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.

中文翻译：

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哒嗪 (o-C4H4N2) 的半实验平衡 (reSE) 和理论结构

已经使用 18 种同位素体的旋转光谱确定了哒嗪 ( o -C ₄ H ₄ N ₂ )的半实验平衡结构 ( r _e ^SE ) 。首次报告了四种同位素体的光谱常数（从 235 到 360 GHz 测量），而先前报告的同位素体的光谱常数通过扩展频率覆盖范围（从 130 到 375 GHz 测量）得到改善。来自每个同位素体的基态旋转常数（A ₀、B ₀和C ₀）的实验值被转换为可确定的常数（A₀ ^″、B ₀ ^″和C ₀ ^″ )，然后使用 CCSD(T)/cc-pCVTZ 计算对振动-旋转相互作用和电子质量分布的影响进行校正。哒嗪的所得r _e ^SE确定键距在 0.001 Å 以内，键角在 0.04° 以内，相对于先前报告的r _e ^SE，统计不确定性至少降低了两倍。尽管结合了新测量的同位素体（[4-² H, 4- ¹³ C]-, [4- ² H, 5- ¹³ C]-, [4- ² H, 6- ¹³ C]-, 和 [4,5- ² H, 4- ¹³ C] -哒嗪）部分负责改进含氢键角的测定。计算出的平衡结构 ( r _e ) (CCSD(T)/cc-pCV5Z) 和平衡结构的“最佳理论估计” ( r _e ) 都与统计实验不确定性 (2σ) 内更新的r _e ^SE结构一致每个结构参数。