Electronic spectra amines

Amine radical-cations have been generated by the treatment of para-substituted anilines with ceric ion (Stone and Waters, 1962 Fox and Waters, 1964). When the para position is free, the initial radical-cation can react further for example, the oxidation of triphenylamine with lead tetra-acetate in the presence of boron trifluoride (Allara ei al., 1965) or with iodine (Stamires and Turkevich, 1963) gives the radical cation Ph3N+ , and, when excess of triphenylamine is used, the former oxidant leads to the radical-cation of A(, .A/, W, A -tetraphenylbenzidine. The only radical observed by the oxidation of dimethylanihne either electrochemically (Mzoguchi and Adams, 1962) or with lead tetraacetate and boron trifluoride (Allara et al., 1965) is the radical-cation of iV, .A, .A7, iV -tetramethylbenzidine. The relatively stable (hindered) anilino radical (40) has been generated from the corresponding aniline by flash photolysis audits e.s.r. spectrum has been measured in n-hexane (Land and Porter, 1961). The electronic spectrum of this radical is very similar to that of the unsubstituted anilino radical, detected during flash photolysis of aniline, but this radical is so short-lived that it has not yet been detected by e.s.r. 

Fig. 2. Dependence of the electronic spectrum of the Co(II)-BSB-mono-amine system on Co(BSB)2 monoamine molar ratio in toluene solution at 20 C.

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

Solvated electrons are known to be formed in amines, amides, dimethyl sulfoxide, and many other liquids that will not be discussed here. Note that, except for the yield and time scale of observation, the production of es itself is not related to polarity. Thus, the es absorption spectrum has indeed been observed in nonpolar liquids both at low temperatures and room temperature (Taub and... 

The protonated amine, because it does not have any -electron, hence the spectrum of such an amine does not show any absorption due to n -> o transition. 

Principally the same, but chemically simpler, sequence was used to prepare arylnitro anion-radicals from arylamines, in high yields. For instance, aqueous sodium nitrite solution was added to a mixture of ascorbic acid and sodium 3,5-dibromo-4-aminobenzenesulfonate in water. After addition of aqueous sodium hydroxide solution, the cation-radical of sodium 3,5-dibromo-4-nitro-benzenesulfonate was formed in the solution. The latter was completely characterized by its ESR spectrum. Double functions of the nitrite and ascorbic acid in the reaction should be underlined. Nitrite takes part in diazotization of the starting amine and trapping of the phenyl a-radical formed after one-electron reduction of the intermediary diazo compound. Ascorbic acid produces acidity to the reaction solution (needed for diazotization) and plays the role of a reductant when the medium becomes alkaline. The method described was proposed for ESR analytical determination of nitrite ions in water solutions (Lagercrantz 1998). 

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