Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, S-10691 Stockholm, Sweden.
Org Biomol Chem. 2011 Aug 7;9(15):5394-406. doi: 10.1039/c0ob01210f. Epub 2011 Jun 7.
We present 2-dimensional potential energy surfaces and optimised transition states (TS) for water attack on a series of substituted phosphate monoester monoanions at the DFT level of theory, comparing a standard 6-31++g(d,p) basis set with a larger triple-zeta (augmented cc-pVTZ) basis set. Small fluorinated model compounds are used to simulate increasing leaving group stability without adding further geometrical complexity to the system. We demonstrate that whilst changing the leaving group causes little qualitative change in the potential energy surfaces (with the exception of the system with the most electron withdrawing leaving group, CF(3)O(-), in which the associative pathway changes from a stepwise A(N) + D(N) pathway to a concerted A(N)D(N) pathway), there is a quantitative change in relative gas-phase and solution barriers for the two competing pathways. In line with previous studies, in the case of OCH(3), the barriers for the associative and dissociative pathways are similar in solution, and the two pathways are equally viable and indistinguishable in solution. However, significantly increasing the stability of the leaving group (decreasing proton affinity, PA) results in the progressive favouring of a stepwise dissociative, D(N) + A(N), mechanism over associative mechanisms.
我们提出了二维势能面和优化过渡态（TS），用于在 DFT 理论水平上研究一系列取代的磷酸单酯单阴离子与水的反应，比较了标准的 6-31++g(d,p)基组与更大的三zeta（augmented cc-pVTZ）基组。使用小的氟化模型化合物来模拟增加离去基团稳定性，而不向体系中添加进一步的几何复杂性。我们证明，尽管改变离去基团几乎不会导致势能面（除了具有最强吸电子离去基团的 CF(3)O(-)系统，其中缔合途径从逐步 A(N) + D(N)途径变为协同 A(N)D(N)途径）产生定性变化，但对于两种竞争途径，气相和溶液相对势垒存在定量变化。与之前的研究一致，在 OCH(3)的情况下，缔合和解离途径的溶液中的势垒相似，两种途径在溶液中都是可行的，并且无法区分。然而，显著增加离去基团的稳定性（降低质子亲和力，PA）导致逐步解离的 D(N) + A(N)机制逐渐优先于缔合机制。
