Knowledge Bank

Using semiempirical methods, the transition energies and oscillator strengths of the four long-wavelength singlet-singlet bands in substituted benzenes have been calculated as a function of the angle of twist of the $\pi$-orbital of a substituent group. The corresponding singlet-triplet energies have also been investigated. Three factors enter importantly into the treatment: (a) The functional dependence of the orbital energies on the twist $angle,1$ (b) the degree of mixing of the excited configurations (leading to states corresponding to the $E1u$, $B1u$, $B2u$ states of benzene), (c) the amount of conjugative and inductive interaction of the twisted substitutent group with the ring. Special reference is made to the dimethylanilines.