| There has been considerable interest in the free radicals which contain N or O atom, both experimentally and theoretically, due to important role of these species in the depletion of ozone in the polar stratospheric. As a result, quantum chemical investigations on the excited states of several important free radicals containing N or O atoms have been carried out using the completed active space self consistent field (CASSCF) method in this thesis. Those compound include (CH3)nCH3-nN(n=0～3), C6H5N and CH3O2, and their positive and negative ions. Potential energy curves (PECs) for O2-loss dissociation from 2A" and 2A′states were also calculated at the CASPT2//CASSCF level. The conclusions made in the present thesis may be helpful for the further theoretical and experimental studies. The main results are summarized as follows:1. CASPT2(multiconfiguration second-order perturbation theory)// CASSCF(Complete active space self-consistent field) calculations with 6-311++g(3df,3pd) basis sets in Cs symmetry were performed for a large number of electronic states of the (CH3)nCH3-nN (n=03). The structures, vibrational frequencies and energies of the ground states and excited states are obtained from the theoretical investigations. The CASSCF geometry for the 13A″( R(C-N)=1.446A and∠HCH= 108.4°) ground state of CH3N is compared with the experimental geometry for the X3A2 ( R(C-N)=1.411A and∠HCH= 106.7°) guound state. The calculated harmonic frequencies of the ground state of CH3N (v1" = 2854 cm-1 (symmetric hydrogen stretch), v2" = 1403 cm-1 (symmetric hydrogen bend), v3" = 1403 cm-1 (C-N stretch), v4" = 2923 cm-1 (asymmetric hydrogen stretch), v5" = 1469 cm-1 (scissors) and v6" = 999 cm-1 (rock)) are in agreement with the experimental data. In the ground state of (CH3)nCH3-nN(n=03), because the CH3 groups delocate the electron between N and the C atom, the C-N bond length get shorted as n increases.The results (CASPT2 adiabatic excitation energies and RASSI oscillator strengths) suggest that the calculated transitions of (CH3)nCH3-nN(n=0～3)at 311, 334, 341 and 346, 356 and 361nm are attributed to the 13A"→13A′and 13A"→23A", respectively. The reason for the red shift is that the CH3 group delocates the electron between N and C atom.CASPT2// CASSCF calculations with 6-311++g(3df,3pd) basis sets in Cs symmetry were performed for a large number of electronic states of the (CH3)nCH3-nN+(n=03) and (CH3)nCH3-nN-(n=03). The structures, vibrational frequencies and energies of the ground states and excited states are obtained from the theoretical investigations. We predict a strong transition in the adiabatic excitation spectrum of CH3N+ atλ= 314nm, a weak transition of CH3N- atλ= 582nm and a weak transition of (CH3)2CNH+ atλ=314nm. We used the optimized geometry of the ground state of the (CH3)nCH3-nN(n=0～3) to calculate the vertical ionization energies which are obtained from the difference of the total energies between the resulting positive ion and the neutral (CH3)nCH3-nN(n=03) radical. The calculated adiabatic ionization energies are obtained from the difference of the total energies between the positive ion and the neutral (CH3)nCH3-nN(n=0～3) radical under their respect optimized geometry. The calculated first vertical/adiabatic ionization energies for the (CH3)nCH3-nN(n=0～3) are 11.17/10.60, 10.43/10.29, 10.41/9.84 and 10.14/9.56eV, which are in agreement with previous experimental data. The other calculated ionization energies are also in agreement with previous experimental data.2. CASPT2// CASSCF calculations with 6-31g(d,p) basis sets were performed for a large number of electronic states of the nitrate free radical C6H5N and their positive and negative ions in C2v symmetry. The structures, vibrational frequencies and energies of the ground states and excited states are obtained from the theoretical investigations. The ground state of C6H5N is 3A2 and is dominantly (core) 1b1213a127b222b121a228b213b11. In the ground state, because the conjugation effect of the N atom with the phenyl, C-N bond length is very short and is 1.326 A, and because the repulse effect of the N atom with the phenyl, the HCH bond angle is smaller than benzene and is 118.8o.The results suggest that the calculated transitions of 13A2→13A1 and 13A2→13B2 at 278 and 284 nm are attributed to theπ(b1)→ny andπ(a2)→ny, respectively, while those of 13A2→13B1 and 13A2→11B1 at 454 and 443nm belong to the mixture of nx→π* (a2) andπ(a2)→nx transitions, these above results are in agreement with previous experimental data.The ionization energies were obtained to compare with the PES data. hese results are in agreement with previous experimental data, too.3. CASPT2// CASSCF calculations with aug-cc-pVTZ basis sets were performed for twelve low-lying electronic states of the methyl peroxy radical CH3O2 and their positive and negative ions in Cs symmetry. All calculated states are valence states and their characters are discussed in detail. The ground state of CH3O2 is 2A" and is dominantly (core)(7a′)2(8a′)2(1a")2(9a′)2(2a")2(10a′)2(3a")1. The calculated transitions of 2A″→2A′at 1.018eV are attributed to the ny→nx, which is in accord with the experimental data of 0.914eV. The calculated affinity energy of CH3O2 is 0.990eV, which is in reasonable agreement with the experimental data of 1.161eV.Potential energy curves (PECs) for O2-loss dissociation from the 2A", 2A′states were calculated at the CASPT2//CASSCF level and the electronic states of the O2 and CH3 as the dissociation products were carefully determined by checking the relative energies and geometries of the asymptote products along the PECs. The O2-loss PEC calculations for CH3O2 indicate that O2-loss dissociation occurs from the 2A" and 2A′states leading to CH3(2A")+O2(3A") and CH3(2A")+O2(1A") , respectively.
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