Theoretical Investigations On The Excited States Of Small Molecules Related To Interstellar Medium And Atmosphere

There are a great many atoms and molecules which are excited states in the atmosphere, interstellar space and combustion processes. HCCO is a critical intermediate in the oxidation of acetylene. Many studies have been reported to characterize the spectroscopy and kinetics of HCCO. It is of significance to study the mechanism of the reaction between HCCO and some molecules and radicals in the atmosphere for environmental protection. The HCNN radical is a highly reactive transient species that plays an important role in combustion chemistry, atmospheric chemistry. HCNN is considered as an intermediate in the reaction between the CH radical and N2 molecule, and provides the reaction pathway for the"promote NO". AlCCH is a photolysis product of the aluminum-acetylene adducts, and has been considered as a molecule with potential interest in astrophysics. The metal-bearing halides, cyanides and isocyanides have been discovered toward circumstellar envelopes of carbon-rich stars such as IRC + 10216 and CRL 2688. AlCN is one of the metal-bearing cyanides. The conversion from AlCN to AlNC needs low activation energy.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 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 CASPT2//CASSCF methods have been used to investigate the low-lying electronic states of the HCCO radical and its anion. The geometries are calculated at the levels of CASSCF/cc-pVTZ,CASSCF/ANO-L and CASPT2/cc-pVTZ, respectively. On the basis of the obtained geometries, the corresponding adiabatic excitation energies are computed at the CASSCF and CASPT2 levels. By comparison, the calculated geometrical parameters at the CASPT2/cc-pVTZ level for the X2A″state of HCCO are in good agreement with experimental values, and theν1 CH stretch (3213 cm-1),ν2 asymmetric CCO stretch (2059 cm-1) andν5 CCH bending (504 cm-1) modes agree with experimental values of 3232, 2023 and 494 cm-1. The barrier to 12П(12A′) is estimated to be 0.069 eV (554 cm-1), which is also close to experimental result (540 cm-1). Moreover, we find that the 22A′state is bent, while 22П(22A″) is linear. The oscillator strength of X2A″→22П(22A″) is 0.0064. The adiabatic excitation energy is 4.054 eV, which accords with the experimental value of 4.14 eV. By comparing the oscillator strengths and adiabatic excitation energies, we consider that the 22П(22A″)←X2A″transition is the most intense among these transitions. The unpaired electrons mainly locate on the C atoms in the 22A′and 22П(22A″) states according to the Mulliken spin populations.On the other hand, the geometrical parameters of the X1A′andΣ+ states of HCCO￣are in good agreement with previous reports. The first adiabatic and vertical detachment energies of HCCO? are 2.210 and 2.362 eV, respectively, which reasonably agree with experimental value of 2.338±0.008 eV. According to vertical detachment energies, the next band of PES is predicted to be around 6.80 eV. Additionally, we explore several higher excited states of the HCCO radical (14A′, 24A″and cis-14A′) and its anion (13A″, 11A″and 13A′), which have not been reported in previous studies.2. We have systemically studied the low-lying electronic states of the HCNN radical and its ions in CS symmetry through CASPT2//CASSCF methods. At the CASSCF/ANO-L level, the geometries and harmonic vibrational frequencies are calculated for the seven electronic states of HCNN, seven electronic states of HCNN+ and the ground state of HCNN￣. The results indicate that the ground state of the HCNN radical has trans-bent planar structures. The calculatedν2 andν4 of the ground state of HCNN are 1842 and 888 cm-1, respectively, which agree with experimental values (1800 and 871 cm-1). The adiabatic excitation energy of the 12A′state is predicted to be 0.578 eV, which is in agreement with the experimental range of 0.522±0.012 ~ 0.675±0.012 eV. The adiabatic excitation energy (2.784 eV) of the 22A″state accords with the experimental report. On the other hand, the first vertical detachment energy of HCNN￣is 1.591 eV, which is in reasonable agreement with the experimental value of 1.685±0.006 eV.3. The low-lying electronic states of AlCCH, cation and anion have been studied by CASPT2//CASSCF methods. The geometrical parameters, electron configurations, excitation energies, oscillator strengths and harmonic vibrational frequencies are calculated in CS symmetry. For the X1Σ+ state of AlCCH, the obtained geometrical parameters are in agreement with those at the CASSCF/VDZ and DFT/aug-cc-pvtz levels, and the calculatedν2 andν4 (1980 and 512 cm-1) are close to the experimental values (1977 and 513 cm-1). The adiabatic and vertical excitation energies of 11Πare 3.65 and 3.77 eV, respectively, which are in agreement with the experimental value of 3.57 eV.Moreover, the geometry of the ground state of AlCCH+ accords with the report at the MP2/6-31G* level. The electron transitions of AlCCH+, X2Σ+→12Π, X2Σ+→22Σ+ and X2Σ+→12Σ￣, are predicted at 2.57, 4.51 and 4.61 eV, respectively. For AlCC￣H, the transition X 2Π→12Σ￣occurs at 3.02 eV. According to the vertival ionization energies, we predict that the three bands of PES are around 8.7, 10.5 and 12.5 eV.4. The CASPT2//CASSCF methods have been used to study the low-lying electronic states of AlCN and its ions. In C2v symmetry, the geometries, leading configurations and configuration interaction coefficients, harmonic vibrational frequencies, adiabatic and vertical excitation energies, and oscillator strengths of AlCN and its ions are calculated with the ANO-L basis sets. For the X1Σ+ state of AlCN, the obtained Al–C and C–N bond lengths are 2.009 and 1.169 ?, respectively, which are in agreement with those at the MRCI/aug-cc-pVTZ and QCISD(T)/cc-pVTZ levels. For 11Πof AlCN, the adiabatic excitation energy is obtained to be 3.65 eV, which agrees well with the experimental value of 3.57 eV, and the X1Σ+→21Πtransition is predicted at 6.98 eV. In addition, for 22Σ+, 12Πand 22Πof AlCN+, the adiabatic excitation energies of are 2.78, 3.85 and 3.91 eV, respectively. In addition, the first adiabatic and vertical ionization energies of AlCN are 9.41and 9.56 eV, respectively.