Theory of the Ultraviolet Absorption Spectrum of Diphenyl

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Abstract
The lower excited energy levels of the molecule diphenyl are calculated by the method of antisymmetrized products of molecular orbitals. The wave functions for diphenyl are constructed from linear combinations of benzene molecular orbitals and the perturbation of the original energy levels of benzene caused by the presence of the second benzene ring in diphenyl is computed. The theory predicts the splitting of the two non‐degenerate singlet levels and the one degenerate singlet level of benzene into eight levels, four of which give allowed electronic transitions to the ground state and four of which do not. Experimentally three strong structureless bands are observed, one between λ = 2200A and λ = 2800A, one at about λ = 1900A, and one below λ = 1700A. The calculated spectrum has allowed transitions at about 2400A, 1900A, 1500A, and 1400A, and thus is in good agreement with the experimental observations.