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# Semi-Experimental Equilibrium (*r*_{e}^{SE}) and Theoretical
Structures of Pyridazine (*o*‑C_{4}H_{4}N_{2})

journal contribution

posted on 2021-09-03, 18:36 authored by Andrew
N. Owen, Maria A. Zdanovskaia, Brian J. Esselman, John F. Stanton, R. Claude Woods, Robert J. McMahonA semi-experimental
equilibrium structure (

*r*_{e}^{SE}) of pyridazine (*o*-C_{4}H_{4}N_{2}) 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 (*A*_{0},*B*_{0}, and*C*_{0}) from each isotopologue were converted to determinable constants (*A*_{0}^{″},*B*_{0}^{″}, and*C*_{0}^{″}), 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-^{2}H, 4-^{13}C]-, [4-^{2}H, 5-^{13}C]-, [4-^{2}H, 6-^{13}C]-, and [4,5-^{2}H, 4-^{13}C]-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.## History

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## Keywords

vibration – rotationcontaining bond angles6 -< sup5 -< sup375 ghz ).360 ghz ),experimental equilibrium (<se 2 13 4 -< supequilibrium structure (<determinable constants (<statistical experimental uncertaintylevel theoretical calculationsc ]- pyridazinepreviously reported isotopologues4 pyridazine (<experimental valuesc theoretical structuresstructure withinstatistical uncertaintiesspectroscopic constantspreviously reportedpcvtz calculationsr b four isotopologues18 isotopologuesc ]-,>‑ c>- cwithin 0two relativestructural parameterrotational spectraprecision appearspartially responsiblelargely duefrequency coveragefirst timedetermined usingccsd (>< sub04 °,001 å)/ cc

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