Bubas Amanda R, Kafle Arjun, Stevenson Brandon C, Armentrout P B
Department of Chemistry, University of Utah, 315 S 1400 E Rm 2020, Salt Lake City, Utah 84112-0850, USA.
J Chem Phys. 2024 Apr 28;160(16). doi: 10.1063/5.0204090.
A guided ion beam tandem mass spectrometer was used to study the reactions of U+ with N2 and NO. Reaction cross sections were measured over a wide range of energy for both systems. In each reaction, UN+ is formed by an endothermic process, thereby enabling the direct measurement of the threshold energy and determination of the UN+ bond dissociation energy. For the reaction of U+ + N2, a threshold energy (E0) of 4.02 ± 0.11 eV was measured, leading to D0 (UN+) = 5.73 ± 0.11 eV. The reaction of U+ + NO yields UO+ through an exothermic, barrierless process that proceeds with 94 ± 23% efficiency at the lowest energy. Analysis of the endothermic UN+ cross section in this reaction provides E0 = 0.72 ± 0.11 eV and, therefore, D0 (UN+) = 5.78 ± 0.11 eV. Averaging the values obtained from both reactions, we report D0 (UN+) = 5.76 ± 0.13 eV as our best value (uncertainty of two standard deviations). Combined with precise literature values for the ionization energies of U and UN, we also derive D0 (UN) = 5.86 ± 0.13 eV. Both bond dissociation energies agree well with high-level theoretical treatments in the literature. The formation of UN+ in reaction of U+ with NO also exhibits a considerable increase in reaction probability above ∼3 eV. Theory suggests that this may be consistent with the formation of UN+ in excited quintet spin states, which we hypothesize are dynamically favored because the number of 5f electrons in reactants and products is conserved.
使用导向离子束串联质谱仪研究了U⁺与N₂和NO的反应。在很宽的能量范围内测量了这两个体系的反应截面。在每个反应中，UN⁺通过吸热过程形成，从而能够直接测量阈能并确定UN⁺的键离解能。对于U⁺ + N₂反应，测得阈能（E₀）为4.02±0.11 eV，得到D₀(UN⁺)=5.73±0.11 eV。U⁺ + NO反应通过一个无势垒的放热过程生成UO⁺，在最低能量下该过程的效率为94±23%。对该反应中吸热的UN⁺截面进行分析得到E₀ = 0.72±0.11 eV，因此D₀(UN⁺)=5.78±0.11 eV。将两个反应得到的值平均，我们报告D₀(UN⁺)=5.76±0.13 eV作为我们的最佳值（两个标准偏差的不确定度）。结合U和UN电离能的精确文献值，我们还推导出D₀(UN)=5.86±0.13 eV。这两个键离解能与文献中的高水平理论处理结果吻合得很好。U⁺与NO反应中UN⁺的形成在~3 eV以上的反应概率也有显著增加。理论表明，这可能与激发五重态自旋态的UN⁺形成一致，我们推测这在动力学上是有利的，因为反应物和产物中5f电子的数量是守恒的。
