Sugar alcohol dehydration in hot water is an important reaction that allows for environmentally friendly biomass conversion without the use of organic solvents. Here, we report a free-energy analysis by metadynamics (MTD) simulations based on ab initio density functional theory and semiempirical density-functional tight-binding method to understand the mechanism of dehydration reactions of d-sorbitol (SBT) in hot acidic water. Comparing the results of ab initio and semiempirical MTD, it was found that the latter gives a reliable free energy surface of SBT dehydration reaction, although the results vary upon the inclusion of dispersion correction. It was found that the reaction proceeds consistently via an S_N2 mechanism, whereby the free energy of protonation of the hydroxyl group created as an intermediate is affected by the acidic species. This mechanism was further verified by real-time trajectories started from the transition state using ab initio molecular dynamics simulations. The free energy barriers of the reaction pathways leading to five-membered ether products are lower than those leading to six-membered ether products, in agreement with experiment. This outcome can be ascribed, in part, to our finding that the reaction barrier of the pathway is correlated to the stability of the SBT confined conformation at the initial stage of the reaction.

中文翻译：

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山梨糖醇在热酸性溶液中脱水的机理

糖醇在热水中脱水是一个重要的反应，可以在不使用有机溶剂的情况下进行环境友好的生物质转化。在这里，我们报告了基于从头算密度泛函理论和半经验密度泛函紧束缚方法的元动力学 (MTD) 模拟进行的自由能分析，以了解d-山梨糖醇 (SBT) 在酸性水中的脱水反应机理。比较 ab initio 和半经验 MTD 的结果，发现后者提供了可靠的 SBT 脱水反应的自由能表面，尽管结果因包含色散校正而异。发现反应通过 S _N一致地进行2 机制，其中作为中间体产生的羟基的质子化自由能受酸性物质的影响。这种机制通过使用 ab initio 分子动力学模拟从过渡态开始的实时轨迹得到进一步验证。与实验一致，生成五元醚产物的反应途径的自由能垒低于生成六元醚产物的反应途径。这一结果可以部分归因于我们的发现，即该途径的反应屏障与反应初始阶段 SBT 受限构象的稳定性相关。