Theoretical spectroscopic characterization at low temperatures of detectable sulfur-organic compounds: Ethyl mercaptan and dimethyl sulfide

Abstract

Microwave and ab initio studies of rare gas-methane van der Waals complexes J. Chem. Phys. 120, 9047 (2004); 10.1063/1.1691743 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Highly correlated ab initio methods are used for the spectroscopic characterization of ethyl mercaptan (CH 3 CH 2 32 SH, ETSH) and dimethyl sulfide (CH 3 32 SCH 3 , DMS), considering them on the vibrational ground and excited torsional states. Since both molecules show non-rigid properties, torsional energy barriers and splittings are provided. Equilibrium geometries and the corresponding rotational constants are calculated by means of a composite scheme based on CCSD(T) calculations that accounts for the extrapolation to the complete basis set limit and core-correlation effects. The ground and excited states rotational constants are then determined using vibrational corrections obtained from CCSD/cc-pVTZ force-field calculations, which are also employed to determine anharmonic frequencies for all vibrational modes. CCSD(T) and CCSD force fields are employed to predict quartic and sextic centrifugal-distortion constants, respectively. Equilibrium rotational constants are also calculated using CCSD(T)-F12. The full-dimensional anharmonic analysis does not predict displacements of the lowest torsional excited states due to Fermi resonances with the remaining vibrational modes. Thus, very accurate torsional transitions are calculated by solving variationally two-dimensional Hamiltonians depending on the CH 3 and SH torsional coordinates of ethyl mercaptan or on the two methyl groups torsions of dimethyl-sulfide. For this purpose, vibrationally corrected potential energy surfaces are computed at the CCSD(T)/aug-cc-pVTZ level of theory. For ethyl mercaptan, calculations show large differences between the gauche (g) and trans (t) conformer spectral features. Interactions between rotating groups are responsible for the displacements of the g-bands with respect to the t-bands that cannot therefore be described with one-dimensional models. For DMS, the CCSD(T) potential energy surface has been semi-empirically adjusted to reproduce experimental data. New assignments are suggested for the methyl torsion bands of ETSH and a reassignment is proposed for the infrared bands of DMS (0 3 → 0 4 and 1 0 → 1 1). Our accurate spectroscopic data should be useful for the analysis of the microwave and far infrared spectra of ETSH and DMS recorded, at low temperatures, either in laboratory or in the interstellar medium.

Laboratory microwave, millimeter wave and far-infrared spectra of dimethyl sulfide

“…The values of these interaction terms are comparable to those found for methyl acetate (34 cm −1 for V 12S and −3.5 cm −1 for V 12C ; Tudorie et al 2011). For DMS, the effective two-dimensional PES derived by Senent et al (2014) at the CCSD(T)-F12/VTZ-F12 level shows only a very small dependence of the torsional modes on the other small amplitude vibrations, after the authors verified the validity of such a 2D Hamiltonian with a full dimensional anharmonic analysis. The effective PES is given by V eff (α 1 , α 2 ) = 712.532 − 354.641 (cos 3α 1 + cos 3α 2 ) + 0.583 cos 3α 1 cos 3α 2 −2.767 (cos 6α 1 +cos 6α 2 ) + 0.719 (cos 3α 1 cos 6α 2 + cos 6α 1 cos 3α 2 ) + 0.263 cos 6α 1 cos 6α 2 + 6.075 sin 3α 1 sin 3α 2 which can be partly compared to the results in our fit V eff (α 1 , α 2 ) = 735.784 − 367.892(cos 3α 1 + cos 3α 2 ) (4) − 32.86 sin 3α 1 sin 3α 2 .…”

“…As stated in Senent et al (2014), the long distance between the two methyl groups in DMS makes the coupling effects associated with this top-top torsional mode near the equilibrium configuration rather small in comparison with its oxygen analog dimethyl ether. As shown in Eq.…”

“…Recently, Senent et al (2014) performed highly correlated ab initio methods combined with large basis sets. This work presented a complete characterization of the rotational parameters at equilibrium and in the ground and excited vibrational states.…”

Electron scattering studies of DMS, DMDS and DMSO homologous series

“…It is quite evident that these sulphur compounds are of prime importance in understanding the interactions occurring in the atmospheric environments. Among many sulphur-organic compounds, DMS conformers have been detected in low temperature regions of interstellar medium [4]. The second molecule of the DMS series, diethyl sulphide (DES), is used in plating baths for gold and silver and as a solvent for anhydrous minerals [5].…”

Communication: Vacuum ultraviolet photoabsorption of interstellar icy thiols

“…28 However, based on the Gibbs free energy difference of 1.92 kJ mol −1 between trans and gauche forms, the relative abundances of trans and gauche forms were estimated to be 19% and 81%, respectively. 29 However, conformerspecific ionization spectroscopy studies on ethanethiol revealed only gauche conformer of ethanethiol to be present in a molecular jet.…”