Kumar Madhu Deepan, Jaccob Madhavan
Department of Chemistry & Computational Chemistry Laboratory, Loyola Institute of Frontier Energy (LIFE), Loyola College, University of Madras, Chennai 600 034, Tamil Nadu, India.
Phys Chem Chem Phys. 2021 Mar 28;23(12):7386-7397. doi: 10.1039/d0cp05090c. Epub 2021 Mar 23.
In this work, density functional theory (DFT) calculations were carried out to study the role of the explicit treatment of four different choline-based ionic liquids (CS, CP, NS, and NP) by utilizing two different cations and anions in the tautomeric equilibrium of ethyl acetoacetate (EAA). The involvement of the acidic N-H proton from the cationic part of NS and NP ionic liquid offers the possibility to have two more additional transition states for the tautomeric equilibrium of EAA. The computed results demonstrated that a high activation free energy barrier (ΔG = 49.4 kcal mol) is associated with the direct enol to keto (E → K) interconversion via a 4-membered ring transition state. Upon explicit involvement of the cationic part of ionic liquids in the tautomeric equilibrium via a 6-membered ring transition state (CAT), ΔG is substantially reduced to 21.88 kcal mol. Further, ΔG is drastically reduced to 10.57 kcal mol upon the involvement of the anionic part of the ionic liquid explicitly via an 8-membered ring transition state (AAT). The W-shaped TS in the CAT pathway causes steric hindrance and increases the energy penalty, while the sickle-shaped TS in AAT facilitates easy proton transfer without the influence of the steric factor. In addition, the RDG scatter graphs predict large negative values of ρ*, which indicate that the hydrogen bonding network in AAT is stronger, enhancing the delocalization of the electron density. The QTAIM analysis substantiated the role of intermolecular hydrogen bonding interactions between the ionic liquid and EAA and within the anion-cation pair in stabilizing the keto group of EAA. Besides, the involvement of the acidic N-H proton in the transition state is the key factor in influencing the energetics of the keto-enol tautomerization reaction. The present study illustrates molecular-level insights into the role of individual ions of ionic liquids and also provides adequate ideas for designing novel ionic liquid-based catalysts for industrially relevant chemical reactions.
在本工作中，进行了密度泛函理论（DFT）计算，以研究在乙酰乙酸乙酯（EAA）的互变异构平衡中使用两种不同的阳离子和阴离子对四种不同胆碱基离子液体（CS、CP、NS和NP）进行显式处理的作用。NS和NP离子液体阳离子部分的酸性N-H质子的参与为EAA的互变异构平衡提供了另外两个额外过渡态的可能性。计算结果表明，通过四元环过渡态直接从烯醇到酮（E→K）的互变具有较高的活化自由能垒（ΔG = 49.4 kcal/mol）。当离子液体的阳离子部分通过六元环过渡态（CAT）明确参与互变异构平衡时，ΔG大幅降低至21.88 kcal/mol。此外，当离子液体的阴离子部分通过八元环过渡态（AAT）明确参与时，ΔG急剧降低至10.57 kcal/mol。CAT途径中的W形过渡态会造成空间位阻并增加能量损失，而AAT中的镰刀形过渡态有利于质子的轻松转移，不受空间因素的影响。此外，RDG散射图预测ρ*的负值较大，这表明AAT中的氢键网络更强，增强了电子密度的离域。QTAIM分析证实了离子液体与EAA之间以及阴离子-阳离子对内部的分子间氢键相互作用在稳定EAA酮基方面的作用。此外，过渡态中酸性N-H质子的参与是影响酮-烯醇互变异构反应能量学的关键因素。本研究阐明了离子液体单个离子作用的分子水平见解，也为设计用于工业相关化学反应的新型离子液体基催化剂提供了充分的思路。
