Coriolis Perturbations in the Infrared Spectrum of Ethylene

Abstract

Rotational structure has been resolved and analyzed in two of the infrared‐active perpendicular bands of C₂H₄ vapor: the Type b fundamental band, ν₁₀, at 826 cm^—1, and the Type c fundamental band, ν₇, at 949 cm^—1. Many of the individual ^PP and ^RR branch lines have been observed. The analysis has been confined to values of the quantum number K≥3, for which energy levels ethylene shows no detectable deviations from a symmetric‐top rotational structure. The analysis reveals a Coriolis interaction between ν₇ and ν₁₀, and between ν₄ and ν₁₀, and values of the Coriolis constants ζ_7,10^z and ζ_4,10^y are obtained; these are related to normal coordinate calculations for the appropriate symmetry species, and force constants are derived to fit the observed zeta constants. The band center of ν₁₀ has been revised from the original figure of 810 cm^—1 to the new value, 826 cm^—1, and the inactive frequency ν₄ is estimated to lie at 1023±3 cm^—1, in good agreement with the previous estimate of 1027 cm^—1. The change in the value of ν₁₀ leads to a suggested change in the value of the Raman‐active fundamental ν₆ from 1236 to 1222 cm^—1. New combination bands have been observed at 2174 cm^—1, assigned as ν₃+ν₁₀; and at 2252 cm^—1, assigned as ν₄+ν₆; also rotational structure has been resolved and analyzed in the ν₆+ν₁₀ band at 2048 cm^—1. The new data obtained for the C₂H₄ molecule are summarized in Table XII, with all of the other data presently available on the vibrational and rotational constants.