Improved line frequencies for the nucleic acid base uracil for a radioastronomical search

Aims. We report new laboratory spectroscopic data on the rotational spectrum of the pyrimidine nucleic acid base uracil (C4H4N2O2). The dataset has been extended both into the microwave region and towards mm-wavelengths with the aim of providing accurate transition rest frequencies for astrophysical searches. Methods. The microwave measurements have been performed with a molecular beam Fourier transform microwave spectrometer in the frequency range from 9-19 GHz and the mm-wave band transitions have been recorded with a Stark-modulated free jet spectrometer up to 100 GHz. The global dataset has been analysed with a standard S-reduced Hamiltonian and precise spectroscopic parameters up to quartic order have been obtained. The hyperfine structure due to the two N-14 nuclei has been resolved in the microwave measurements and the nuclear quadrupole coupling constants could be derived to high accuracy. Results. Based on the new laboratory data and analysis, highly precise rotational transition rest frequencies are available for astrophysical important lines of uracil up to 300 GHz.