北京理工大学谢静团队取得一项新突破。他们的最新研究揭示了水微滴增强的化学反应——一个Diels-Alder反应案例。这一研究成果发表在2024年11月12日出版的国际学术期刊《美国化学会杂志》上。

通常，与相应的本体相相比，微滴中的化学反应明显加快。虽然确定精确的催化因素仍然具有挑战性，但界面电场（IEF）和部分溶剂化是两个被广泛提出的因素，它们解释了微滴中许多反应的加速或开启。

与此形成鲜明对比的是，这一计算和实验相结合的研究表明，这两个关键因素对促进微水滴中，环戊二烯和丙烯腈之间的Diels-Alder（DA）模型反应的影响可以忽略不计。相反，DA反应的加速似乎是由限域效应和蒸发引起的浓度增加所驱动。量子化学计算和从头算分子动力学模拟结合增强的采样技术预测，空气-水界面在该反应中表现出比本体更高的自由能势垒，而外电场略微降低了势垒。

值得注意的是，IEF在水微滴表面的催化能力，在很大程度上受到其波动特性的影响。微滴反应的质谱评估证实了这些发现，表明IEF并没有促进DA反应，因为增加喷雾电位通过促进底物氧化来抑制DA产物。

虽然DA反应在水微滴中表现出表面偏好，但同样的反应往往主要发生在乙腈微滴的核心内，这表明部分溶剂化不一定是微滴中加速该反应的关键因素。此外，实验表明，微滴的快速蒸发和随后在微滴可达的受限体积内的试剂富集，导致了微滴中DA反应的加速。

附：英文原文

Title: Revisiting the Enhanced Chemical Reactivity in Water Microdroplets: The Case of a Diels–Alder Reaction

Author: Ke Gong, Abhijit Nandy, Zhexuan Song, Quan-Song Li, Ali Hassanali, Giuseppe Cassone, Shibdas Banerjee, Jing Xie

Issue&Volume: November 12, 2024

Abstract: Often, chemical reactions are markedly accelerated in microdroplets compared with the corresponding bulk phase. While identifying the precise causative factors remains challenging, the interfacial electric field (IEF) and partial solvation are the two widely proposed factors, accounting for the acceleration or turning on of many reactions in microdroplets. In sharp contrast, this combined computational and experimental study demonstrates that these two critical factors have a negligible effect on promoting a model Diels–Alder (DA) reaction between cyclopentadiene and acrylonitrile in water microdroplets. Instead, the acceleration of the DA reaction appears to be driven by the effect of confinement and the concentration increase caused by evaporation. Quantum chemical calculations and ab initio molecular dynamics simulations coupled with enhanced sampling techniques predict that the air–water interface exhibits a higher free-energy barrier of this reaction than the bulk, while external electric fields marginally reduce the barrier. Remarkably, the catalytic capability of the IEF at the water microdroplet surface is largely hampered by its fluctuating character. Mass spectrometric assessment of the microdroplet reaction corroborates these findings, suggesting that the DA reaction is not facilitated by the IEF as increasing the spray potential suppresses the DA products by promoting substrate oxidation. While the DA reaction exhibits a surface preference in water microdroplets, the same reaction tends to occur mainly within the core of the acetonitrile microdroplet, suggesting that the partial solvation is not necessarily a critical factor for accelerating this reaction in microdroplets. Moreover, experiments indicate that the rapid evaporation of microdroplets and subsequent reagent enrichment within the accessible confined volume of microdroplets caused the observed acceleration of the DA reaction in water microdroplets.

DOI: 10.1021/jacs.4c09400

Source: https://pubs.acs.org/doi/abs/10.1021/jacs.4c09400