2-7 June 2019
Simon Fraser University
America/Vancouver timezone
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FTIR Synchrotron Spectroscopy of the Lower Modes of Methyl-D3 Mercaptan (CD3SH) – Where is the C-S Stretch?

6 Jun 2019, 12:00
BLU 10011 (Simon Fraser University)

BLU 10011

Simon Fraser University

Oral (Non-Student) / Orale (non-étudiant(e)) Division of Atomic, Molecular and Optical Physics, Canada / Division de la physique atomique, moléculaire et photonique, Canada (DAMOPC-DPAMPC) R1-4 Molecular sciences: outreach, teaching and research (DPE/DAMOPC) | Sciences moléculaires: rayonnement, enseignement et recherche (DEP/DPAMPC)


Dr Ronald M. Lees (University of NB Saint John)


The infrared Fourier transform spectrum of the lower vibrational modes of CD$_{3}$SH has been recorded in the 400-1200 cm$^{-1}$ region using synchrotron radiation at the FIR beamline of the Canadian Light Source in Saskatoon. Torsion-rotation assignments have been made for a relatively strong parallel band centred at 644 cm$^{-1}$ and a weaker perpendicular band centred at 727 cm$^{-1}$. Comparison with the spectra for the normal CH$_{3}$SH species as well as the analogous CD$_{3}$OH and CH$_{3}$OH methanol molecules would suggest an obvious association of the 644 cm$^{-1}$ band with the C-S stretching mode, with the 727 cm$^{-1}$ mode likely to be the out-of-plane methyl rock. However, a previous vibrational normal mode analysis [Byler & Gerasimowicz, J. Mol. Struct. 112 (1984) 207-219] showed strong coupling between the C-S stretch and CSH bending modes. They assign the 644 cm$^{-1}$ band to the latter, and attribute the C-S stretch instead to a supposed feature at 688 cm$^{-1}$ that we find no clear evidence for in our spectrum. For normal CH$_{3}$SH, the CSH bend is very weak and lies between the strong C-S stretch and CH$_{3}$-rocking bands. A Gaussian quantum chemistry calculation was carried out to explore this mystery, and indeed there is a mode predicted to lie in between our two observed bands with almost vanishing intensity and a reduced mass and effective force constant corresponding closely to those calculated for the C-S stretch for normal CH$_{3}$SH. This apparent dramatic extinction of the normally very strong C-S stretching band is quite remarkable!

Primary authors

Mr Craig W. Beaman (University of NB Saint John) Dr Ronald M. Lees (University of NB Saint John) Dr Li-Hong Xu (University of NB Saint John) Dr Brant E. Billinghurst (Canadian Light Source Inc.)

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