Millimeter-wave and high-resolution infrared spectroscopy of the low-lying vibrational states of pyridazine isotopologues

The gas-phase rotational spectrum from 8 to 750 GHz and the high-resolution infrared (IR) spectrum of pyridazine (o-C4H4N2) have been analyzed for the ground and four lowest-energy vibrationally excited states. A combined global fit of the rotational and IR data has been obtained using a sextic, centrifugally distorted-rotor Hamiltonian with Coriolis coupling between appropriate states. Coriolis coupling has been addressed in the two lowest-energy coupled dyads (nu(16), nu(13) and nu(24), nu(9)). Utilizing the Coriolis coupling between the vibrational states of each dyad and the analysis of the IR spectrum for nu(16) and nu(9), we have determined precise band origins for each of these fundamental states: nu(16) (B-1) = 361.213 292 7 (17) cm(-1), nu(13) (A(2)) = 361.284 082 4 (17) cm(-1), nu(24) (B-2) = 618.969 096 (26) cm(-1), and nu(9) (A(1)) = 664.723 378 4 (27) cm(-1). Notably, the energy separation in the nu(16)-nu(13) Coriolis-coupled dyad is one of the smallest spectroscopically measured energy separations between vibrational states: 2122.222 (72) MHz or 0.070 789 7 (24) cm(-1). Despite nu(13) being IR inactive and nu(24) having an impractically low-intensity IR intensity, the band origins of all four vibrational states were measured, showcasing the power of combining the data provided by millimeter-wave and high-resolution IR spectra. Additionally, the spectra of pyridazine-d(x) isotopologues generated for a previous semi-experimental equilibrium structure (r(e)(SE)) determination allowed us to analyze the two lowest-energy vibrational states of pyridazine for all nine pyridazine-d(x) isotopologues. Coriolis-coupling terms have been measured for analogous vibrational states across seven isotopologues, both enabling their comparison and providing a new benchmark for computational chemistry.