Abdel Azeim Safwat, van der Rest Guillaume
Laboratoire des Mécanismes Réactionnels, CNRS UMR 7651, Ecole Polytechnique, 91128 Palaiseau Cedex, France.
J Phys Chem A. 2005 Mar 24;109(11):2505-13. doi: 10.1021/jp045425c.
The thermochemical properties of some small clusters such as the (H2O)2*+ dimer have already been investigated by both experimental and theoretical methods. The recent method to selectively prepare the ammonia-water ionized dimer [NH3, H2O]+ (and not its proton transfer isomer [NH4+, OH]) allowed us to study its chemical reactivity. This study focuses on the charge and proton transfer pathways: Ion-molecule reactions in the cell of an FT-ICR mass spectrometer were carried out with a range of organic compounds. Examination of the reactivity of the [NH3, H2O]+ ionized dimer versus ionization energy and proton affinity of the neutral reagents shows a threshold in the reactivity in both instances. This leads to a bracketing of thermochemical properties related to the dimer. From these experiments and in agreement with ab initio calculations, the adiabatic recombination energy of the [NH3, H2O]+ dimer was evaluated at -9.38 +/- 0.04 eV. The proton affinity bracketing required the reevaluation of two reference gas-phase basicity values. The results, in good agreement with the calculation, lead to an evaluation of the proton affinity of the [NH2*, H2O] dimer at 204.4 +/- 0.9 kcal mol(-1). These two experimental values are respectively related to the ionization energy of NH3*+ and to the proton affinity of NH2* by the difference in single water molecule solvation energies of ionized ammonia, of neutral ammonia, and of the NH2* radical.
一些小团簇的热化学性质，比如(H2O)2*+二聚体，已经通过实验和理论方法进行了研究。最近选择性制备氨 - 水离子化二聚体[NH3, H2O]+（而非其质子转移异构体[NH4+, OH]）的方法，使我们能够研究其化学反应性。本研究聚焦于电荷和质子转移途径：在傅里叶变换离子回旋共振质谱仪的离子池中，用一系列有机化合物进行了离子 - 分子反应。对[NH3, H2O]+离子化二聚体与中性试剂的电离能和质子亲和性的反应性研究表明，在这两种情况下反应性都存在一个阈值。这导致了与该二聚体相关的热化学性质的界定。通过这些实验并与从头算计算结果一致，[NH3, H2O]+二聚体的绝热重组能评估为-9.38±0.04 eV。质子亲和性的界定需要重新评估两个参考气相碱度值。结果与计算结果高度吻合，得出[NH2*, H2O]二聚体的质子亲和性为204.4±0.9 kcal mol(-1)。这两个实验值分别与NH3*+的电离能以及NH2的质子亲和性相关，其关联方式是通过离子化氨、中性氨以及NH2自由基的单个水分子溶剂化能的差异来实现。
