Aniline ionization energy

Substituent on aniline Ionization energy (eV) (method) Proton affinity (kJ mol 1)... 

Upon ionization, the second type of complex in which ANI plays the role of hydrogen donor, becomes more stable and in fact the only complex found (Figure 42). Due to the difference between the ionization energies of water and ANI, the complex is formed from interaction of neutral water with ionized aniline. The resulting complexation energy amounts to A hb(ANI-H20+) = —72 kJmol-1, which represents a substantial stabilization of about 50 kJmol-1, with respect to the neutral complex formation. 

TABLE 1. Ionization energies (IE) and proton affinities (PA) of aniline and selected derivatives... 

We turn now to the ionization process of a series of methyl mono-substituted ring systems pyridines, anilines, phenols, and substituted benzoic acids. Table 2 shows the free energies, A G, the heats, A H, and entropies of ionization, A S, for a number of these compounds. The compounds fall into two classes, according to the different ionic nature of the ionization process. The pyridine and aniline ionizations are of the type... 

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. 

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

Many other aliphatic or aromatic amines produce the same reaction. However if N-alkyl (or aryl)2-aniline is made to react with tris-dimethylaminophosphine, tricoordinated phosphorus compounds are obtained. Their analyses and mass spectra show that they are tetramers (5) when the energy of the ionizing electron beam is nearly 70 eV the mass spectra of the oligomers show an intense [M] peak which corresponds to the monomer ion (n = 1) but when the spectra are obtained by field desorption m.s. the molecular ion peak corresponds to a tetramer (n = 4) one sees also smaller peaks for ions n = 3, 2 or 1. 

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... 

Contribution from the three structures V, VI, and VII stabilizes the amine in a way that is not possible for the ammonium ion resonance thus lowers the energy content of aniline more than it lowers the energy content of the anilinium ion. The net effect is to shift the equilibrium in the direction of less ionization, that is, to make smaller (Fig. 23.1). (See, however, the discussion in Sec. 18.11.)... 

It is of interest to mention some detailed mass spectrometric results that were obtained by performing distinct experiments (a) ion-molecule reactions in mixtures of haloben-zenes and ammonia under chemical ionization conditions (b) ion-molecule reactions of mass-selected ionized halobenzenes toward ammonia in a quadrupole collision cell of a hybrid tandem mass spectrometer (c) ion-molecule reactions of phenyl diazonium cations with ammonia in the same quadrupole collision cell and, finally, (d) electrospray ionization of anilines in a hybrid quadrupole-time of flight mass spectrometer (QTof). Characterization of the product ions relies on collisional activation experiments in the low or high kinetic energy regime98. 

TABLE 4. Relative intensities of some significant peaks in the high energy CA spectra of m/z 94 ions generated by ionization of mixtures of ammonia and aniline, chlorobenzene, bromobenzene or iodobenzene in a Cl source. The most significant differences are indicated in bold... 

FIGURE 13. (a) CID spectrum of protonated aniline (m/z 94) generated in ESI conditions (20 eV collision energy) and (b) low energy CID spectrum of protonated aniline generated in a chemical ionization source... 

Ionized aniline may exist in a conventional-ion form as well as in isomeric distonic forms, and gas-phase MS experiments performed by Chyall and Kenttamaa134,135 have also been used to form and to study the intrinsic reactivity of these high-energy isomers. Via collision-induced dissociation (CID) of protonated 2-, 3- and 4-iodoanilines in a dual-cell Fourier transform ion cyclotron resonance spectrometer, the 2-, 3- and 4-dehydroanilinium ions were formed as the result of loss of an iodine atom (Scheme 41). Ion/molecule reactions and energy-resolved CID experiments demonstrated that these three ions as well as ionized aniline behave distinctively. The reactivity of three distonic ions... 

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