Gutowski Keith E, Holbrey John D, Rogers Robin D, Dixon David A
Department of Chemistry and Center for Green Manufacturing, Shelby Hall, Box 870336, University of Alabama, Tuscaloosa, Alabama 35487, USA.
J Phys Chem B. 2005 Dec 15;109(49):23196-208. doi: 10.1021/jp053985l.
A computational approach to predict the thermodynamics for forming a variety of imidazolium-based salts and ionic liquids from typical starting materials is described. The gas-phase proton and methyl cation acidities of several protonating and methylating agents, as well as the proton and methyl cation affinities of many important methyl-, nitro-, and cyano-substituted imidazoles, have been calculated reliably by using the computationally feasible DFT (B3LYP) and MP2 (extrapolated to the complete basis set limit) methods. These accurately calculated proton and methyl cation affinities of neutrals and anions are used in conjunction with an empirical approach based on molecular volumes to estimate the lattice enthalpies and entropies of ionic liquids, organic solids, and organic liquids. These quantities were used to construct a thermodynamic cycle for salt formation to reliably predict the ability to synthesize a variety of salts including ones with potentially high energetic densities. An adjustment of the gas phase thermodynamic cycle to account for solid- and liquid-phase chemistries provides the best overall assessment of salt formation and stability. This has been applied to imidazoles (the cation to be formed) with alkyl, nitro, and cyano substituents. The proton and methyl cation donors studied were as follows: HCl, HBr, HI, (HO)2SO2, HSO3CF3 (TfOH), and HSO3(C6H4)CH3 (TsOH); CH3Cl, CH3Br, CH3I, (CH3O)2SO2, CH3SO3CF3 (TfOCH3), and CH3SO3(C6H4)CH3 (TsOCH3). As substitution of the cation with electron-withdrawing groups increases, the triflate reagents appear to be the best overall choice as protonating and methylating agents. Even stronger alkylating agents should be considered to enhance the chances of synthetic success. When using the enthalpies of reaction for the gas-phase reactants (eq 6) to form a salt, a cutoff value of -13 kcal mol(-1) or lower (more negative) should be used as the minimum value for predicting whether a salt can be synthesized.
本文描述了一种计算方法，用于预测由典型起始原料形成各种咪唑基盐和离子液体的热力学性质。通过使用计算上可行的密度泛函理论（DFT，B3LYP）和二阶微扰理论（MP2，外推至完全基组极限）方法，可靠地计算了几种质子化和甲基化试剂的气相质子和甲基阳离子酸度，以及许多重要的甲基、硝基和氰基取代咪唑的质子和甲基阳离子亲合势。这些精确计算的中性分子和阴离子的质子和甲基阳离子亲合势，与基于分子体积的经验方法相结合，用于估算离子液体、有机固体和有机液体的晶格焓和熵。这些量被用于构建盐形成的热力学循环，以可靠地预测合成各种盐（包括具有潜在高能量密度的盐）的能力。对气相热力学循环进行调整以考虑固液相化学性质，可对盐的形成和稳定性提供最佳的整体评估。这已应用于具有烷基、硝基和氰基取代基的咪唑（待形成的阳离子）。所研究的质子和甲基阳离子供体如下：HCl、HBr、HI、(HO)2SO2、HSO3CF3（三氟甲磺酸，TfOH）和HSO3(C6H4)CH3（对甲苯磺酸，TsOH）；CH3Cl、CH3Br、CH3I、(CH3O)2SO2、CH3SO3CF3（三氟甲磺酸甲酯，TfOCH3）和CH3SO3(C6H4)CH3（对甲苯磺酸甲酯，TsOCH3）。随着阳离子被吸电子基团取代的增加，三氟甲磺酸试剂似乎是作为质子化和甲基化试剂的最佳整体选择。应考虑使用更强的烷基化试剂以提高合成成功的机会。当使用气相反应物的反应焓（式6）来形成盐时，-13 kcal mol(-1)或更低（更负）的截止值应作为预测盐是否可以合成的最小值。
