical shifts of protons attached to, and in particular of the carbons in, heterocyclic systems, can be taken as relating to the electron density at that position, with lower fields corresponding to electron-deficient carbons. For example, in the H spectrum of pyridine, the lowest-held signals are for the a-protons (Table 2.5), the next lowest is that for the y-proton and the highest-held signal corresponds to the p-protons, and this is echoed in the corresponding C shifts (Table 2.6). A second generality relates to the inductive... 

For a consistent quantitative description of rearrangements it seems reasonable to divide the whole set of degenerate carbocations into series of structurally related ones within which the changes in rearrangement rates can be treated as caused mainly by one structural effect. There is every reason to believe (cf. Ref. ) that in many cases the extent of electron deficiency in the carbonium centre may serve as a valuable parameter for such a purpose. For the arenonium ions with an aromatic transition state the decisive factor may prove to be the value of jt-electron deficiency which for a series of related structures may be characterized by the chemical shift of the carbonium carbon (5C ) in the C NMR spectrum 259,270)... 

The most appropriate parameter seems to be the chemical shift of the carbon atom — the carbocation centre (SC ). Its main advantage is its close relationship with the value of -electron deficiency in the carbocation centre which can be estimated by the MO method. For the series of structurally related arenonium ions - a linear correlation is found between the free activation energy of the... 

Since, as shown in Sect. III.2, the values of the deficiency of rc-electron density at sp -hybridized carbons linearly depend on the chemical shifts of these atoms in the NMR- C spectra the above relation can be rewritten in the form... 

Here the coefficient of proportionality between the change of the chemical shift and the deficiency of K-electron density on the sp -hybridized carbon turns out to equal 187 ppm/electron. 

The chemical shifts of the sp -hybridized carbons for methylbenzenium ions were compared with the jt-electron (q,) and the total (q = q, + q ) electron defkioM calculated by the Htickel LCAO MO and CNDO/2 methods. To judge by the values of root-mean-square deviations (s) and of correlation coefficients (r) the chemical shifts of the sp -hybridized carbons of the ions under consideration correlate better with the Tc-electron deficiency calculated by the simple Hiickel MO method... 

Relationship between the activation parameters of the 1,2-hydrogen shift and the deficiency of electron density Characteristing the electron deficiency with the chemical shift of the carbon atom (8 ) to which the hydrogen migrates, the author found for the free activation energies of the 1,2-hydrogen shifts in the l-H-1,2,3,4,5,6-hexamethylbenzenium and 3,6-di-substituted 9-H-9,10-dimethylphenanthrenium ions the simple linear relationships, ... 

Figure 1. Comparison of calculated (vertical axis) and experimental "B chemical shifts [Used by permission of John Wiley Sons, Inc., from Buhl and Schleiy (1991) In Electron Deficient Boron and Carbon Clusters. GA Olah (ed). Fig 4.2, p. 123].

Buhl M, Schleyer PVR (1991) Ab initio geometries and chemical shift calculations for neutral boranes and borane anions. In Electron Deficient Boron and Carbon Clusters. Olah, GA (ed), p 113-142... 

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