Extending the rotational spectrum of cyclopentadiene towards higher frequencies and vibrational states

Cyclopentadiene (c-C5H6) is a cyclic pure hydrocarbon that was already detected astronomically towards the prototypical dark cloud TMC-1 (Cernicharo et al. 2021, Astron. Astrophys. 649, L15). However, accurate predictions of its rotational spectrum are still limited to the microwave region and narrow quantum number ranges. In the present study, the pure rotational spectrum of cyclopentadiene was measured in the frequency ranges 170-250 GHz and 340-510 GHz to improve the number of ground vibrational state assignments by more than a factor of 20, resulting in more accurate rotational parameters and the determination of higher-order centrifugal distortion parameters. Additionally, vibrational satellite spectra of cyclopentadiene in its eight energetically lowest vibrationally excited states were analyzed for the first time. Coriolis interactions between selected vibrational states were identified and treated successfully in combined fits. Previous microwave work on the three singly 13C substituted isotopologues was extended significantly also covering frequency ranges up to 250 GHz. The new data sets permit reliable frequency predictions for the isotopologues and vibrational satellite spectra far into the sub-mm-wave range. Finally, the experimental rotational constants of all available isotopologues and calculated zero-point vibrational contributions to the rotational constants were used to derive a semi-experimental equilibrium structure of this fundamental ring molecule.