Assignment and analysis of the rotational spectra of the v7Â =Â 1, v12Â =Â 1 and v13Â =Â 1 vibrational states of CH3CCCCH

We have used the theoretical predictions from anharmonic force field calculations reported in our previous work [J. Mol. Spectrosc. 253 (2009) 106] to assign the pure rotational spectra in the v12 and v13 doubly degenerate bending modes of the main isotopic species of methyldiacetylene. The key experimentally determined spectroscopic constants are found to be in good agreement with calculation. The rotational spectrum in the lowest singly degenerate mode, the v7 stretching mode, has also been assigned. The corresponding rotational constants require a revision of the previously reported computational results and point to the possibility of an interaction between ν7 and 2ν13.

Highlights►Rotational spectrum of methyldiacetylene in vibrational excited states: the v12 = 1 and v13 = 1 bending modes and the v7 = 1 stretching mode. ► Assignment and analysis of rotational spectra in vibrational excited states: determination of the spectroscopic constants. ► Quantum-chemical calculations at the coupled-cluster level of spectroscopic parameters: the prediction of vibration-rotation interaction constants. ► Third-rank resonance between ν7 and 2ν13: Coriolis and/or Fermi interaction.