7c-Electron densities

FIGURE 5. Total net charges and 7c-electron densities (in parenthesis) from CNDO/S calculations for vinylamine, acrolein and 3-dimethylaminoacrolein. Experimental dipole moments are from References 17, 105 and 106, respectively... 

C6o solubility in thiophenol is more than 2 times greater than that in toluene as opposed to pyridine. Improved C6o solubility in thiophenol is attributable to the increase in the overall 7i-electron density of the benzene core. When acted upon by a positive mesomeric effect of a SH-group, 7c-electron density is localized mainly in the ortho- and / ara-positions, i.e. in the reaction centers for interaction between thiophenol molecules and C60. 

The chemical shifts of the ring carbons of 173 and 174 fit quite well into a correlation with 7c-electron density. This correlation has as its limiting members 175 and 176 whose ring carbon chemical shifts range over more than 120 ppm with the corresponding n-electron density values varying between ca. 0.5 and 1.20 . ... 

The effects on the 2-substituted compound of alkyl, aryl, and halogen substituents in the imidazole ring have also been examined, and although the rates for the 5-substituents were represented satisfactorily by quantum-mechanical calculations of 7c-electron densities for the imidazole neutral molecule predict the order of substitution as 5 > 4 > 2, but the tautomeric equivalence of the 4- and 5-positions is not taken into account. In addition, there are probably few occasions on which electrophilic substitution takes place with the neutral molecule the conjugate acid or conjugate base may be the reactive species. 

The principal feature of the chemical reactivity of QBA is the addition of a nucleophile to the iminium bond C=N (Scheme 1). The carbon atom C-6 displays the lowest 7C-electron density [97,98]. This process is associated with a number of significant alterations in the constitution, physical appearance, solubility, spectral properties, etc. The quaternary cation is a brightly coloured, polar, water-soluble species. The tertiary-nitrogen adduct has lost the colour and is non-polar and water insoluble. In the case of aminoacetal and aminal derivatives (Scheme 1, Nu = OR, NHR), the reaction is essentially reversible, i.e. the action of acid immediately converts the adduct back to the quaternary salt. Viewed from another perspective the emergence of colour is a sensitive indicator of the presence of some acid and ipso facto of deconq)osition of the adduct. 

If only the electron density of the highest occupied molecular orbital (HOMO) is taken into account, an electrophilic attack is said to be regulated by the frontier electron density index (54JCP1433 79FCF1). In nucleophilic substitutions, the aromatic substrate tends to accept an electron pair in the transition state, and so the frontier orbital is taken as the lowest unoccupied molecular orbital (LUMO). In this case, the frontier electron density is assumed to be as the electron distribution that would be present in the LUMO if it were occupied by two electrons. In contrast to arguments based on the charge or 7c-electron densities, both nucleophilic and electrophilic substitution occur preferentially at the atom with the highest electron density within the appropriate frontier orbital, i.e., LUMO or HOMO, respectively. 

Application of this equation to some aromatic hydrocarbons affords chemical shifts in good agreement with experimental values. Alger et al. have extended this theory by inclusion of a- as well as 7C-electron density terms. The chemical shifts in biphenyl,... 

Hawthorne and coworkers. However, the interatomic distances of Cs to the carborane cages are such that it could be regarded as a cesium-carborane complex in which some degree of interaction exists between the metal and the 7C-electron density on the carborane cage. Since this cesium compound can also be prepared by an ion-exchange reaction directly from lithium, sodium or potassium salts of the C B -cage (see Figure 6b), further study of this and related compounds in solvent extraction of radioactive cesium metal ( Cs) from nuclear waste is envisioned. 

From the pairing theorem, the 7c-electron density at any position in any neutral AH (even AH or odd AH radical) is unity. If atom i is at such a position in both reactants and products, replacing it by a heteroatom will have no first-order effect on the equilibrium. If, however, atom i is active in a reaction involving odd AH ions, the final term in equation (4.92) will not vanish and so the equilibrium will be disturbed by introduction of the heteroatom. 

The 7r-electron densities in the benzyl anion have already been calculated (see p. 81). The position meta to methylene is inactive and the 7c-electron density is accordingly unity, whereas at the para position the additional electron density q is 1/7. One would therefore expect a heteroatom at the meta position to have little effect on the equilibrium of equation (4.95), while a heteroatom at the para position should favor formation of the conjugate anion. y-Picoline is in fact a much stronger acid than toluene or )5-picoline, the equilibrium of equation (4.97) being pushed toward the right by the gain in TT-electron density at the nitrogen atom. 

The atom transfer reaction (1) tends to proceed via a planar HONO transition state, since the OH fragment is generated with its 7c-electron density preferentially localised in the plane of rotation. A similar preference is found in the OH fragment generated by photodissociation of nitrous acid (reaction (2)) [19] but here the intermediate state is a planar photo-excited molecule of electronic symmetry A". 

Figure 9. Excess 7C-electron density (7C-electron densities minus 1) in the 2 Ag state of hexadecaoctaene that follow from a fit of the polyene 2iAg and l Bu excitation energies by the simple model described in Section 4. The net charge on a carbon atom is the negative of the excess 7t-electron density.

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