2015. 56, 2 UDC 541.6:548.737 THEORETICAL STUDIES ON THE SPECTROSCOPIC PROPERTIES OF METHYL MERCAPTAN (CH3SH) A.-Y. Yu1, Y.-P. Yang2 1Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China E-mail: ayyu@imr.ac.cn 2State Key Laboratory of Organic and Inorganic Composites, Beijing University of Chemical Technology, Beijing, China E-mail: yypcnbe@126.com Received July, 20, 2013 Revised — November, 4, 2013 The properties of the ground and low-lying excited states of methyl mercaptan (CH3SH) are studied by using quantum chemistry methods. <...> The geometric parameters and energies of the ground state and the triplet excited state are calculated in this work. <...> It is shown that the calculated geometries of the ground and triplet state are stable and the triplet state potential energy surface has the repulsive feature. <...> The calculated vertical excitation energies and vertical ionization potentials of methyl mercaptan are in best agreement with the experimental values. <...> In addition, the vertical electron affinity and the adiabatic electron affinity of the CH3SH molecule are also calculated. <...> Keywords: methyl mercaptan (CH3SH), ground state, triplet state, excitation energy, ionization energy, electron affinity. <...> INTRODUCTION Methyl mercaptan (CH3SH) is a very important sulfur compound both in atmosphere and industry [ 1 ]. <...> Both bands correlate with the excitation from the ground state to the first two excited singlet states, which are the 1, 2 1A’ states. <...> Despite some theoretical and experimental efforts on the spectroscopic studies of CH3SH [6, 7 ], the investigations of the properties of the ground and low-lying excited states of methyl mercaptan (CH3SH) are still needed. <...> In this work, the geometric parameters, energies, and the harmonic vibrational frequencies of the ground state and the triplet state are obtained by quantum chemistry methods. <...> The vertical excitation energies of the singlet excited states of CH3SH are also calculated and the theoretical results are compared with available experimental results. <...> In addition, the vertical ionization energy, adiabatic ionization energy, the vertical electron affinity and the adiabatic electron affinity of the methyl mercaptan are calculated, as well. <...> These results could serve as a guide for the experimental and theoretical studies in the future. <...> THEORY AND CALCULATION DETAILS It is found that the MP2/aug-cc-pVTZ level of theory <...>