Keeffe James R, Gronert Scott, Colvin Michael E, Tran Ngoc L
Department of Chemistry and Biochemistry, San Francisco State University, 1600 Holloway Avenue, San Francisco, California 94132, USA.
J Am Chem Soc. 2003 Sep 24;125(38):11730-45. doi: 10.1021/ja0356683.
Identity proton-transfer reactions between 21 acids, Y-X-H, and their conjugate bases, (-)X-Y, were studied according to the reaction scheme, Y-X-H + (-)X-Y --> (Y-X-H...(-)X-Y)(cx) --> Y-X...H...X-Y --> (Y-X..H-X-Y)(cx) --> Y-X(-) + H-X-Y, where cx indicates an ion-molecule complex and ts indicates the proton-transfer transition state. All species were optimized at the MP2/6-311+G level, and these geometries were used for single-point calculations by other methods: coupled-cluster, DFT (gas phase), and a polarizable continuum aqueous solvent model (COSMO). All methods gave enthalpies of deprotonation which correlate well with experimental measurements of deltaH(ACID) (gas) or pK(a) (aq). Calculated gas-phase enthalpies of deprotonation (deltaH(ACID)) and enthalpies of activation (deltaH(#)) are poorly correlated except for small, carefully selected sets. This result stands in contrast to the many aqueous phase Brönsted correlations of kinetic and equilibrium acid strength. On the other hand, gas-phase enthalpies of complexation and deltaH(#) are well correlated, indicating that factors which stabilize the transition state are at work in the bimolecular ion-molecule complex although to a smaller degree. We infer that intermoiety electrostatic and other interactions, similar within the complex and the transition state, but absent in the separated reactants (products), cause the lack of correlation between deltaH(ACID) and the other two quantities. Such differences are strongly attenuated in water because reactants and products do interact with polar/polarizable matter (the solvent) if not with each other. Charge distributions (NPA) were computed, allowing calculation of Bernasconi's "transition state imbalance parameter". Such measures provide intuitively satisfactory trends, but only if the reaction termini, X, are kept the same. As X is made more electronegative, the magnitude of the apparent imbalance increases, a result of greater negative charge on X in the transition state. This result gives additional support for the importance of the ion-triplet structure, [YX(-)...H(+)...(-)XY], to the stability of the transition state. Additional qualitative support for this conclusion is provided by the inverse relationship between the activation barrier and the charge on the in-flight hydrogen in the transition state, and by the dominance of polar over resonance substituent effects on the stability of the transition state. Calculations also show that the "nitroalkane anomaly", well established in solution, does not exist in the gas phase. The COSMO model partly reproduces this anomaly and performs adequately except when strong, specific intermolecular forces such as hydrogen bonding between solvent and anions are important.
根据反应式Y-X-H + (-)X-Y --> (Y-X-H...(-)X-Y)(cx) --> Y-X...H...X-Y --> (Y-X..H-X-Y)(cx) --> Y-X(-) + H-X-Y，研究了21种酸Y-X-H与其共轭碱(-)X-Y之间的质子转移反应，其中cx表示离子-分子复合物，ts表示质子转移过渡态。所有物种均在MP2/6-311+G水平上进行了优化，并将这些几何结构用于其他方法的单点计算：耦合簇方法、密度泛函理论（气相）以及可极化连续介质水溶剂模型（COSMO）。所有方法给出的去质子化焓与实验测得的ΔH(ACID)（气相）或pK(a)（水溶液）具有良好的相关性。除了经过精心挑选的小数据集外，计算得到的气相去质子化焓(ΔH(ACID))和活化焓(ΔH(#))之间的相关性较差。这一结果与许多关于动力学和平衡酸强度的水相布朗斯特相关性形成对比。另一方面，络合的气相焓和ΔH(#)具有良好的相关性，这表明稳定过渡态的因素在双分子离子-分子复合物中起作用，尽管程度较小。我们推断，复合物和过渡态中存在但分离的反应物（产物）中不存在的分子间静电和其他相互作用，导致了ΔH(ACID)与其他两个量之间缺乏相关性。在水中，这种差异会大大减弱，因为反应物和产物即使彼此不相互作用，也会与极性/可极化物质（溶剂）相互作用。计算了电荷分布（NPA），从而可以计算贝尔纳斯科尼的“过渡态不平衡参数”。这些量度提供了直观上令人满意的趋势，但前提是反应末端X保持不变。随着X的电负性增强，表观不平衡的幅度增大，这是过渡态中X上负电荷增加的结果。这一结果进一步支持了离子三联体结构[YX(-)...H(+)...(-)XY]对过渡态稳定性的重要性。活化能垒与过渡态中飞行中氢上的电荷之间的反比关系以及极性取代基效应相对于共振取代基效应在过渡态稳定性上的主导地位，为这一结论提供了额外的定性支持。计算还表明，在溶液中已得到充分证实的“硝基烷烃异常”在气相中不存在。COSMO模型部分重现了这种异常，并且表现良好，除非溶剂与阴离子之间的强特异性分子间力（如氢键）很重要。
