title: Theoretical spectroscopic characterization at low temperatures of S-methyl thioformate and O-methyl thioformate.
abstract: Highly correlated ab initio methods are employed to determine spectroscopic properties at low temperatures of two S-analogs of methyl formate: S-methyl thioformate CH3-S-CHO (MSCHO) and O-methyl thioformate CH3-O-CHS (MOCHS). Both species are detectable and they are expected to play an important role in Astrochemistry. Molecular properties are compared with those of the O-analog, methyl formate. Both isomers present two conformers cis and trans. cis-CH3-S-CHO represents the most stable structure lying 4372.2 cm(-1) below cis-CH3-O-CHS. The energy difference between the cis and trans forms is drastically lower for MSCHO (1134 cm(-1)) than for MOCHS (1963.6 cm(-1)). Harmonic and anharmonic fundamentals and the corresponding intensities, as well as the rotational constants for the ground vibrational and first excited torsional states and the centrifugal distortions constants, are provided. Low torsional energy levels have been obtained by solving variationally a two dimensional Hamiltonian expressed in terms of the two torsional degrees of freedom. The corresponding 2D potential energy surfaces have been computed at the CCSD(T)/aug-cc-pVTZ level of theory. The methyl torsional barriers V3(cis) are determined to be 139.7 cm(-1) (CH3-S-CHO) and 670.4 cm(-1) (CH3-O-CHS). The A/E splitting of ground torsional state has been estimated to be 0.438 cm(-1) for CH3-S-CHO and negligible for CH3-O-CHS.
authors.affiliation: Departamento de Química y Física Teóricas, Instituto de Estructura de la Materia, IEM-C.S.I.C., Serrano 121, Madrid 28006, Spain.
authors.forename: M L
authors.initials: ML
authors.lastname: Senent
authors.affiliation: Dipartimento di Chimica G. Ciamician, Università di Bologna, Via F. Selmi 2, I-40126 Bologna, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Puzzarini
authors.affiliation: Laboratoire de Modélisation et Simulation Multi Echelle, Université Paris-Est, MSME UMR 8208 CNRS, 5 boulevard Descartes, 77454 Marne-la-Vallée, France.
authors.forename: M
authors.initials: M
authors.lastname: Hochlaf
authors.affiliation: Departamento de Ingeniería Civil, Cátedra de Química, E.U.I.T. Obras Públicas, Universidad Politécnica de Madrid, Madrid, Spain.
authors.forename: R
authors.initials: R
authors.lastname: Domínguez-Gómez
authors.affiliation: Departamento de Física Aplicada, Facultad de Ciencias Experimentales, Unidad Asociada IEM-CSIC-U.Huelva, Universidad de Huelva, 21071 Huelva, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Carvajal