Effect of Electron Density

Although the Lamb expression in Eq. 4.2 applies strictly to an isolated atom, it can be evaluated approximately for the local contribution of a proton in a molecule to give 

FIGURE 4.2 Approximate chemical shift ranges for protons in various functional groups. Refer-ence TMS (internal). Chemical shift highly dependent on hydrogen bonding. 

Equation 4.13 suggests that there ought to be some sort of correlation between the shielding factor and the electron density around the hydrogen. For example, a more acidic proton, such as the OH proton in phenol, should be less shielded than the corresponding less acidic proton in an alcohol. This is indeed found to be the case, the chemical shift for the OH proton of phenol occurring 

FIGURE 4.4 Approximate chemical shift ranges for 14N and 15N in various functional groups. Reference Liquid NH3 (external). 

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Reduction of Other Aldehydes. We examined the reduction of anisaldehyde, p-CH30C6H4CH0 and tolualdehyde, p-CH3(C6H<,)CH0 to examine the effect of electron density on aldehyde reduction. In addition, we also investigated one ketone, acetophenone, C6H5C0CH3. The results of these experiments are given in Table 2. 

The complexity of the formation of poly(amic acid) was highlighted by Denisov et al. [41], who studied the structures formed by condensation of four different aromatic dianhydrides with both p-diphenylenediamine and benzidine, using solution-state NMR. The authors assigned the complex spectra of the mixtures of poly(amic acid) isomers by comparison with the spectra of model compounds, and the consideration of the well-known effects of substituents on the chemical shifts of aromatic carbons. For example, it was found that the poly(amic acid) formed by reaction of pyromellitic dianhydride with both diamines was 60% in the cis-isomer and 40% in the rra 5-isomer. More complex spectra were obtained for the poly(amic acid)s formed from the asymmetric dianhydrides. In all cases, the proportion of isomers was independent of the type of diamine and depended only on the dianhydride. A correlation was made between the effect of electron density on the relative rates of formation of the respective isomers, and the difference in chemical shifts of the quaternary aromatic carbons attached to the anhydride. 

Thompson, H. W. McPherson, E. Lences, B. L. Stereochemical Control of Reductions. 5. Effects of Electron Density and Solvent on Group Haptophilicity /. Of. Chem. 1976, 4i, 2903-2906. 

A much less basis set dependent method is to analyze the total electron density. This is called the atoms in molecules (AIM) method. It is designed to examine the small effects due to bonding in the primarily featureless electron density. This is done by examining the gradient and Laplacian of electron density. AIM analysis incorporates a number of graphic analysis techniques as well as population analysis. The population analysis will be discussed here and the graphic techniques in the next chapter. 

The NDDO (Neglect of Diatomic Differential Overlap) approximation is the basis for the MNDO, AMI, and PM3 methods. In addition to the integralsused in the INDO methods, they have an additional class of electron repulsion integrals. This class includes the overlap density between two orbitals centered on the same atom interacting with the overlap density between two orbitals also centered on a single (but possibly different) atom. This is a significant step toward calculatin g th e effects of electron -electron in teraction s on different atoms. 

Differences in reactivity of the double bond among the four isomers are controlled by substitution pattern and geometry. Inductive effects imply that the carbons labeled B in Table 3 should have less electron density than the A carbons. nmr shift data, a measure of electron density, confirm this. 

Notice that the MO picture gives the same qualitative picture of the substituent effects as described by resonance structures. The amino group is pictured by resonance as an electron donor which causes a buildup of electron density at the /3 carbon, whereas the formyl group is an electron acceptor which diminishes electron density at the /3 carbon. 

These calculations indicate that both the methyl group and the nitrogen atom increase the electron density around the carbon atom in the double bond which is /3 to the substituent (models 143 and 144). Therefore, when both of these groups are bonded to the same carbon atom of the double bond, this increase of electron density about the jS-carbon atom is intensified (as in model 146 and compound 142). This type of compound, then, is more strongly held by the stationary phase, and hence its retention time is longer than that of compound 141, where the effects of the methyl substituent and the nitrogen counteract each other (model 145). 

In the last few years, methods based on Density Functional Theory have gained steadily in popularity. The best DFT methods achieve significantly greater accuracy than Harttee-Fock theory at only a modest increase in cost (far less than MP2 for medium-size and larger molecular systems). They do so by including some of the effects of electron correlation much less expensively than traditional correlated methods. 

Taking into account the high structural similarity of dppf and cdpp, their different influence on the reaction s selectivity has to be attributable to electronic effects. The electron density at the phosphorus atoms is significantly lower in the case of cdpp, due to the electron-withdrawing effect of the formal cobalt(III) central atom... 

Deshielding (Section 13.2) An effect observed in NMR that causes a nucleus to absorb downfield (to the left) of tetramethylsilane (TMS) standard. Deshielding is caused by a withdrawal of electron density from the nucleus. 

The molecular chemisorption of CO on various alkali-modified metal surfaces has been studied extensively in the literature. It is well established that alkali modification of the metal surface enhances both the strength of molecular chemisorption and the tendency towards dissociative chemisorption. This effect can be attributed to the strongly electropositive character of the alkali, which results in donation of electron density from the alkali to the metal and then to the adsorbed CO, via increased backdonation into the... 

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

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