Polarization propagator study of electronic excitation in key heterocyclic molecules I. Pyrrole Original Research Article

The electronic excitation spectrum of pyrrole is studied using a polarization propagator method referred to as the second-order algebraic-diagrammatic construction (ADC(2)), along with a simple model for vibrational excitation accounting for all totally symmetric modes. The method describes the optical absorption profile of pyrrole with an expected accuracy of 0.2 – 0.4 eV for the vertical excitation energies. The vibrational analysis provides for detailed additional spectroscopic information. In the singlet spectrum, besides the ns, np and nd (n = 3,4) Rydberg excitations, three π-π∗ valence transitions, V′(1A1), V(1B2) and V(1A1) can clearly be distinguished. No evidence is found for Rydberg-valence interaction near the equilibrium geometry. Substantial vibrational widths and distinct vibrational excitation patterns are predicted for the Rydberg series converging to the first and second ionization thresholds. Some new assignments of major spectral features are proposed. The long-wave absorption maximum in the 5.6 – 6.6. eV region is explained exclusively by the presence of Rydberg transitions, while the most intense absorption in the short-wave band system (7.0 – 8.3 ev) predominantly originates from the V(1B2) and V(1A1) valence transitions.