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

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 IF, to ionisation of the inner pi-level (1b(1)) is correct and that most other assignments to ionisation of a sigma-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: 1a(2)(-1) < 2b(1)(-1) < 1b(1)(-1) < 9a(1)(-1) < 6b(2)(-1) < 5b(2)(-1) < 8a(1)(-1) < 7a(1)(-1) < 4b(2)(-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 (1)pi pi* states of symmetry (1)A(1) and B-1(2). The range of calculated valence states of pi pi*-type has been extended to include those from 1b(1). (C) 1998 Elsevier Science B.V. All rights reserved.