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

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.