Direct Observation of the Infrared Torsional Spectrum of C2H6, CH3CD3, and C2D6

Abstract

The torsional spectra of gaseous C₂H₆, CH₃CD₃CD₃, and C₂D₆ have been observed directly in the infrared under high‐pressure‐path‐length conditions (30‐70 atm·m). The fundamental was observed for all three species, at 289, 253, and 208 cm⁻¹, respectively. Hot bands were observed for C₂H₆ ($255{\mathrm{cm}}^{\text{−1}}\mathrm{[A]}$ , $258{\mathrm{cm}}^{\text{−1}}\mathrm{[E]}$ ) and CH₃CD₃ (228 cm⁻¹) as was the first overtone for CH₃CD₃ (482 cm⁻¹). All the experimental frequencies can be very well accounted for by use of a single internal rotation potential function with a barrier height $V_3\text{\hspace{0.17em}=\hspace{0.17em}2928}\mathrm{cal}\mathrm{\hspace{0.17em}/\hspace{0.17em}}\mathrm{mole}$ . This implies that $V_6$ (and higher terms) are negligibly small. Two alternative explanations for the occurrence of these spectra in the infrared, one in terms of Coriolis interaction with an infrared‐active vibration and one in terms of a ${\nu }_4{\mathrm{\hspace{0.17em}+\hspace{0.17em}\nu }}_x{\mathrm{\hspace{0.17em}-\hspace{0.17em}\nu }}_x$ vibration, are discussed. A more reliable determination of ${\nu }_{12}$ , the “uncertain frequency” of C₂H₆, has been obtained from the ${\nu }_{12}{\mathrm{\hspace{0.17em}-\hspace{0.17em}\nu }}_9$ difference band, giving ${\nu }_{12}\text{\hspace{0.17em}=\hspace{0.17em}1206}{\mathrm{cm}}^{\text{−1}}$ .