Effect of Oxygen on the Ultraviolet Spectra of Benzene

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Abstract
The ultraviolet spectrum of benzene induced by oxygen was studied in the wavelength range 2200–3000 A. From the experimental results obtained, the electronic absorption of this system is interpreted as a charge‐transfer absorption induced during a chance contact between benzene and oxygen without formation of stable complexes. The general formula obtained by Tsubomura and Mulliken was used together with experimentally obtained energy of the charge‐transfer state (about 5.6 ev) to calculate the intensity of the charge‐transfer absorption borrowed from the 1 A 1g —1 B 1u and 1 A 1g —1 E 1u absorptions of benzene. The calculated intensity is in excellent agreement with experimental data, indicating that it is the interaction of the charge‐transfer state with the benzene 1 B 1u and 1 E 1u states which is responsible for the observed intensity of the charge‐transfer absorption. Experimental evidence is also presented which indicates that the two previously observed weak absorptions in benzene at 4.2 and 4.9 ev, which several workers have conjectured might be the transitions to the higher triplet states, are not multiplicity‐forbidden transitions. The absorption at 4.2 ev has been interpreted as due to an impurity and the absorption at 4.9 ev has been tentatively ascribed as due to a solvent effect whose nature is not yet understood.