Ionization potential, alternating

In the HMO or extended Hiickel approach, the individual ionization potentials should be set equal to orbital energies. The inadequacy of the HMO treatment is apparent with odd alternant hydrocarbons (e.g., allyl, benzyl), where a constant value is obtained, in disagreement with the experiment. Streitwieser and Nair (105) showed, however, that reasonable results can be obtained with the co technique. 

To get an approximate expression for the chemical hardness, start with an expression for the electronic chemical potential. Let a hypothetical atom have an energy, UQ. Subtract one electron from it. This costs I = ionization energy. Alternatively, add one electron to it. This yields A = electron affinity. The derivative = electronic chemical potential = p = AU/AN = (I + A)/2. The hardness is the derivative of the chemical potential = r = Ap/AN = (I - A)/2. 

An alternative measure of the electron-donor properties is obtained from the energetics of electron detachment in the gas phase the ionization potentials (IP) of many organic donors have been experimentally determined from the photoelectron spectra obtained by their photoionization in the gas phase. Thus, the values of the ionization potential IP differ from the oxidation potential x by solvation,66 i.e.,... 

An alternative approach to acidities is via a thermodynamic cycle using the bond dissociation energy (DH°), electron affinity (EA) and ionization potential (IP) as follows ... 

This is known as the ionization potential theorem. Equation 7.7 between max and ionization potential I can also be obtained alternatively by looking at the asymptotic behavior of the density of a many-electron system. For atoms and molecules, the asymptotic decay of the density is given as [6-11]... 

Krishna, B. and Gupta. co-Type calculations on n-electron systems with incltrsion of overlap charges. I. Ionization potential of some alternant hydrocarbons, J. Am. Chem. Soc., 92(25) 7247-7248, 1970. 

A more important application of the local ionization potential is as an alternative to the electrostatic potential as a graphical indicator of electrophilic reactivity. This is in terms of a property map rather than as an isosurface. Further discussion is provided later in this chapter. 

Such a representation is referred to as a local ionization potential map. Local ionization potential maps provide an alternative to electrostatic potential maps for revealing sites which may be particularly susceptible to electrophilic attack. For example, local ionization potential maps show both the positional selectivity in electrophilic aromatic substitution (NH2 directs ortho para, and NO2 directs meta), and the fact that TC-donor groups (NH2) activate benzene while electron-withdrawing groups (NO2) deactivate benzene. 

V. D. Parker [56] obtained in acetonitrile the oxidation and reduction potentials (EQx and ERea) of alternant aromatic hydrocarbons (AAH) by cyclic voltammetry and examined how those potentials are related to the ionization potential (IP) and the electron affinity (EA) of the compounds (Table 8.8). As expected, he found linear relations of unit slopes between E0x and IP and between ERed and EA. Moreover, he found that E0x and ERed of each AAH was symmetrical with respect to a common potential MAAH (-0.31 V vs SCE). The values of (E0x-MAAH) and (ERed Maa ) are correlated with the values of IP and EA, obtained in the vacuum, by E0x-Maah = IP- +AGsV+ and ERed-MAAII = liA-r/t-AG, respectively (Fig. 8.21). Here, is the work function of graphite and equal to 4.34 eV, and AGj v+ and AG v are the differences in solvation energies for the 0/+1 and 0/-1 couples of AAH. Experimentally, AG°V+ and AG°V were almost equal, not depending on the species of AAH, and were equal to -1.94 eV in AN. 

Tab. 8.8 Oxidation potentials, reduction potentials, ionization potentials and electron affinities of alternant aromatic hydrocarbons (AAHs) ...

The free valence number therefore provides a good guide to reactivity in alternant hydrocarbons unfortunately this approach cannot easily be extended to compounds of other types. It can be used only when the contribution to 6E due to changes in the qt is the same for different aromatic systems and this is so only if coulomb integrals a, are the same for all the atoms present. Equation (33) shows that this is not so for molecules containing heteroatoms the ionization potential Wt is different for different atoms. Equation (33) shows that non-alternant hydrocarbons such as azulene must also be excluded here the... 

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