Microwave, infrared, and Raman spectra, r0 structural parameters, conformational stability, and vibrational assignment of allyl thiol

FT-microwave spectrum of allyl thiol, H2CCHCH2SH, has been recorded, and 19 transitions have been assigned for the most abundant isotopologue of Gg conformer, and the rotational constants have been determined; A = 20,041.439 (4), B = 2795.830 (1), C = 2701.084 (1). From the determined microwave rotational constants and ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r0 parameters are reported with distances (Å): rCC = 1.343 (3), rC–C = 1.496 (3), rC–S = 1.827 (3) and angles (°) ∠CCC = 123.4 (5), ∠CCS = 112.5 (5), and τCγCβCαS = 118.7 (5). Variable temperature (−55 to −100 °C) infrared spectra (3600–400 cm−1) were recorded of allyl thiol in liquid xenon and the Gg conformer was determined to be the most stable form. The enthalpy differences relative to the Gg form are for Cg 120 ± 9 cm−1 (1.44 ± 0.11 kJ/mol), for Gg′ 337 ± 34 cm−1 (4.03 ± 0.41 kJ/mol), and for Gt 360 ± 36 cm−1 (4.31 ± 0.43 kJ/mol). The relative amounts present at ambient temperature are Gg 52 ± 1%, Cg 29 ± 1%, Gg′ 10 ± 1%, and Gt 9 ± 1%. The conformational stabilities have been predicted from ab initio calculations with many basis sets up to aug-cc-pVTZ and the predicted stabilities are in agreement with the experimentally determined order. Vibrational assignments are reported with support by ab initio predictions and results are discussed.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Vibrational data indicates the presence of a four conformers. ► The Gauche-gauche conformer is shown to be stable in the infrared spectra of the solid. ► Enthalpy differences have been determined for the four observed conformers. ► Rotational constants and centrifugal distortion constants have been determined from microwave spectra. ► The adjusted r0 structural parameters have been obtained for the Gauche–gauche conformer.