posted on 2021-09-03, 18:36authored byAndrew
N. Owen, Maria A. Zdanovskaia, Brian J. Esselman, John F. Stanton, R. Claude Woods, Robert J. McMahon
A semi-experimental
equilibrium structure (reSE) 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 reSE 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 reSE. 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
(re) (CCSD(T)/cc-pCV5Z) and a “best
theoretical estimate” of the equilibrium structure (re) both agree with the updated reSE structure
within the statistical experimental uncertainty (2σ) of each
structural parameter.