Spectroscopic studies on the protonation of s-triaminobenzene

Abstract

The protonation of s-triaminobenzene (TAB) has been studied by measuring IR, UV and NMR spectra, special attention being paid to the structure of the single protonation product (TABH +) of TAB. The IR spectrum at room temperature shows that the NH stretching vibrations due to the neutral amino groups of TAB remain unaltered in TABH +. Furthermore, in the NMR spectrum of TABH +, the proton signal of the methylene group appears at τ = 6·7 ppm. These facts and other supporting evidence, demonstrate that the first protonation at room temperature mainly occurs on the ring C atom and produces the triaminobenzenium ion. It was further concluded, from the detailed analysis of the temperature dependence of UV spectra, that TAB in aqueous solution of pH 4·3 consists of two components, the triaminobenzenium ion and the amino ▪ overwhelmingly predominant at room temperature, the amount of the latter increases with the decreasing temperature. The peaks at 27,400, 36,800 and 45,500 cm −1 observed for TABH + at room temperature have been ascribed to the triaminobenzenium ion, since these peak positions coincide with the transition energies calculated for this ion by the Pariser-Parr-Pople + CI method.