Assignment of the electronic states of pyrazole by ab initio multi-reference configuration interaction calculations

Abstract

The gas-phase VUV absorption spectrum of pyrazole, which we reported recently, has been further assigned in the light of multi-reference multi-root CI calculations, using basis sets of varying size up to quadruple zeta quality, and containing both valence and Rydberg type functions. A very intense VUV band centred near 7.8 eV appears to arise from the summation of three calculated bands of ππ* character, of which the first and third are the most intense. The window resonance near the band maximum is ascribed to mutual annihilation of a Rydberg state and valence state, and a probable assignment is discussed. The electron energy loss (EEL) spectrum also obtained previously, showed low-lying triplet states at about 3.9 and 5.1 eV, respectively; the present computations suggest that two triplet (3ππ*) states lie within the 3.9 eV band, and identifies the species involved. The assignment of the UV-photoelectron spectrum has been reconsidered, but the identity of the first three IPs as π3<π2<LPN (lone pair) is confirmed; as a consequence most of the excited singlet and triplet states below 10 eV are likely to be of ππ* and πσ* character with both valence and Rydberg types. However, a number of σσ* and σπ* states including those derived from the lone pair electrons were obtained from the CI study, which give predictive values for the onset of such excitations. The highest pair of occupied orbitals (π3 and π2) both interact strongly with the lowest pair of valence VMOs (π4* and π5*) leading to a wide variety of valence states, and as such the molecule behaves much more like a system with degenerate pairs of π-MOs. Evidence is presented that (short lived) anions formed by electron impact are in fact 2Σ(2A′) rather than 2Π(2A″), and that this is likely to be true for the related anions of pyrrole, furan and isoxazole. The calculated vibration frequencies and a number of 1-electron properties of the ground state have been compared with experimental data.