Infrared and UV–vis spectra of phenoxonium cations produced during the oxidation of phenols with structures similar to vitamin E

Several phenols with structures similar to vitamin E were oxidised and the intermediate species produced were characterised by in situ infrared and UV–vis spectroscopies. The Fourier transform infrared (FTIR) measurements were performed by chemically oxidising the phenols with 2 mol equiv. of NO+SbF6− in CH3CN and recording the spectra between 1900 and 1300 cm−1 with an attenuated total reflectance (ATR) probe utilising a fiber conduit and a diamond composite sensor. The compounds that formed long-lived phenoxonium cations displayed two IR absorbances at 1665 (±15) cm−1 and one at 1600 (±10) cm−1 associated with the carbonyl, symmetric ring stretch and asymmetric ring stretch modes. The para-quinones are one of the long-term products of oxidation of the phenols, and displayed solution phase IR absorbances at 1650 (±10) cm−1. In situ electrochemical UV–vis experiments performed during the oxidation of the phenols led to the detection of bands due to the phenoxonium cations at 295 (±5) and 440 (±15) nm and due to the para-quinones at 260 (±10) nm. The concentration of the substrate and the water content of the solvent had a major effect on the yields of the intermediates and products that were produced during the oxidation reactions.