The electronic states of isoxazole studied by VUV absorption, electron energy-loss spectroscopies and ab initio multi-reference configuration interaction calculations

The VUV absorption spectrum of isoxazole (5-10.8 eV, 250-115 nn) has been recorded for the first time. The molecule has also been probed using electron impact with electrons of different incident energies and the He(I) photoelectron spectrum has been re-measured. Electronic excitation energies for valence and Rydberg-type states have been computed using multi-reference multi-root CI methods. Calculated energies for Rydberg states are close to those expected, but the precision of the calculated (1)pipi* states is more variable, especially for the lowest members. More than 30 valence excited states having finite oscillator strengths are computed to lie between 6 and 12 eV, but most of the intensity in the VUV absorption spectrum is from excitation of states of (1)pipi* character. From the results of the calculations, it is concluded that the first two (1)pipi* states lie at about 6 and 7 eV, respectively, and are separated by a state of type (1)sigmapi*, where a is nitrogen lone pair; dominant higher bands near 8 and 9 eV are also largely (1)pipi* in character. The lowest-lying triplet states, located by experiment at about 4.1 eV and 5.3 eV, are calculated to be (3)pipi*. Short-lived anionic states (electron-molecule resonances) have been detected in both inelastic scattering and dissociative electron attachment channels. Some one-electron properties derived from the ground state wavefunction of the molecule have also been computed for comparison with experiment. (C) 2003 Elsevier B.V. All rights reserved.