Microwave Spectrum, Structure, Dipole Moment and Internal Rotation of Ethanethiol. II. Gauche Isomer

Abstract

The microwave spectra of gauche-ethanethiol and its isotopic species were studied. Most of the observed spectra for the species having the plane of symmetry exhibited doublet structures with large spacings due to the internal rotation of the mercapto group. The rs structure of the gauche isomer was determined from the observed moments of inertia. The gauche isomer whose dihedral angle τ(CCSH) is 61°45′±58′ has structural parameters close to those for the trans isomer except the angles around the carbon atom in the methylene group. The difference in angle values around the carbon atom in the methylene group between the trans and gauche isomers can be explained by the 3°7′ tilt of the methylene group towards the lone pair electrons on the sulfur atom. The direction of the dipole moment in the molecule was discussed on the basis of data given by Schmidt and Quade. The potential barrier of the mercapto internal rotation was obtained from splittings of the observed spectra. The Fourier coefficients of this barrier are V1=−207±67, V2=−386±27, and V3=1305±20 cal/mol. The spectra due to the excited methyl and mercapto torsional states were also measured for the trans and gauche isomers. The potential barrier of the methyl internal rotation for the trans isomer was found to be 3260±30 cal/mol from splittings of the spectra due to the first excited methyl torsional state.

Simple models for the quick estimation of ground state hydrogen tunneling splittings in alcohols and other compounds

“…75 The extension of the alkyl chain is calculated to have only a small impact on the torsional properties. Consequently, the predictions obtained here are similar to ethanol and one order of magnitude larger than the one by Kawashima et al In analogy, the splittings of ethanethiol, 129 1-propanethiol 130 and 1-butanethiol 47 are similar to each other as well.…”

Conformations and Low-Frequency Intramolecular Motions of 1-Butanol, 1-Butanethiol, Iso-butanol, and Iso-butanethiol Investigated by Fourier Transform Microwave Spectroscopy Combined with Quantum Chemical Calculations

“…The energy difference ΔE obtained in the present study for TTg of 1-BuSH and Tg of iso-BuSH may be compared with those for the g of ethanethiol and deuterated isotopomer: 1754 (9) and 70.2 (60) MHz, respectively, 26 for the Tg of propanethiol and deuterated isotopomer: 1613.0 (3) and 58.2 (36) MHz, respectively, 6 for the g of isopropanethiol and deuterated isotopomer: 562.38 (10) and 10.06 (20) MHz, respectively, 8 for the g of ethanol and deuterated isotopomer: 96748.8164 (69) 28 and 17096.8 29 MHz, respectively, and for the g of isopropanol and deuterated isotopomer: 46798.50 (11) 7 and 4431.4613 (17) 30 MHz, respectively. The value for the Tg of propanol has not been reported.…”

“…The r s structures derived for TTg and TGg′ are compared with those calculated using MP2/ 6-311++G(d,p) and cam-B3LYP/6-311++G(d,p) in Table S14. r s (C−S) is 1.834 and 1.826 Å in TTg and TGg′, respectively, which are longer than 1.819 Å in CH 3 SH, 25 1.814 Å for g and 1.820 Å for t in C 2 H 5 SH, 26,27 and 1.814 Å for Tg in C 3 H 7 SH. 6 r s (S−H) is 1.338 and 1.363 Å in TTg and TGg′, respectively; however, r s (S−H) of TGg′ is different from r(S− H) in related molecules: 1.32−1.34 Å.…”

Spectroscopic Characterization and Detection of Ethyl Mercaptan in Orion

“…A schematic potential energy diagram for the -SH group torsion is shown in Figure 1-b. The tunneling process removes the vibrational degeneracy and the vibrational ground state is split into two substates labeled as 0 + and 0 − with an energy separation ∆E = E − − E + of about 1750 MHz (Quade et al 1975, Nakagawa et al 1976. In order to facilitate the assignments of the gauche-and transethyl mercaptan millimeter-wave spectra, the microwave data from Quade et al (1975) were used for initial predictions of the rotational transitions.…”

“…Hence, the thiol equivalent of ethanol, ethyl mercaptan (CH 3 CH 2 SH), could also be present in space. Initial studies of the Stark modulated microwave spectra by Imanov et al (1967), Hayashi et al (1970), Hayashi et al (1973), Quade et al (1975), andNakagawa et al (1976) provided the first values of the spectroscopic constants for the gauche and trans conformers of ethyl mercaptan. These microwave laboratory data, however, cannot be used to accurately predict their frequencies in the millimeter and submillimeter-wave domains.…”