Theoretical Investigations On The Spectroscopic Properties Of Molecules Related To Interstellar Medium And Air Pollution

The small molecules and radicals including Si atom and the hydrocarbon molecule with high dipole like cumulene carbenes which are detected in the interstellar medium have received much attention. These molecules and radicals not only participate in many photochemistry and photophysical reactions but have been involved in combustion chemistry and interstellar chemistry as well. There have been considerable interests in the sulphur compounds, both experimentally and theoretically, due to important role of these species in the air pollution. As a result, quantum chemical investigations on the excited states of these molecules and free redicals is becoming necessary and meaningful, so in this thesis high level ab initio method CASPT2//CASSCF have been used to perform investigation on H2CCC, H2CCSi, HSiCN, HSiNC and CH3SS molecule radicals. The main results are summarized as follows:1.CASPT2(multiconfiguration second-order perturbation theory)// CASSCF(Complete active space self-consistent field) calculations with ANO-S basis set in C2v symmetry were performed for several electronic states of H2CCC and H2CCSi. The geometries, vibrational frequencies and energies of the ground states and low-lying excited states are obtained from the theoretical investigations. The electronic configurations are used to explain the geometrical alteration in detail. The ground states of H2CCC and H2CCSi turned out to be X1A1. The CASSCF geometry for the ground state of H2CCC and H2CCSi are compared with the experimental geometry. And the calculated harmonic freqencies of the H2CCC and H2CCSi are in agreement with the experimental data. The singlet-triplet energy gap of H2CCC(106.1 kJ/mol) is larger than that of H2CCSi(98.4 kJ/mol).We have presented results from a fully correlated ab initio investigation of the electronic spectrum of H2CCC and H2CCSi neutral radical by means of using CASPT2//CASSCF method, a well-established procedure for accurated calculations of electronic spectra of molecules. The results (CASPT2 vertical excitation energies and RASSI oscillator strengths) suggest that the calculated most intensive transition of H2CCC and H2CCSi at 476.4 kJ/mol and 348.2 kJ/mol are attributed to the X1A1→21A1 and X1A1→1B1, respectively. The most intensive absorption peak is red shifted.CASPT2//CASSCF calculations with ANO-S basis set in C2v symmetry were also performed for a large number of electronic states of the H2CCC+ and H2CCSi+. Based on the corresponding results, the PES of the H2CCC and H2CCSi are assigned. The vertical and adiabatic ionization energies assigned to 15 and 16 ionic states for H2CCC and H2CCSi at the CASPT2 level, respectively. The computed first VIP/AIP of H2CCC(993.9/979.3 kJ/mol) are larger than that of H2CCSi( 779.5/773.8 kJ/mol).Due to H2CCC neutral radical's high dipole moment, all excited H2CCC- negative ion states can be characterized as dipole-bound states, but its homolog H2CCSi has valence anion excited state. The first adiabatic electronic affinity of H2CCC(160.1 kJ/mol) is larger than that of H2CCSi(100.1 kJ/mol). 2. Equilibrium geometries of low-lying electronic states of cyanosilylene (HSiCN), isocyanosilylene (HSiNC), and their ions have been investigated using the complete active space self-consistent field (CASSCF) approach. The harmonic vibrational frequencies on the optimized geometries were calculated using the multiconfiguration linear response (MCLR) method. Taking the further correlation effects into account, the complete active space perturbation theory of second-order (CASPT2) was carried out for the energetic correction. The CASPT2 calculations have been performed to obtain the vertical excitation energies of selected low-lying excited states of HSiCN and HSiNC. Computed results show that the singlet-triplet splitting energies are calculated to be 0.99 eV and 1.30 eV for HSiCN and HSiNC, respectively. The vertical excitation energies of the lowest singlet and triplet excited states in HSiCN are lower than those in HSiNC. The first vertical ionization energy of HSiCN (10.04 eV) is higher than that of HSiNC (9.97 eV). The ground-state adiabatic electron affinities are found to be rather high, and the value of HSiCN (1.87 eV) higher than that of HSiNC (1.86 eV). The existences of dipole-bound excited negative ion states have been discovered within HSiCN and HSiNC.3. CASPT2//CASSCF calculations with ANO-S basis sets were performed for 5 and 8 low-lying electronic states of CH3SS and CH3SS+ in Cs symmetry, respectively. All calculated states are valence states and their character are discussed in detail. The ground state of CH3SS is X2A", which has a leading electronic configuration (core)(12a')2(3a")2(13a')2 (14a')2(15a')2(4a")2(16a')2 (5a")α. The calculated the harmonic frequency of the ground stateν7(a') = 582 cm-1(S-S stretch)are in great agreement with the experimental data(610±160 cm-1).The electronic configurations are used to explain the geometrical alteration in detail.The ionization energies are obtained to compare with the PES data. The results are in agreement with previous experimental data.