Honma Kenji, Armentrout P B
Department of Material Science, University of Hyogo, Kamigori, Hyogo, 678-1297 Japan.
J Chem Phys. 2004 Nov 1;121(17):8307-20. doi: 10.1063/1.1802391.
Reactions of protonated water clusters, H(H(2)O)(n) (+) (n=1-4) with D(2)O and their "mirror" reactions, D(D(2)O)(n) (+) (n=1-4) with H(2)O, are studied using guided-ion beam mass spectrometry. Absolute reaction cross sections are determined as a function of collision energy from thermal energy to over 10 eV. At low collision energies, we observe reactions in which H(2)O and D(2)O molecules are interchanged and reactions where H-D exchange has occurred. As the collision energy is increased, the H-D exchange products decrease and the water exchange products become dominant. At high collision energies, processes in which one or more water molecules are lost from the reactant ions become important, with simple collision-induced dissociation processes, i.e., those without H-D exchange, being dominant. Threshold energies of endothermic channels are measured and used to determine binding energies of the proton bound complexes, which are consistent with those determined by thermal equilibrium measurements and previous collision-induced dissociation studies. A kinetic scheme that relies only on the ratio of isomerization and dissociation rate constants successfully accounts for the kinetic energy dependence observed in the branching ratios for H-D and water exchange products in all systems. Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations confirm the feasibility and establish the details of this kinetic model.
利用导向离子束质谱法研究了质子化水团簇H(H₂O)ₙ⁺（n = 1 - 4）与D₂O的反应及其“镜像”反应，即D(D₂O)ₙ⁺（n = 1 - 4）与H₂O的反应。确定了从热能到超过10 eV的碰撞能量范围内的绝对反应截面。在低碰撞能量下，我们观察到H₂O和D₂O分子发生交换的反应以及发生H-D交换的反应。随着碰撞能量增加，H-D交换产物减少，水交换产物占主导。在高碰撞能量下，反应物离子失去一个或多个水分子的过程变得重要，简单的碰撞诱导解离过程（即没有H-D交换的过程）占主导。测量了吸热通道的阈值能量，并用于确定质子结合复合物的结合能，这与通过热平衡测量和先前的碰撞诱导解离研究所确定的结果一致。一个仅依赖于异构化和解离速率常数比值的动力学方案成功地解释了在所有系统中H-D和水交换产物分支比中观察到的动能依赖性。赖斯-拉姆齐格-卡塞尔-马库斯理论和从头算计算证实了该动力学模型的可行性并确定了其细节。
