FTIR synchrotron spectroscopy of the lower modes of methyl-d3 mercaptan (CD3SH) ‒ where is the C-S stretch?

Lees, Ronald M.

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https://hdl.handle.net/2142/104468 Copy

Description

Title

FTIR synchrotron spectroscopy of the lower modes of methyl-d3 mercaptan (CD3SH) ‒ where is the C-S stretch?

Author(s)

Lees, Ronald M.

Contributor(s)

• Billinghurst, Brant E.
• Xu, Li-Hong
• Beaman, Craig W.

Issue Date

2019-06-17

Keyword(s)

• Large amplitude motions
• Internal rotation

Abstract

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 centered at 644 cm$^{-1}$ and a weaker perpendicular band centered 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 and 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 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 of the vibrational frequencies and transition moments was carried out, 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 of normal CH$_{3}$SH. This apparent dramatic extinction of the normally very strong C-S stretching band is quite remarkable!

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

eng

Permalink

http://hdl.handle.net/2142/104468

DOI

https://doi.org/10.15278/isms.2019.MI06

Copyright and License Information

Copyright 2019 Ronald M. Lees

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