Laboratory for Theoretical and Applied Chemometrics, Institute of Chemistry, University of Campinas - UNICAMP, Campinas, SP 13084-971, POB 6154, Brazil.
J Phys Chem A. 2010 Apr 22;114(15):5187-94. doi: 10.1021/jp9106316.
A theoretical study of propylene oxide acid-catalyzed hydrolysis was performed by investigation of the S(N)1 and S(N)2-like mechanisms. By using chemometric tools, hierarchical cluster analysis (HCA), and principal component analysis (PCA), the MP2/6-311++G** level of theory was selected from HF, MP2, and DFT as the best method to describe the geometry of the basic skeleton (oxirane). At this level of theory, geometry optimizations, vibrational frequencies, intrinsic reaction coordinate (IRC), and other thermodynamic calculations have shown that the borderline S(N)2 mechanism is more favorable than pure S(N)2 and S(N)1 mechanisms in the gas phase. In the S(N)1 mechanism, the existence of the typical carbocation was not observed, and furthermore, the possibility of epoxide conversion to a protonated aldehyde was indicated, even in the presence of a water molecule (nucleophile). The Chelpg charge distribution of the reactants, steric hindrance, synchronous bond breaking-formation and trajectory angle of nucleophilic attack are discussed for pure and borderline S(N)2 mechanisms. Solvation effect calculations indicate that the pure S(N)2 mechanism is more favorable than borderline S(N)2 and S(N)1 mechanisms. This is discussed in terms of hydrogen bond formation.
采用 S(N)1 和 S(N)2 类似机制对环氧丙烷酸催化水解进行了理论研究。通过使用化学计量学工具，层次聚类分析 (HCA) 和主成分分析 (PCA)，从 HF、MP2 和 DFT 中选择了 MP2/6-311++G** 理论水平作为描述基本骨架（环氧化物）几何形状的最佳方法。在该理论水平上，几何优化、振动频率、内禀反应坐标 (IRC) 和其他热力学计算表明，在气相中，边界 S(N)2 机制比纯 S(N)2 和 S(N)1 机制更有利。在 S(N)1 机制中，没有观察到典型碳正离子的存在，并且甚至在存在水分子（亲核试剂）的情况下，也表明了环氧化物转化为质子化醛的可能性。讨论了反应物的 Chelpg 电荷分布、空间位阻、同步键断裂-形成和亲核进攻的轨迹角，用于纯和边界 S(N)2 机制。溶剂化效应计算表明，纯 S(N)2 机制比边界 S(N)2 和 S(N)1 机制更有利。这是根据氢键形成来讨论的。
