Asatryan Rubik, Hudzik Jason M, Bozzelli Joseph W, Khachatryan Lavrent, Ruckenstein Eli
Department of Chemical and Biological Engineering , University at Buffalo, The State University of New York , Buffalo , New York 14226 , United States.
Department of Chemistry and Environmental Science , New Jersey Institute of Technology , Newark , New Jersey 07102 , United States.
J Phys Chem A. 2019 Apr 4;123(13):2570-2585. doi: 10.1021/acs.jpca.9b00185. Epub 2019 Mar 20.
Cinnamyl alcohols such as p-coumaryl alcohol ( p-CMA) are lignin models and precursors (monolignols) and the most important primary products of lignin pyrolysis. However, the detection of monomers is not straightforward since they either undergo secondary transformations or repolymerize to contribute to the char formation. Both concerted-molecular and free-radical pathways are involved in these processes. Our recent fundamentally based theoretical and low-temperature matrix-isolation-EPR studies of cinnamyl alcohols highlighted the role of side-chain reactivity in diversity of pyrolysis products and provided a network of the chemically activated H + p-CMA reactions ( Asatryan J. Phys. Chem. A, 2017 , 121 , 3352 - 3371 ). The readily available hydroxyl radicals also can trigger a cascade of free-radical processes. Here, we present a comprehensive potential energy surface (PES) analysis of the OH + p-CMA reaction using various DFT and ab initio protocols. Since the p-CMA involves both an alkyl OH-group and a side-chain double bond, the title reaction can also serve as a relevant model for reactions of unsaturated alcohols with hydroxyl radicals to form various oxygenates including polyhydric alcohols which are abundant in nature. The newly identified pathways suggest certain alternatives to the known radical reactions. Of particular interest are the roaming-like low-energy dehydration reactions to generate a variety of O- and C-centered intermediate radicals, which are primarily transformed into the phenolic compounds observed in pyrolysis experiments. Several concerted unimolecular decomposition pathways for p-CMA are also revealed, not considered previously, such as the migration of terminal OH-group, and/or its splitting over the ipso-C and ortho-C atoms of the benzene ring to form bicyclic oxispiro- and chromene compounds represented in natural lignin.
肉桂醇，如对香豆醇（p-CMA），是木质素模型和前体（单木质醇），也是木质素热解最重要的初级产物。然而，单体的检测并非易事，因为它们要么经历二次转化，要么重新聚合以促进焦炭的形成。这些过程涉及协同分子途径和自由基途径。我们最近基于基础理论以及对肉桂醇进行的低温基质隔离-电子顺磁共振研究，突出了侧链反应性在热解产物多样性中的作用，并提供了化学活化的H + p-CMA反应网络（阿萨特良，《物理化学杂志A》，2017年，121卷，3352 - 3371页）。容易获得的羟基自由基也可以引发一系列自由基过程。在此，我们使用各种密度泛函理论（DFT）和从头算方法，对OH + p-CMA反应进行了全面的势能面（PES）分析。由于p-CMA既包含烷基羟基又包含侧链双键，该反应也可作为不饱和醇与羟基自由基反应形成各种含氧化合物（包括自然界中丰富的多元醇）的相关模型。新发现的途径为已知的自由基反应提供了某些替代方案。特别有趣的是类似漫游的低能脱水反应，可生成各种以氧和碳为中心的中间自由基，这些自由基主要转化为热解实验中观察到的酚类化合物。还揭示了几种之前未考虑过的p-CMA的协同单分子分解途径，例如末端羟基的迁移，和/或其在苯环的本位碳和邻位碳上的分裂，以形成天然木质素中存在的双环氧化螺和色烯化合物。
