Millimeter/Submillimeter-wave Spectroscopy and the Semi-experimental Equilibrium (<i>r</i>_<i>e</i>^SE) Structure of 1<i>H</i>‑1,2,4-Triazole (<i>c</i>‑C₂H₃N₃)

The millimeter/submillimeter spectrum of 1H-1,2,4-triazole is reported from 70 to 700 GHz, providing spectral frequencies directly comparable to radio telescopes and enabling an astronomical search. Using four deuteriated samples of 1,2,4-triazole, we measured, assigned, and least-squares fit transitions for 26 isotopologues to sextic A- and S-reduced Hamiltonians. An accurate and precise semi-experimental (reSE) structure from 50 independent moments of inertia has been obtained. Structural parameters are provided with 2σ uncertainties within 0.0009 Å for bond distances and 0.09° for bond angles. The structural parameters are in quite good agreement with the best theoretical estimate (BTE) obtained using CCSD(T)/cc-pCV5Z, where an agreement within the 2σ uncertainty is observed for all but one case. Despite the large number of isotopologues already included in this structure, more may be useful. One isotopologue, [1,3-2H]-1H-1,2,4-triazole, is observed to closely approach the oblate asymmetric-top limit, resulting in a clear breakdown of the A-reduction Hamiltonian. The highly accurate reSE structure and subsequent analysis demonstrates that the S-reduction is also unable to adequately model the spectrum of this isotopologue.本研究报道了1H-1,2,4-三唑（1H-1,2,4-triazole）在70至700 GHz频段内的毫米波/亚毫米波光谱（millimeter/submillimeter spectrum），所得光谱频率可直接与射电望远镜（radio telescopes）观测数据比对，为天文搜寻提供支撑。本研究采用1,2,4-三唑的4份氘代样品（deuteriated samples），对26种同位素异构体（isotopologues）的跃迁谱线进行了测量、指认，并基于六次A约化哈密顿量与S约化哈密顿量（sextic A- and S-reduced Hamiltonians）完成最小二乘拟合。基于50个独立转动惯量矩（moments of inertia），我们获得了高精度的半实验（semi-experimental, reSE）结构。结构参数的键长不确定度（2σ）不超过0.0009埃，键角不确定度（2σ）不超过0.09°。该结构参数与采用CCSD(T)/cc-pCV5Z方法得到的最优理论估算值（best theoretical estimate, BTE）吻合度较高，除一组数据外，其余均在2σ不确定度范围内一致。尽管本结构已纳入大量同位素异构体，进一步扩充数据集仍具备研究价值。其中一种同位素异构体——[1,3-2H]-1H-1,2,4-三唑（[1,3-2H]-1H-1,2,4-triazole）——的光谱特性极为接近扁型不对称陀螺极限（oblate asymmetric-top limit），导致A约化哈密顿量（A-reduction Hamiltonian）出现明显失效。高精度的reSE半实验结构及后续分析表明，S约化哈密顿量（S-reduction Hamiltonian）同样无法准确模拟该同位素异构体的光谱特征。