FTIR synchrotron spectroscopy of lower modes of methyl-D3 mercaptan (CD3SH)

Abstract

The infrared Fourier transform spectrum of the lower vibrational modes of the CD3SH isotopologue of methyl mercaptan has been recorded using synchrotron radiation at the FIR beamline of the Canadian Light Source. Torsion-rotation structure has been analyzed for a strong parallel band centred at 644 cm−1 and a weaker perpendicular band centred at 727 cm−1, identified respectively with a mixed {in-plane-CD3-rocking/CSH-bending} mode and the out-of-plane CD3 rock. The torsional energies for the latter mode follow well-behaved curves, inverted with respect to the ground state, but those for the former do not display a systematic pattern. Surprisingly, no clear evidence was seen for the C-S stretching fundamental band, in contrast to CH3SH and the analogous CD3OH and CH3OH methanol species for which the corresponding C-S or C-O stretching bands are major features of the spectrum. This mystery prompted an ab initio calculation to compare the CD3SH and CH3SH vibrational frequencies and transition moments. A mode lying in the expected position was indeed predicted for CD3SH but with almost vanishing intensity. The calculated reduced mass and effective force constant for this mode corresponded closely to those for the C-S stretch of normal CH3SH, indicating that an anticipated strong C-S stretching band is in fact almost completely extinguished for CD3SH.