Investigation of rotational spectra of isotopic species of trans-formic acid: a test case for the interplay between experiment and theory

Trans-formic acid is the simplest organic acid and the rst that has been identied in the interstellar medium. Its astrophysical relevance has motivated this spectroscopic study. To provide highly accurate rest frequencies as well as to improve the accuracy of the spectroscopic and hyperne parameters available in the literature, the rotational spectra of various isotopic species, namely, HCOOH, DCOOH, HCOOD, DCOOD, have been investigated. The Lamb-dip technique has been exploited to record the rotational spectra at sub-Doppler resolution in the millimeter- and submillimeter-wave frequency ranges and, when possible, to resolve the hyperne structure due to the hydrogen and/or deuterium nuclei. Doppler-limited THz measurements (1.0-1.6 THz; with an accuracy of about 50-70 kHz) have also been carried out in order to extend the predictive capabilities of our spectroscopic parameters to higher frequencies. The experimental investigation of the hyperne structure has been supported by high-level quantum-chemical calculations of the parameters involved. The calculations have been carried out at the CCSD(T) level together with basis sets of quadruple- up to sextuple-zeta quality. Vibrational corrections have also been included as obtained by means of vibrational second-order perturbation theory.