A preliminary determination of the A₀ rotational constant of propyne

Abstract

The A ₀ rotational constant of a symmetric top molecule can be reached by combining data from three infrared bands, ν_t, (ν_t+ν_t′)^±2 and ν_t+ν_t′−ν_t where ν_t and ν_t′, are two degenerate modes. This method is applied to propyne CH₃−C≡CH, with ν_t=ν₁₀ and ν_t′=ν₈. Unfortunately only one set of K values, corresponding to the difference between K=3 and 0, is available at present. A detailed study of the two K. ΔK=1 subbands present in ν₈+ν₁₀−ν₁₀ gives A ₀=5·3082 (22)cm⁻¹ but when better spectra of ν₁₀ are analysed, the uncertainty can be decreased by an order of magnitude.