Electronic structure of the ground and excited states of beta-carboline.

Coupled-cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of beta-carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects.