Aniline, ionization potential

Rate constants for quenching of triplet erythrosin (an acidic dye of the xanthene series) by a series of anilines were found to correlate with the aniline ionization potential (32). 

TABLE 9. Ionization potentials IP (eV) and orbital energies e (eV) of aniline... 

This is the same order of the catalytic activity of transition metal in exchanged zeolites for aniline formation. Irving and Williams (15, 16) pointed out also that there is a clear correlation between complex stability and the second ionization potential. As a matter of fact, a good correlation was found between the catalytic activity and the second ionization potential of divalent ions. (We thank the reviewer for pointing out this correlation.)... 

There remains the choice of Up values. As a starting point we have adopted Uc = —9.5 eV, which is known to give satisfactory values for the ionization potentials in hydrocarbons86 and determined U0, Ux, UNH> and t/NHl so as to reproduce the ionization potentials of formaldehyde, pyridine, pyrimidine, pyrrole, and aniline. Moreover,... 

Talamoni and coworkers17 found that in methanol, allylamine has no influence on the yield of 0-Ps. Other amines, such as w-propylamine and triethylamine, increase the yield of the o-Ps. The authors ascribe this difference to the higher ionization potential of allylamine, making positive charge transfer from the solvent ions (the holes) ineffective. The ionization potential of allylamine (9.6 V) is considerably higher than that of other amines (w-propylamine 8.8 eV, triethylamine 7.8 eV, aniline 7.7 eV). They found a correlation between the ionization potentials and the enhancement factors. In water, allylamine also enhances the formation of 0-Ps, due to the much higher ionization potential of water, 12.6 eV (while the value for methanol is only 10.8 eV). 

Alternatively, it has been assumed that the linear or nearly linear portions of the ionization efficiency curves of compound and standard observed at a few volts above threshold (Fig. 14) are of equal slope (Warren, 1950). In this method it is assumed that the ionization probabilities of compound and standard are equal. Now, if is the cross section at electron beam energy, E, of a compound of ionization potential, Eq, then the ionization probability, C, is given by Qion = G E — Eq)", where the number of electrons leaving the reaction complex is n+. However, the relative ionization probabilities of benzene, nitrobenzene, and aniline for example, were found to be 200,132, and 389 (Deverse and King, 1964), and therefore ionization potentials will be in... 

In a recent important study, Tamon and coworkers [733-735] implicitly took the approach that orbital control is dominant. They analyzed the adsorption and desorption characteristics of a series of aromatic compounds at uncontrolled pH and used the semiempirical MINDO/3 method to determine the HOMO energy levels ( ii) for these adsorbates and the LUMO levels ( l) for several adsorbents (including chemically modified activated carbons). The range of calculated ii — El I values for adsorbates ranging from / -nitrobenzoic acid to aniline on a model activated carbon was 6.7-9.0 eV, as shown in Table 38. Low values (<7.3 eV) coincided with those systems for which irreversible adsorption was found those adsorbates that possess electron-donating groups (e.g.. aniline), i.e., with the highest HOMO levels (or lowest ionization potentials), could not be... 

Workentin et al. have recently reported the results of an extensive laser flash photolysis investigation of the reactions of the cation radicals of 9-phenyl- and 9,10-diphenylanthracene (PA and DPA, respectively) with amines. Primary amines react with both cation radicals via nucleophilic addition with rate constants which reflect both the amine basicity and a steric requirement for bond formation. Steric effects are more pronounced for addition of DPA " vs. PA ", presumably due to the presence of substituents at both the 9- and 10-position. Tertiary amines and anilines react with PA " and DPA " via electron transfer with rate constants which correlate with amine ionization potentials. Rate constants for nucleophilic addition of primary amines are faster in acetonitrile than in acetonitrile/water solution. The rate-retarding effect of water is attributed to an equilibrium between the fiee amine (reactive) and hydrated amine (unreactive). The beneficial effect of water on preparative ET-sen itized photoamination may reflect its role as a catalyst for the proton transfer processes which follow C-N bond formation (Scheme 2). Hydration of the amine also should render it less reactive in primary and secondary electron transfer processes which can compete with the formation of the arene cation radical. 

Photoelectron spectra of amines, nitroso and nitro compounds TABLE 10. Ionization potentials (eV) of substituted anilines"... 

Similar experiments reveal that ionized N-heterocycles such as pyridines eliminate HCN, rather than HNC148. This distinction offers a potentially useful means of distinguishing between isomeric anilines and pyridines. 

Non-Reversible Processes. —Reactions of the non-reversible type, i.e., with systems which do not give reversible equilibrium potentials, occur most frequently with un-ionized organic compounds the cathodic reduction of nitrobenzene to aniline and the anodic oxidation of alcohol to acetic acid are instances of this type of process. A number of inorganic reactions, such as the electrolytic reduction of nitric acid and nitrates to hydroxylamine and ammonia, and the anodic oxidation of chromic ions to chromate, are also probably irreversible in character. Although the problems of electrolytic oxidation and reduction have been the subject of much experimental investigation, the exact mechanisms of the reactions involved are still in dispute. For example, the electrolytic reduction of the compound RO to R may be represented by... 

Murray, Politzer, and their co-workers have developed several descriptors based on features of the molecular electrostatic potential surface (EPS) that can be used to characterize a variety of chemical and physical properties, including pK s [26,199,231]. In studies of the acidities of substituted azoles and anilines they showed that values of the most negative surface potentials (Vrma) and the minimum local ionization energy on the molecular surface (Is,min) showed strong correlations (r 0.97) with the pK s of these compounds. Later, Ma et al. [27] found that Is,jjn and several other EPS descriptors provided good models of the pK variations in substituted phenols and benzoic acids. Sakai and co-workers [232] have shown that Vmin yields an excellent fit (r = 0.996) for the aqueous pK s of a set of 22 amines. These studies demonstrate that features of the molecular electrostatic potential surfaces of acids can offer useful guides for pK, estimatioa... 

In 1992 Haeberlein et al. [199] showed that two descriptors obtained from the molecular electrostatic potential surface, the minimum ionization energy Is and the minimum of the molecular electrostatic potential Vs min> correlated with the pK s of para-substituted anilines. These descriptors also showed strong correlations with the Hammett constants. More recently Sakai et al. have shown that is an exceptionally good descrip-... 

M. Haeberlein, J.S. Murray, T. Brinck, and P. Politzer, Calculated electrostatic potentials and local surface ionization energies of para-substitituted anilines as measures of substituent effects. Can. J. Chem. 70 (1992), pp. 2209-2214. 

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