The electronic states of pyrrole studied by optical (VUV) absorption, near-threshold electron energy-loss (EEL) spectroscopy and ab initio multi-reference configuration interaction calculations Original Research Article

A reinvestigation and extension of the observed VUV and EEL spectrum of pyrrole has been carried out, and the spectra assigned by means of high level multi-reference multi-root CI studies. A similar reasssessment of the UV-photoelectron spectrum leads to the conclusion that our previous assignment of IP3 to ionisation of the inner π-level (1b1) is correct and that most other assignments to ionisation of a σ-electron are incorrect. This is supported by the calculation of a wide range of Rydberg states derived from the five least bound orbitals, in the IP order: 1a2−1 < 2b1−1 < 1b1−1 < 9a1−1 < 6b2−1 < 5b2−1 < 8a1−1 < 7a1−1 < 4b2−1. It is concluded that each of the two strong absorption bands centred around 6 and 7.5 eV respectively owe their intensity to excitation of two 1ππ∗ states of symmetry 1A1 and 1B2. The range of calculated valence states of ππ∗-type has been extended to include those from 1b1.