Theoretical Investigations On The Excited States Of Several Transient Species Related To Atmosphere And Interstellar

There are a great many atoms and molecules which are excited states in the interstellar space. With the continuous development of science, more and more heteroatom-doped carbon-chain radicals were discovered in interstellar spaces IRC + 10216, CRL 2688 and TMC-1, such as double-doped MgNC, MgCN, NaCN, KCN, AlNC, AlCCH and HCnN and single-atom doping CCCH, CCCN, CCCO and CCCS, therefore, the study of heteroatom-doped carbon-chain radicals in interstellar spaces is an important job for us. On another hand, there are also much more free radicals in the atmosphere and combustion processes, such as methyl free radical, which is very unstable, has a high degree of importance in combustion chemistry and atmospheric chemistry since it is a critical intermediate in many oxidations, so that, many studies have been reported to characterize the spectroscopy and kinetics of methyl free radical. Thus, it is of significance to study the mechanism of the reaction between methyl free radical and some molecules and radicals in the atmosphere for environmental protection.In this article, the low-lying electronic states of several molecules and radicals and their ions which are in relation to interstellar medium and atmosphere, have been studied by using high level quantum chemistry method complete active space self-consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2). The present results might not only provide a theoretical guidance to the absorption spectrum and photoelectron spectrum of the molecules and radicals, but also help to further explore theoretical and experimental works. The main results are summarized as follows:1. The low-lying electronic states of the CCCF radical and its ionic states have been investigated systematically using the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods in conjunction with ANO-RCC-TZP basis set. To investigated the Renner-Teller effect on the CCCF radical, Cs symmetry was used for CCCF. The CCCF has been found to have a X²A' ground state with rotational constant B = 4550.5 MHz, which is in good agreement with the experimental values of 4555.8043 MHz. The calculations of vertical excitation energies of CCCF at 2.973 and 3.782 eV are attribute to the X²A'→3²A″and X²A'→5²A', respectively, which has larger oscillator strengths.AThe vertical excitation energies are predicted at 2.973 and 3.782 eV for X²A'→3²A″and→5²A' of CCCF. The structure of ground state X¹A' for CCCF+ is to be linear, while the structure of ground state X¹A' for CCCF- is strongly bent. For CCCF-, the Ta values of the 1¹A″and 2¹A' states have been predicted at 1.689 and 2.833 eV. The magnitude of the dipole moment is larger for the anionic states. The first AEA of CCCF is calculated to be 2.550 eV. The AIPs and VIPs at the CASSCF/CASPT2 levels are more than 9 eV. According to the VIPs values, we predicted the three bands of PES around 10.7, 12.5, and 14.0 eV.On the other hand, we also give a theoretical study on CCCX (X=F, Cl, and Br) and we fond that In the ground state, the CCCX radicals have an allenic (C=C= CA A X) electronic structure with unpair electron on C3 atom, but are more appropriately described as having an acetylenic (ACAC≡CAX) structure in the first excited states, with the unpaired electron localized on the C1 atom. The larger Renner-Teller effect for CCX bending compared to CCC bending in all these radicals is a direct result of the larger dipole moment component generated by displacing the X atom from linearity. The Renner-Teller stabilization energies (ΔE) are smaller as atomic weight of X increase.2. The low-lying electronic states of the CCCN radical and its ionic states have been investigated systematically using the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods in conjunction with ANO-RCC-TZP basis set. The CCCN has been found to have a X²Î£⁺ ground state with a linear structure and the band lengths R₁=1.214 A, R₂=1.363 A, R₃=1.162 A, which is in good agreement with the experimental values of 1.2116, 1.3746, 1.1609 A. The calculations of vertical excitation energies of CCCN at 0.632, 4.044 and 5.490 eV are attribute to the X²Î£⁺→1²Î , X²Î£⁺→2²Î and X²Î£⁺→2²Î£⁺, respectively, which has larger oscillator strengths. The ionization potentials of CCCN are computed in order to provide a theoretical guidance to the photoelectron spectrum (PES) of the CCCN radical. The first AEA of CCCN is predicted to be 4.158 eV. A comparison of the geometries and bonding among the CCCN radicals presents the characters of the bonds in the ground state X²Î£⁺ of CCCN neither doubly nor triply, actually, they are between them.3. Electronic states of the CH3Se and its cation CH₃Se⁺ have been studied using the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods in conjunction with ANO-RCC(TZP) basis set. To investigate the Jahn-Teller effect on the CH3Se radical, Cs symmetry was used for CH3Se in calculations. The results show that the Jahn-Teller effect is very small (69 cm^-1) and the 1²A' state is slightly more stable than 1²A"(8 cm-1). The CH3Se has been found to have a 1²A' ground state with C-Se bond distance of 1.973 A. The computed C-Se stretching ν6(a') frequency is 554.1 cm^-1 which is in good agreement with the experimental values of 600±60 cm-1. The calculations of CH₃Se at 3.621 and 5.307 eV are attribute to the 1²A'→2²A'(1²A₁) and 1²A'→2²A", respectively. The vertical and adiabatic ionization energies were obtained to compare with the PES data.