Propadienedione, C₃O₂, is a quasilinear molecule. That is a molecule which in the equilibrium configuration is non-linear, but with a small barrier to linearity. The pattern of the low-lying vibrational levels of C₃O₂ is between that of a linear molecule and that of an asymmetric rotor – slightly closer to the former, see below. Traditionally, the spectrum of C₃O₂ is viewed as that of a linear molecule with a very anharmonic potential, and all l-states of a given v₇ manifold a treated separately.
The ν₇¹ ← 0 lines (K_a = 1 ← 0 in the asymmetric rotor language) with a band origin of 18.256 cm^–1 were taken from
(1) E. N. Karyakin, A. F. Krupnov, and S. M. Shapin, 1982, J. Mol. Spectrosc. 98, 283. Transitions belonging to the 2ν₇² ← ν₇¹ band (K_a = 2 ← 1 in the asymmetric rotor language) with a band origin of 27.850 cm^–1, and also published in that work, were also used in the fit.
In the limit of a linear molecule, the origin would be in the vicinity of 18 cm^–1 whereas in the asymmetric rotor limit it would be close to 55 cm^–1.
Additional high-J transitions from the FT-FIR spectroscopic investigation in
(2) J. vander Auwera, J. W. C. Johns, and O. L. Polyanski, 1991, J. Chem. Phys. 95, 2299
contributed marginally to the improvement of the spectroscopic parameters.
Transitions belonging to the 2ν₇⁰ ← ν₇¹ band, also published in the latter work, were included in the fit for better calculations of the partition function at slightly elevated temperatures (30 – 40 K). Contributions of low-lying vibrational states to the partition function are available.
Transitions with experimental uncertainties larger than 100 kHz have not been merged.
The dipole moment was taken from a high level ab initio calculation by
(3) J. Koput, 2005, private communication.