The observed rovibrational and rotational transition frequencies have been described in
(1) H. S. P. Müller, P. Pracna, and V.-M. Horneman, 2002, J. Mol. Spectrosc. 216, 397, in
(2) P. Pracna, H. S. P. Müller, S. Klee, and V.-M. Horneman, 2004, Mol. Phys. 102, 1555, and in references therein.
The ν₉ band is the second lowest and the strongest fundamental in propyne. The ν₉ band is a doubly degenerate vibration. State number 1 describes k values ≤ 0, while state number 2 indicates states with k > 0.
The data have been subjected to a combined fit consisting of v₁₀ = 0, 1, 2 and v₉ = 1 pure rotational data as well as ν₁₀, ν₉, and 2ν₁₀ – ν₁₀ rovibrational data. A strong, isolated line has an estimated precision of 0.0002 cm^–1; the accuracies are probably slightly worse because of calibration uncertainties.
The Coriolis interactions of v₉ = 1 with v₁₀ = 1 and the Fermi interaction with v₁₀ = 2 described in (1) and (2), have been taken into account. The latter occurs at K = 1 between l = –1 of v₉ = 1 and l = +2 of v₁₀ = 2, causing non-zero intensity in some transitions of 2ν₁₀; these transitions have the upper state number 3. Its interactions with states of the next higher polyad have not been taken into account in this prediction and the Coriolis interaction with v₁₀ = 2 at higher K have not been considered in the current model. Therefore, predictions for transitions with k ≤ –10 or k ≥ 13 should be viewed with caution.
The transition dipole moment has been derived from intensity measurements of individual rovibrational transitions by
(3) G. Blanquet, J. Walrand, and M. Dang-Nhu. 1992, Spectrochim. Acta, 48A, 1231.
Besides the ground vibrational state, the partition function takes into account the states v₁₀ = 1, v₉ = 1, and v₁₀ = 2. Their respective contributions are given in parentheses as long as they are different from 0 within the quoted digits.