化学
电泳剂
吉布斯自由能
计算化学
渲染(计算机图形)
反应机理
溶剂
质子
组合化学
缩合反应
分子
光化学
有机化学
热力学
催化作用
物理
计算机图形学(图像)
量子力学
计算机科学
作者
János Daru,András Stirling
摘要
One of the most widespread synthetic routes to coumarins is the condensation of esters and phenols via the Pechmann reaction. Despite the industrial and technological importance of the reaction, its mechanism is still poorly understood. We have explored several possible reaction paths by DFT calculations at the M05–2X/6-31+G* level. Amphoteric groups and the solvent have a crucial role in the frequent proton-transfer steps of the mechanisms; therefore, we have employed a mixed solvent model, where we combined the implicit PCM model together with an explicit water molecule placed at the actual proton transfer region. The Gibbs free-energy profiles of the possible routes suggest that three parallel channels (featuring water elimination, trans-esterification, and electrophilic attack) operate simultaneously. Enolic routes have prohibitively high activation barriers rendering these paths untenable. The calculated profiles indicate that in each feasible route the first elementary step has the highest activation energy. Reaction intermediates identified on the free-energy profiles can explain several experimental observations.
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