Direct observation of the infrared torsional spectrum of C ₂H₆, CH₃CD₃ and C₂D ₆

The torsional spectra of gaseous C₂H₆, CH ₃CD₃, 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 C ₂H₆ (255 cm^-1 [A], 258 cm^-1[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 V₃ = 2928 cal/mole. This implies that V₆ (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 v₄+v_x-v_x vibration, are discussed. A more reliable determination of v₁₂, the "uncertain frequency" of C₂H₆, has been obtained from the v ₁₂-v₉ difference band, giving v₁₂= 1206 cm ^-1.