Rotational spectroscopy of pyridazine and its isotopologs from 235–360 GHz: Equilibrium structure and vibrational satellites

Abstract

The rotational spectrum of pyridazine (o-C4H4N2), the ortho disubstituted nitrogen analog of benzene, has been measured and analyzed in the gas phase. For the ground vibrational state of the normal isotopolog, over 2000 individual rotational transitions have been identified between 238 and 360 GHz and have been fit to 13 parameters of a 6th-order centrifugal distortion Hamiltonian. All transitions in this frequency region can now be predicted from this model to near experimental accuracy, i.e., well enough for the purpose of any future radio-astronomical search for this species. Three isotopologs, [3-(13)C]-C4H4N2, [4-(13)C]-C4H4N2, and [1-(15)N]-C4H4N2, have been detected in natural abundance, and several hundred lines have been measured for each of these species and fit to 6th-order Hamiltonians. Ten additional isotopologs were synthesized with enhanced deuterium substitution and analyzed to allow for a complete structure determination. The equilibrium structure (Re) of pyridazine was obtained by correcting the experimental rotational constants for the effects of vibration-rotation coupling using interaction constants predicted from CCSD(T) calculations with an ANO0 basis set and further correcting for the effect of electron mass. The final Re structural parameters are determined with excellent accuracy, as evidenced by their ability to predict 28 independent moments of inertia (Ia and Ib for 14 isotopologs) very well from 9 structural parameters. The rotational spectra of the six lowest-energy fundamental vibrational satellites of the main isotopolog have been detected. The rotational spectra of the five lowest-energy vibrational satellites have been assigned and fit to yield accurate rotational and distortion constants, while the fit and assignment for the sixth is less complete. The resultant vibration-rotation interaction (α) constants are found to be in excellent agreement with ones predicted from coupled-cluster calculations, which proved to be the key to unambiguous assignment of the satellite spectra to specific vibration modes.

The 130–370 GHz rotational spectrum of phenyl isocyanide (C6H5NC)

“…The 250-370 GHz spectrum was collected using the instrument described previously. [39][40][41] The 130-230 GHz frequency spectrum was collected on the same apparatus using a Virginia Diodes amplification and multiplication chain (WR5.1AMC 140-220 GHz) and a Virginia Diodes zero-bias detector (WR5.1ZBD 140-220 GHz). The spectra were combined into a single broadband spectrum using Assignment and Analysis of Broadband Spectra (AABS) software.…”

The heavy atom structures and 33S quadrupole coupling constants of 2-thiophenecarboxaldehyde: insights from microwave spectroscopy

“…Several alternatives have been proposed to overcome the problems of the rs structure, one of them is the semi-experimental equilibrium re SE structure. Many investigations in the literature have proven that the re SE structure is a reasonable choice for structure determination [46][47][48][49][50][51][52].…”

Rotation-vibration interactions in the spectra of polycyclic aromatic hydrocarbons: Quinoline as a test-case species

“…Due to the presence of these low-lying vibrational modes, the density of states is rapidly increasing (see, e.g., Basire et al 21 for calculations of the density of states in large molecular systems) and should in contrast lead to numerous perturbations in the ES rotational structures. To date, very few spectroscopic studies of large molecules deal with rotationally perturbed spectra; and even in the case of monocyclic species, perturbations of the ES have been ignored in the rotational analysis (see, e.g., Esselman et al 22 ). The purpose of the present study is thus to provide a thorough understanding of the rotational structure of the two lowest vibrational modes of quinoline C 9 H 7 N (ν 45 and ν 44 ), a simple two ring PANH molecule, using complementary high resolution experimental techniques.…”