Theoretical Investigation On The Excited States Of Several Halogen Compounds

There has been considerable interest in the halogen compounds, bothexperimentally and theoretically, due to important role of these species inthe depletion of ozone in the polar stratospheric. As a results, quantumchemical investigations on the excited states of several important halogencompounds have been carried out using the completed active space selfconsistent field (CASSCF) method in the thesis. Those compound includeOClO,RBr(R=CH₃,C₂H₅) and XONO2 (X=F,Cl). Important informationsuch as structures, vibrational frequencies and energies of the ground statsand excited states are obtained from the theoretical investigations. Someconclusions that are made in the present thesis may be helpful for the furthertheoretical and experimental studies. The main results are summarized asfollows:1.Using the complete active space self-consistent field (CASSCF)method with large atomic natural orbital (ANO-L) basis set, ten, thirteenand nine electronic states of the OClO radical, OClO+ cation and OClO^-anion were calculated, respectively. Taking the further correlation effectsinto account, the second-order perturbation (CASPT2) calculations werecarried out for the energetic calibration. The adiabatic ionization energiesand electron affinity are obtained by the energy of each state. Furthermore,based on the Franck-Condon principle, when vertical electron ionizations ofOClO and OClO– are considered, the optimized OClO structures should bekept in computing OClO+ and the OClO– structures need to be kept incomputing OClO, respectively. The photoelectron spectroscopy of the OClOradical and OClO? anion are in detail assigned in the both case of theadiabatic and vertical ionization energies. Furthermore, the Rydberg statesof the OClO radical were investigated by using multi-configurationalCASPT2 (MS-CASPT2) theory under the basis set of ANO-L functionsaugmented with an adapted 1s1p1d Rydberg functions that have speciallybeen built for this study. Sixteen Rydberg states are obtained and thetransition of 3b1 to the 4s and 4p Rydberg orbitals are consistent of theexperimental results. Meanwhile, the results of transition to the 3d Rydbergorbitals provide evidence to explain the controversy of assignment.2. The ground state and excited states of CH3Br and C2H5Br molecule,cation and anion are investigated at the CASSCF/AN-S level. In order tocompare the effect of basis set, the electronic states of C2H5Br are studiedwith the effective core potential (ECP) basis set. As a result, the optimizedgeometry of the ground state with ANO basis set agrees better with theexperimental values than ones with ECP basis set. Meanwhile, taking intoaccount the dynamic correlation effects, the second-order perturbation(CASPT2) method has been used to obtain more reliable energies. Theadiabatic ionization energies and electron affinity are obtained by the energyof each state. Although the first two states of C2H5Br cation are degeneratedusing the ANO basis set, the adiabatic ionization energies are 9.945 and10.207eV at CASPT2 level if the interaction of spin and orbital are included,respectively. The energy split, 0.262eV, is compare with the experimentalvalue 0.308eV. Meanwhile, the electron affinity of CH3Br and C2H5Br are0.87 and 0.734eV at CASPT2 level, respectively.3. Using the CASSCF method with ANO-S basis set, the ground stateand low-lying excited states of fluorine and chlorine nitrate and their cationhave been studied. First, the ground state is studied using the different activespace in order to choose the optimal active space. Meanwhile, the effects ofbasis set are investigated in the ground state. The stable geometry of groundstate was the planar structure with the Cs symmetry, but the stable structuresof excited states had been changed to the nonplanar structure. However, thesimilar change of the cation geometry had not been observed. Furthermore,the vertical ionization potential of the fluorine and chlorine nitratemolecular has been studied basing on the optimized geometry of groundstate. Taking into account the dynamic correlation effects, the second-orderperturbation (CASPT2) method has been used to obtain more reliableenergies. The assignment of first two bands agreed excellent with theexperimental data and the theoretical results, but in the higher energy region,the different assignments had been completed basing on the CASPT2 results.Furthermore, the mechanisms of ClONO2 dissociation reaction are studiedat CASSCF level, and ClONO2→ClO+ NO2 dissociation is controlled bythe triplet electronic state.