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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Mechanisms of Ligand Exchange Reactions, A Quantum Chemical Study of the Reaction UO_2~(2+)(Aq) + HF~(Aq) → UO_2F~+(Aq) + H~+(Aq)
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Mechanisms of Ligand Exchange Reactions, A Quantum Chemical Study of the Reaction UO_2~(2+)(Aq) + HF~(Aq) → UO_2F~+(Aq) + H~+(Aq)

机译：配体交换反应的机理，反应UO_2〜（2 +）（Aq）+ HF〜（Aq）→UO_2F〜+（Aq）+ H〜+（Aq）的量子化学研究

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The thermodynamics and the reaction mechanism for the reaction UO_2~(2+)(Aq) + HF~(Aq) → UO_2F~+(Aq) + H~+(Aq) in water solution has been studied using quantum chemical methods. The solvent was modeled using the polarized medium method (CPCM) with additional water molecules in the second coordination sphere of the complexes studied. The overall reaction was divided into three steps that were analyzed separately. The quantum chemical study was made on the reaction step [UO_2(H_2O)_5~(2+)], HF(H_2O)_n → [UO_2F (H_2O)_4+], H_2O+ (H_2O)_n, with n = 1 and 2, where the species in the second coordination sphere are located outside the square brackets. The formation of the precursor complex and dissociation of the successor complex were described by the Fuoss equation. The geometry of the different precursor and successor complexes was in good agreement with known bond distances, and strong F---H---O, and/or O---H---O hydrogen bonds are an important structure element in all of them. The Gibbs energy, enthalpy, and entropy of reaction was calculated using the electronic energy at the MP2 level in the solvent, with thermal functions calculated at the SCF/B3LYP levels using the gas-phase geometry. The calculated gibbs energy of reaction for n = 2 at 298.15 K was -35 kJ/mol at the HF and -25 kJ/mol at the B3LYP level after correction for a known systematic error in the HF bond energy; this compares favorably with the experimental value, -11 kJ/mol. The ligand exchange mechanism was explored by identification of a transition state where HF from the second sphere enters the first coordination sphere in an associative reaction. It was not possible to identify the same transition state from the successor side, indicating that the reaction mechanisms consists of at least two steps. We suggest that the rate determining step is the entry of HF from the second to the first coordination sphere, with practically no bond-breaking as indicated by the small change in the H-F distance between precursor and transition state. This suggestion is supported by the experimentally observed reverse H/D isotope effect. The quantum chemical activation energy △U~≠ was 34 kJ/mol, close to the experimental activation enthalpy △H~≠ = 38 kJ/mol.
机译：利用量子化学方法研究了水溶液中UO_2〜（2 +）（Aq）+ HF〜（Aq）→UO_2F〜+（Aq）+ H〜+（Aq）的热力学及反应机理。使用极化介质方法（CPCM）对溶剂进行了建模，并在研究的配合物的第二配位领域中添加了水分子。整个反应分为三个步骤，分别进行分析。在反应步骤[UO_2（H_2O）_5〜（2 +）]，HF（H_2O）_n→[UO_2F（H_2O）_4 +]，H_2O +（H_2O）_n的反应步骤上进行了量子化学研究，n = 1和2，第二个协调领域中的物种位于方括号之外的位置。前体复合物的形成和后继复合物的解离由Fuoss方程描述。不同的前驱体和后继配合物的几何形状与已知的键距非常吻合，强的F --- H --- O和/或O --- H --- O氢键是其中重要的结构元素。他们全部。使用溶剂中MP2处的电子能计算反应的Gibbs能量，焓和熵，并使用气相几何结构计算SCF / B3LYP处的热函数。在校正了已知的HF键能量系统误差后，在298.15 K下，n = 2时，计算得出的反应吉布斯能量在HF下为-35 kJ / mol，在B3LYP水平下为-25 kJ / mol。这与实验值-11 kJ / mol相比具有优势。通过鉴定过渡态的配体交换机制，其中HF从第二个球进入缔合反应时进入第一个配位球。从后继侧不可能确定相同的过渡状态，这表明反应机理至少包括两个步骤。我们建议速率确定步骤是HF从第二个到第一个配位域的进入，实际上没有键断裂，如前体和过渡态之间的H-F距离的微小变化所表明。这一建议得到了实验观察到的反向H / D同位素效应的支持。量子化学活化能△U〜≠为34 kJ / mol，接近实验活化焓△H〜≠= 38 kJ / mol。

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