The absorption spectra of aromatic carbonium ions in HF solution

Abstract

The absorption spectra between 2200 Å and 10 000 Å have been measured of the conjugate acids of a number of aromatic hydrocarbons dissolved in oxygen-free, anhydrous HF and HF-BF₃ mixtures, using a specially constructed cell. All spectra show very intense and characteristic absorption bands in the visible or near-ultra-violet region. In order to interpret these spectra, self-consistent field molecular orbital calculations have been made for the ground and excited states. The calculations, based on a π-electron model, were performed on an electronic digital computer. It could be shown in a number of cases that the aromatic hydrocarbon (in the absence of oxygen) is present in the acid solution as one type of protonated species. In these cases good agreement is found between the experimental and calculated spectra. Conversely, the theoretical predictions allowed of the interpretation of other cases where the aromatic hydrocarbon, when dissolved in the acid, forms a mixture of different ions, corresponding to a more complicated spectrum. Some of the solutions, if contacted with oxygen, display fundamental changes of spectrum. For perylene these are shown to be due to the mono-valent positive ion of the hydrocarbon.