Multireference Study On The Small Molecule Radical Containing S

There are a great many atoms and molecules which are excited statesin the atmosphere, interstellar space and combustion processes, in whichthe sulfur-containing compounds, especially alkylthio radicals areimportant intermediates in the process of combustion and atmosphericchemistry, they have important influence to the sulfur cycle in theatmosphere, now for this type of alkythio radicals, the spectroscopicresearch has drawn much attention in both experiment and theory.Therefore it is of significance to study the properties of the ground andexcited states of this kind of free radicals for environmental protection.（CH₃）₂CHS is an alkylthio radical that less reported plays an importantrole in combustion chemistry, atmospheric chemistry. Through thehigh-temperature pyrolysis isopropyl disulfide C3H7SSC3H7can obtainthe （CH₃）₂CHS.In this article, the ground and excited states of （CH₃）₂CHS and itscation have been systematically studied by using complete active spaceself-consistent field （CASSCF） and multiconfigurational second-orderperturbation theory （CASPT2）. The calculation results might provide atheoretical guidance to the absorption spectrum and photoelectronspectrum of （CH₃）₂CHS, and through the theoretical predictions of some unknown data could be used to further explore theoretical andexperimental works. The main results are summarized as follows:In CSsymmetry, we have systemically studied the low-lyingelectronic states of （CH₃）₂CHS and its cation by CASSCF/CASPT2methods. At the CASSCF/ANO-RCC-VTZP level, the geometries andharmonic vibrational frequencies are calculated for the five electronicstates of （CH₃）₂CHS, six electronic states of （CH₃）₂CSH⁺. The resultsindicate that the electronic state X2A′was identified as the ground state of（CH₃）₂CHS, and the calculated C-S stretching mode is in good agreementwith experimental reports. The result of our calculations suggest that thecalculated transition of （CH₃）₂CHS at17712,370,201,205, and204nmare attributed to X²A′→1²A″, X²A′→2²A′, X²A′→2²A″, X²A′→3²A′,and X²A′→3²A″, respectively.On the other hand, the result suggest that the electronic state X3A″was identified as the ground state of （CH₃）₂CSH⁺.The first verticalionization potential is8.98eV, which reasonably agree with experimentalvalue of8.95eV. Remarkably, we explore several cation statesadiabatically and find that11A′state is not stable and convert to（CH₃）₂CSH⁺. Moreover, the vertical and adiabatic ionization energieswere obtained to compare with the photoelectron spectroscopic data.