The electronic states of the azines. III. Pyrimidine, studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration calculations

Abstract

The electronic states of pyrimidine have been investigated by VUV absorption and near-threshold electron energy-loss spectroscopy, together with ab initio multi-reference configuration interaction calculations. This approach has allowed the experimental assignment of all the low-lying valence-excited nπ* and ππ* singlet states. In contrast to ππ* excitations, some of which are difficult to compute within 0.5 eV, the nπ* states are more accurately calculated, as are the lowest Rydberg excited states. The experimental identification of many low-lying Rydberg states is not yet possible. The UV-photoelectron spectral assignments by large-scale CI are similar to earlier Green's function results, except for the inner π level (1b 1) ionisation; many shake-up states are computed below 17 eV and it is clear that the TDA method, with the same basis set, leads to a much later onset.