Esterification of citric acid (CA) with the primary alcohols and the hydroxyl groups of cellulose chain (n = 1-2) in parched condition were investigated by using density functional theory (DFT) method and a two-layer ONIOM approach. Geometry and energy of reactants, products, and transition state (TS) structures were optimized at B3LYP/6-311g (d, p) level and ONIOM (B3LYP/6-311g (d, p):PM3MM) level. The computational results show that the esterification occurs in the two main steps: the first step is the dehydration reaction of CA to form anhydrides of 5-membered ring and 6-membered ring and the second step is the ring opening reaction with the hydroxyl –OH groups to form the ester products. The energy barrier of dehydration reaction step is much higher than that of ring opening reaction step. Effect of substituent R in primary alcohol R-CH2OH (R: CH=CH2, CH2NHCH3, CH2OCH3, CH2Cl) and cellulose chain (1G, 2G) on the reactivity, which has negative inductive effect –I, is significant. The combination of calculation data and experiment data were applied to make findings more rigorous. The activation energy of CA was determined by using differential scanning calorimetry (DSC) and thermal gravimetric (TG) analysis to be = 47.8 kcal/mol; the experimental data favoured the dehydration reaction step of CA.

中文翻译：

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柠檬酸与纤维素链（n = 1-2）的伯醇和羟基在干燥条件下酯化的理论和实验研究

使用密度泛函理论 (DFT) 方法和两层 ONIOM 方法研究了在干燥条件下柠檬酸 (CA) 与伯醇和纤维素链的羟基 (n = 1-2) 的酯化。反应物、产物和过渡态 (TS) 结构的几何形状和能量在 B3LYP/6-311g (d, p) 水平和 ONIOM (B3LYP/6-311g (d, p):PM3MM) 水平上进行了优化。计算结果表明，酯化发生在两个主要步骤中：第一步是CA脱水反应生成5元环和6元环酸酐，第二步是与羟基-OH的开环反应基团形成酯产物。脱水反应步骤的能垒远高于开环反应步骤。伯醇R-CH2OH中取代基R的影响（R：CH=CH2, CH2NHCH3, CH2OCH3, CH2Cl) 和纤维素链(1G, 2G) 对反应性有显着的负诱导效应-I，是显着的。计算数据和实验数据相结合，使研究结果更加严谨。用差示扫描量热法（DSC）和热重法（TG）分析确定CA的活化能为=47.8 kcal/mol；实验数据有利于CA的脱水反应步骤。