Fernandez-Lima F A, Ponciano C R, Chaer Nascimento M A, da Silveira E F
Physics Department, Pontifícia Universidade Católica, Rua Marques de São Vicente 225, 22543-970 Rio de Janeiro, Brazil.
J Phys Chem A. 2006 Aug 24;110(33):10018-24. doi: 10.1021/jp0619944.
Positive and negatively charged ammonia clusters produced by the impact of (252)Cf fission fragments (FF) on an NH(3) ice target have been examined theoretical and experimentally. The ammonia clusters generated by (252)Cf FF show an exponential dependence of the cluster population on its mass, and the desorption yields for the positive (NH(3))(n)NH(4)(+) clusters are 1 order of magnitude higher than those for the negative (NH(3))(n)NH(2)(-) clusters. The experimental population analysis of (NH(3))(n)NH(4)(+) (n = 0-18) and (NH(3))(n)NH(2)(-) (n = 0-8) cluster series show a special stability at n = 4 and 16 and n = 2, 4, and 6, respectively. DFT/B3LYP calculations of the (NH(3))(0)(-)(8)NH(4)(+) clusters show that the structures of the more stable conformers follow a clear pattern: each additional NH(3) group makes a new hydrogen bond with one of the hydrogen atoms of an NH(3) unit already bound to the NH(4)(+) core. For the (NH(3))(0)(-)(8)NH(2)(-) clusters, the DFT/B3LYP calculations show that, within the calculation error, the more stable conformers follow a clear pattern for n = 1-6: each additional NH(3) group makes a new hydrogen bond to the NH(2)(-) core. For n = 7 and 8, the additional NH(3) groups bind to other NH(3) groups, probably because of the saturation of the NH(2)(-) core. Similar results were obtained at the MP2 level of calculation. A stability analysis was performed using the commonly defined stability function E(n)(-)(1) + E(n)(+1) - 2E(n), where E is the total energy of the cluster, including the zero point correction energy (E = E(t) + ZPE). The trend on the relative stability of the clusters presents an excellent agreement with the distribution of experimental cluster abundances. Moreover, the stability analysis predicts that the (NH(3))(4)NH(4)(+) and the even negative clusters [(NH(3))(n)NH(2)(-), n = 2, 4, and 6] should be the most stable ones, in perfect agreement with the experimental results.
通过（252）Cf裂变碎片（FF）撞击NH₃冰靶产生的带正电和负电的氨簇已通过理论和实验进行了研究。由（252）Cf FF产生的氨簇显示出簇数量对其质量的指数依赖性，并且正（NH₃）ₙNH₄⁺簇的解吸产率比负（NH₃）ₙNH₂⁻簇的解吸产率高1个数量级。对（NH₃）ₙNH₄⁺（n = 0 - 18）和（NH₃）ₙNH₂⁻（n = 0 - 8）簇系列的实验数量分析分别显示在n = 4和16以及n = 2、4和6时具有特殊稳定性。（NH₃）₀⁻⁸NH₄⁺簇的DFT/B3LYP计算表明，更稳定构象体的结构遵循清晰的模式：每个额外的NH₃基团与已经与NH₄⁺核心结合的NH₃单元的一个氢原子形成新的氢键。对于（NH₃）₀⁻⁸NH₂⁻簇，DFT/B3LYP计算表明，在计算误差范围内，n = 1 - 6时更稳定构象体遵循清晰的模式：每个额外的NH₃基团与NH₂⁻核心形成新的氢键。对于n = 7和8，额外的NH₃基团与其他NH₃基团结合，可能是因为NH₂⁻核心饱和。在MP2计算水平上也获得了类似的结果。使用通常定义的稳定性函数E(n)⁻¹ + E(n + 1) - 2E(n)进行了稳定性分析，其中E是簇的总能量，包括零点校正能量（E = E(t) + ZPE）。簇相对稳定性的趋势与实验簇丰度的分布非常吻合。此外，稳定性分析预测（NH₃）₄NH₄⁺和偶数负簇[（NH₃）ₙNH₂⁻，n = 2、4和6]应该是最稳定的，这与实验结果完全一致。
