Theoretical study of the vertical excited states of benzene, pyrimidine, and pyrazine by the symmetry adapted cluster-configuration interaction method

The ground state and the excited states of benzene, pyrimidine, and pyrazine have been examined by using the symmetry adapted cluster-configuration interaction (SAC-CI) method. Detailed characterizations and the structures of the absorption peaks in the vacuum ultraviolet (VUV), low energy electron impact (LEEI), and electron energy loss (EEL) spectra were theoretically clarified by calculating the excitation energy and the oscillator strength for each excited state. We show that SAC-CI has the power to well reproduce the electronic excitation spectra (VUV, LEEI, and EEL) simultaneously to an accuracy for both the singlet and the triplet excited states originated from the low-lying pi -> pi*, n -> pi*, pi -> sigma* and n -> sigma* excited states of the titled compounds. The present results are compared with those of the previous theoretical studies by methods, such as EOM-CCSD(T), STEOM-CCSD, CASPT2 and TD-B3LYP, etc. (C) 2007 Wiley Periodicals, Inc

Resonance Formation in Electron Collisions With Pyrimidine-Like Targets

In this thesis we apply the R-matrix method to study low-energy (<15 eV) elastic and inelastic electron collisions with the diazine molecules (pyrazine, pyrimidine and pyridazine), (2 and 4) oxo-pyrimidines and the RNA nucleobase uracil. We use these molecules as models for the pyrimidinic DNA nucleobases. The main goal of this work is to study resonance formation in these targets. We test various standard scattering models (Static Exchange, Static Exchange plus Polarization, Close-Coupling) to identify those that produce the most accurate results; we also study simplified versions of these models that allow us to provide detailed insights into the resonance formation. We compare our elastic (for pyrazine and pyrimidine) and inelastic (for pyrimidine) cross sections with available experimental data and find a good agreement. A methodology based on the time-delay analysis is applied that allowed us to find and characterize in detail many new resonances not previously identified. We find an unexpectedly large number of resonances. Most of the new resonances have core-excited shape character and many of them do not enhance (significantly) the elastic or inelastic cross sections for collisions with the molecule in the ground state. These resonances, however, significantly enhance the elastic cross sections for collisions with molecules in electronic excited states. The discovery of a large number of these resonances and their unusual properties are novel and unexpected results. We find that oxygen substitution of the pyrimidine ring does not lead to significant changes in the resonance formation: the resonances in pyrimidinic molecules are associated with the ring structure. We conclude that the picture of resonance formation in pyrimidines is much more complex than expected. Most of the work presented in this thesis has been published in peer-reviewed journals