Elucidation of rate variations for a Diels-Alder reaction in ionic liquids from QM/MM simulations
Elucidation of rate variations for a Diels-Alder reaction in ionic liquids from QM/MM simulations
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DOI：
10.1021/ct6002753
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发表时间：
2007-01-01
影响因子：
5.5
通讯作者：
Evanseck, Jeffrey D.
中科院分区：
文献类型：
--
作者：
Acevedo, Orlando;Jorgensen, William L.;Evanseck, Jeffrey D.
采用QM/MM方法研究了酸性和碱性离子液体1-乙基-3-甲基咪唑氯化物（EMIC）熔体对环戊二烯和丙烯酸甲酯Diels-Alder反应速率的影响.之前已经提出了离子液体作为氢键供体（阳离子效应）的能力，由其氢键接受能力（阴离子效应）调节，以解释观察到的内/外型比。然而，赋予离子液体以其速率增强潜力的分子因素仍然未知。新的OPLS-AA力场参数与来自Monte Carlo模拟（MC/FEP）中的自由能微扰计算的平均力（PMF）的潜力被用来计算活化能。QM/MM模拟使用一个周期性的离子盒再现相对速率增强EMIC熔体相比，水和1-氯丁烷，再现动力学实验。在沿着反应坐标的关键固定点上分析了酸性和碱性离子液体熔体中溶质-溶剂相互作用。发现反应速率在酸性熔体中比在碱性熔体中更大，这是由于在酸性熔体中不太占优势的离子配对，使得EMI阳离子能够更好地与过渡态的亲二烯体配位。模拟表明，EMI阳离子的C2上的氢并不有助于过渡态的稳定，如先前所认为的，并且与C4和C5上更多空间暴露的氢的相互作用发挥更大的作用。此外，通过与EMI阳离子在酸性熔体中的静电相互作用的过渡态的相对稳定性也大于通过与水溶液中的水分子的氢键结合提供的较弱的路易斯酸效应所提供的相对稳定性。
The impact of acidic and basic ionic liquid 1-ethyl-3-methylimidazolium chloride (EMIC) melts upon cyclopentadiene and methyl acrylate Diels-Alder reaction rates has been investigated using QM/MM calculations. The ability of the ionic liquid to act as a hydrogen bond donor (cation effect), moderated by its hydrogen bond accepting ability (anion effect), has been proposed previously to explain observed endo/exo ratios. However, the molecular factors that endow ionic liquids with their rate enhancing potential remain unknown. New OPLS-AA force field parameters in conjunction with potentials of mean force (PMF) derived from free energy perturbation calculations in Monte Carlo simulations (MC/FEP) are used to compute activation energies. QM/MM simulations using a periodic box of ions reproduce relative rate enhancements for the EMIC melts compared to water and 1-chlorobutane that reproduce kinetic experiments. Solute-solvent interactions in acidic and basic ionic liquid melts have been analyzed at key stationary points along the reaction coordinate. The reaction rate was found to be greater in the acidic rather than the basic melt due to less-dominant ion-pairing in the acidic melt, enabling the EMI cation to better coordinate to the dienophile at the transition state. The simulations suggest that the hydrogen on C2 of the EMI cation does not contribute to stabilization of the transition state, as previously believed, and the interactions with the more sterically exposed hydrogens on C4 and C5 play a larger role. In addition, the relative stabilization of the transition state through electrostatic interactions with the EMI cation in the acidic melt is also greater than that afforded by the weaker Lewis-acid effect provided by hydrogen bonding with water molecules in aqueous solution.