The electronic states of 1,2,5-oxadiazole studied by VUV absorption spectroscopy and CI, CCSD(T) and DFT methods

Abstract

The 1,2,5-oxadiazole VUV absorption spectrum in the range 5–11.5eV, shows broad bands centred near 6.2, 7.1, 8.3, 8.8, 10.6 and 11.3eV. Rydberg states associated with three ionisation energies (IE) were identified in the complex fine structure above 8.7eV. Electronic vertical excitation energies for singlet and triplet valence, and Rydberg states were computed using ab initio multi-reference multi-root CI methods. There is generally a good correlation between the envelope of the theoretical intensities and the experimental spectrum. The nature of the more intense calculated Rydberg states, and positions of the main valence and Rydberg bands are discussed. The lowest triplet, singlet and Rydberg 3s excited states have equilibrium structures that are non-planar with CS symmetry, in a chair-like orientation where the O and H atoms lie out of the NCCN plane. This finding is consistent with the doubling of the low energy UV spectral lines [B.J. Forrest, A.W. Richardson, Can. J. Chem., 50 (1972) 2088].The nearly degenerate IE of the UV-photoelectron spectrum (UV–PES, Palmer et al. 1977) makes analysis of the VUV spectrum difficult, leading to the necessity for reinvestigation. Vertical studies (IEV) using CI, Tamm–Dancoff (TDA) and Green’s Function (GF) methods all gave similar results, with near degeneracy of the first 3IEV confirming the earlier study. Studies of the adiabatic IE (IEA) using CCSD(T) and B3LYP methods, showed the energy sequence 2A2<2B1<2B2, but these states are all saddle points, in contrast to the 4th state (2A1) which is a minimum. In contrast, MP2 study of the 2B2 state showed a minimum, with only two saddle points.Complete minima were found after minor twisting of the structures. The lowest energy cationic state is 2A″ (CS), which closely resembles the 2B2 state. The O–N–C–C skeleton is twisted by 8°. The corresponding 2A′ state (CS) is effectively identical to the 2B1 state. Attempts to find minima for other symmetry states were unsuccessful.