Semi-Experimental Equilibrium (r_e^SE) and Theoretical Structures of Pyridazine (o-C₄H₄N₂)

A semi-experimental equilibrium structure (r_e^SE) of pyridazine (o-C₄H₄N₂) has been determined using the rotational spectra of 18 isotopologues. Spectroscopic constants of four isotopologues are reported for the first time (measured from 235 to 360 GHz), while spectroscopic constants for previously reported isotopologues are improved by extending the frequency coverage (measured from 130 to 375 GHz). The experimental values of the ground-state rotational constants (A₀, B₀, and C₀) from each isotopologue were converted to determinable constants (A₀^″, B₀^″, and C₀^″), which were then corrected for the effects of vibration-rotation interactions and electron-mass distributions using CCSD(T)/cc-pCVTZ calculations. The resultant r_e^SE for pyridazine determines bond distances to within 0.001 Å and bond angles within 0.04°, a reduction in the statistical uncertainties by at least a factor of two relative to the previously reported r_e^SE. The improvement in precision appears to be largely due to the use of higher-level theoretical calculations of the vibration-rotation and electron-mass effects, though the incorporation of the newly measured isotopologues ([4-²H, 4-¹³C]-, [4-²H, 5-¹³C]-, [4-²H, 6-¹³C]-, and [4,5-²H, 4-¹³C]-pyridazine) is partially responsible for the improved determination of the hydrogen-containing bond angles. The computed equilibrium structure (r_e) (CCSD(T)/cc-pCV5Z) and a "best theoretical estimate" of the equilibrium structure (r_e) both agree with the updated r_e^SE structure within the statistical experimental uncertainty (2σ) of each structural parameter.