Aniline radical cations

The radical cation formed upon ionization of ANI has been studied by different spectrometric techniques, including photoelectron, two-color photoionization, ZEKE70,80,199,212-226 and mass107,227-232 spectrometries. In most cases, the technique used has been coupled with infrared spectroscopy, which allowed the fine vibrational spectrum of the ion to be determined, in both line position and intensity. For example, the ZEKE photoelectron spectrum216 was recorded by exciting to the neutral S ( 52) excited state, and well-resolved vibrational bands of the cation were observed. In conjunction with quantum chemical calculations of fundamental frequencies, an assignment of the observed vibrational bands can thus be made. A few theoretical studies56,107,218,233,234 have also been devoted to the radical cation. 

FIGURE 26. Selected geometrical parameters of the aniline radical cation (2 B ). Bond lengths in angstroms and bond angles in degrees were obtained from UB3LYP/6-311++G(d,p) optimization 

FIGURE 27 (PLATE 3). Differential density Ap(r) of the vertical aniline radical cation, calculated using UB3LYP/6-311++G(d,p) as the difference between electron densities of both ion and neutral forms at the neutral geometry 

It has been noted that alkyl substitution at the C4-position reduces the IE of aniline by 0.25-0.30 eV225 as the length of the alkyl chain increases, even though the effects on other geometrical properties are not important. 

The ANT+ ion exhibits an electronically excited state whose adiabatic IE was found to be 8.94 eV212 from He I PES. Two-color laser photoelectron spectroscopy219 via both the S0 and S) states of ANI revealed that there is an optical transition from the ( S state to the 2A2 state of ANT+. The IE of the latter state was found at 9.031 eV, which is slightly higher than the PES value. This implies that the 2A2 (2E) state constitutes the lowest-lying excited state of ANI +, and the 2A2 2B energy gap of ANI + amounts 

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We have found in our work that these parameters give excellent agreement with experimental spin densities for trimethylsilyl-and trimethylgermyl-substituted aniline radical cations (29, 28). 

Monomolecular C-X bond cleavage energies are accessible by thermochemical cycle calculations such bond dissociation processes are often induced by nucleophiles, at least with X = SiR3 [163, 171]. As a consequence, rates are much faster than expected on the basis of thermochemical data. Detailed data are available for aniline radical cation desilylation [170]. 

Ionized aniline possesses an isomeric stable species, the 4- (or 3-) dehydroanilinium distonic ions (m/z 93). It is thus expected that their identification could afford some additional pieces of information related to the localization of the initial site of protonation. The distonic ions can readily be prepared by protonation of iodoanilines followed by collisional deiodination. Such a protonation-deiodination sequence is also readily performed in the quadrupole-time of flight instrument and the CID spectra of all the m/z 93 ions are collected in Table 5. As a reference for aniline radical cations, AZ-methylaniline was used and was observed to intensively expel a methyl radical after protonation and excitation (high cone voltage). 

It is clear from the recorded CID data that aniline radical cations are produced in the protonation of aniline, followed by loss of a hydrogen atom. The distonic species are differentiated by a significant signal at m/z 76 for benzyne ions. 

For the ANI radical cation, an acidity constant pKa = 6.4 was obtained182 however, no experimental BDE of this ion has been reported. A linear correlation of the oxidation potentials of anilines versus the acidities of the corresponding radical cations was observed. Recent ab initio calculations198 derived a value of BDE(N+—H) = 418 10 kl mol-1 for the aniline radical cation, relative to the Ph—N—H+ cation in its singlet ground state. Removal of an electron reinforces the strength of the N—H bond, due to the electron delocalization. 

FIGURE 28 (PLATE 4). Summary of electronic distribution in aniline radical cation (a) Bond distances (A), NBO charges [bracket, in au] and Wiberg indices (parentheses, in au). (b) Topology of the electron density determined from AIM p(r) = electron density, L = Laplacian of the density defined as L(r) = — V2p(r) and e = ellipticity of the bond critical point, (c) Isosurfaces of the electron localization function, ELF = 0.87 the values s are the populations of the valence basins... 

TABLE 11. Fundamental vibrational frequencies (cm 1) and associated potential energy distributions of the aniline radical cation in its lowest-lying doublet state. Calculated values were obtained from scaled UB3LYP/6-311++G(3df,2p) harmonic frequencies... 

FIGURE 30. Normal displacements of vibrational modes of aniline radical cation (251). The assignment of the normal vibrations and associated frequencies are presented in Table 12. The numbers given within the ring correspond to the normal modes numbered from Q1 to Q36 in Table 11... 

Decomposition of aniline radical cation HNC versus HCN elimination... 

FIGURE 38. The mechanism proposed for the decomposition of aniline radical cation leading to a HNC formation... 

In their study, Stassen and Hambitzer also demonstrated that dimers undergo dealkylation. Scheme 4 illustrates the mechanism of that process, and the resulting aniline radical cation undergoes further reaction to the corresponding dimer. The peaks at m/z= 199 were attributed to the dealkylated dimers for which the structures 28 and 29 can be shown. 

The oxidation of polyhaloanilines in acidic aqueous solutions at a concentration of sulfuric acid higher than 1.0 M is different in two aspects from those found in ACN. Firstly, the electroactive substrate exists in the protonated form, e.g. 110 in the case of 2,4-dihaloanilines, as is evident from pKa values150 as well as from the fact that voltam-metric peak potentials of the main process (I) are independent of acid concentrations145,146. Secondly, as a consequence, no aniline radical cations are formed and their dimerization is... 

By comparison with phenoxyl radicals, the isoelectronic anUinyl radicals protonate much more readily pATa values in the range 4-7 have been reported for the equilibria between various anilinyl radicals and their corresponding aniline radical cations 45i... 

Stejskal that aniline radical cations adsorb on the substrate surface during an induction period before bulk polymerization is observed. The adsorbed species promote the formation of a dense film initially on the substrate surface. Once this film has formed, further growth occurs on the PAn surface in a manner similar to the growth of localized globules during electropolymerization. The surface roughness of in situ PAn films may be further increased by the incorporation of PAn precipitates from the bulk solution. 

The pKj of monosubstituted benzene radical cations vary from 7 for CjHjNHj " to -2 for C5H5OH. CgHgCHj is even more acidic but the p has not been determined experimentally. In table 4, the pK s of a number of substituted phenol- and aniline radical cations are given. The pK s of aniline radical cations have been determined by pulse radiolysis as previously described[12,13,57,58] and the pK,s of phenol radical cations have been measured by ESR in mixtures of sulphuric acid and water. [59,60]... 

As for the one-electron reduction potentials, the pK s of substituted aniline radical cations seem to be linearly dependent on the Brown substituent constant. In figure 5, the pK s of 4-substituted aniline radical... 

This equation can also be extended to hold for multisubstituted aniline radical cations.[13]... 

For a number of different substituted benzenes, the substituents are involved themselves in the redox chemistry. Among these substituent-active compounds, we find phenolic substances, aromatic amines, chalcogenide substituted benzenes, benzyUc substances, benzoic adds, thiobenzoic adds and benzyl alcohols. As has already been discussed, phenol radical cations, aniline radical cations and toluene radical cations are more or less acidic and can deprotonate to form the corresponding neutral radical. Chedcogenide substituted benzenes (S, Se and Te) are usually characterized by the fact that their redox properties are determined by the chalcogenides rather than by the substituent pattern. 

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