First high resolution analysis of the absorption spectrum of propane in the 6.7 μm to 7.5 μm spectral region

The rotational structure of the v19 (CH2− wagging, A-type band), v18 (CH3− s-deformation, A-type band), v24 (CH3− d-deformation, C-type band), and v4 (CH3− d-deformation, B-type band) vibrational fundamentals of propane, were recorded using a molecular beam coupled to a Fourier-transform spectrometer working at 0.005 cm−1 resolution. The analysis was performed using Watson Hamiltonians for the 191 and 181 states. However, to properly calculate the rotational levels of the 41 and 241 vibrational levels, it is necessary to include in the Hamiltonian matrix not only the strong A-type Coriolis interaction which couples them but also the various interactions (Coriolis or Fermi-type) which link them to the levels of the dark 51 and 171 vibrational states. It is then possible to calculate the upper-state levels to within an average uncertainty of 2 to 5×10−3 cm−1 depending on the state. These results are satisfactory given the fact that (i) possible perturbations with nearby combination states were not considered and (ii) several torsional splittings were not accounted for. The band centers derived from the fits are ṽ0(v19)=1338.965 cm−1, ṽ0(v18)=1376.850 cm−1, ṽ0(v24)=1471.874 cm−1, and ṽ0(v4)=1476.710 cm−1. The standard uncertainties of these values are estimated to be about 0.002 cm−1, which includes the calibration errors as well as the statistical uncertainty of the fittings.