Investigation of the ν8 and ν21 bands of propane CH3CH2CH3 at 11.5 and 10.9 µm: evidence of large amplitude tunnelling effects

Abstract

ABSTRACTWe present the first investigation of the ν8 band (C–C symmetric stretch at 870.3137 cm−1), together with an extended analysis of the neighbouring ν21 band (CH3 rock at 921.3756 cm−1) of propane (C3H8). Our previous investigation of the ν21 A-type band [A.Perrin, F. Kwabia-Tchana, J.M.Flaud, L.Manceron, P.Groner, W.J.Lafferty. J. Mol. Spectrosc. 315, 55 (2015)] revealed that the rotational energy levels of 211 are split because of interactions with the internal rotations of the methyl groups, leading to the identification of AA, EE, AE and EA torsional components. In this work, a similar behaviour was observed for the B-type ν8 band and the analysis of the ν21 band was greatly extended. One of the results of the present study is to show that these torsional splittings are due to the existence of anharmonic and Coriolis resonances, coupling the 211 and 81 rotational levels to nearby highly excited levels of the two internal rotations of the methyl groups. Accordingly, an effective ‘vibration – torsion- rotation’ Hamiltonian model was built in the G36 symmetry group which accounts for both types of resonances. In parallel, a code computing the line intensities was developed to allow unambiguous torsional component assignments. The line assignments were performed using a high resolution (0.0015 cm−1) infrared spectrum of propane, recorded with synchrotron radiation at the SOLEIL French light source facility coupled to a Bruker IFS-125 Fourier transform spectrometer. Finally, a linelist of positions and intensities which can be used for the detection of propane in the Earth and outer planets atmospheres was produced.