Vacuum ultra-violet absorption spectra of various mono-substituted benzenes

Abstract

The vacuum ultra-violet spectra of various mono-substituted benzenes were measured in the wavelength region of 1550 Å to 2200 Å in the vapour phase by a recording vacuum ultra-violet spectrophotometer. The compounds studied here are phenols (phenol, anisole, phenetole and thiophenol), halogenobenzenes (fluorobenzene, chlorobenzene, bromobenzene and iodobenzene), and toluene. It was found that four π→π* transition bands appear in the wavelength region of 1550 Å to 3000 Å for phenol, anisole, phenetole, fluorobenzene, chlorobenzene and bromobenzene. On the other hand, six π→π* transition bands were found in the same wavelength region for thiophenol and iodobenzene. The absorption spectra of the former group are similar to that of benzene itself. On the other hand, the absorption spectra of the latter group are very different from that of benzene and rather similar to those of the anilines studied in a previous paper. From this point of view, the former group may be regarded as the molecules with weak substituents, and the latter as the molecules with substituents. Theoretical studies of π-electron structures have been carried out with the phenols and halogenobenzenes by considering configurational interactions among the ground, locally excited, and intramolecular charge-transfer configurations. Good agreement was obtained between the experimental and theoretical values for both transition energies and oscillator strengths. It was concluded that the energies of charge-transfer configurations have a great effect upon the absorption spectra of the mono-substituted benzene molecules. That is to say, the charge-transfer configurations lie intermediate between the locally excited configurations for the molecules with strong substituents, while for the molecules with weak substituents they have higher energies than the locally excited configurations. Intramolecular charge-transfer bands were observed for the molecules with strong substituents. The electron affinity of benzene was determined to be -1·1 ± 0·3 eV from the energy of the charge-transfer configuration estimated in such a way as to explain as well as possible the observed absorption spectra of the mono-substituted benzenes under consideration.