阿拉伯糖醇自由基β-消除反应的理论研究

Theoretical Study on The β-Elimination Reaction of Arabitol Radical

自由基机理是纤维素老化降解和生物质能热解的重要反应途径,本文采用密度泛函理论方法,在B3LYP/6-311++G**//B3LYP/6-31G**水平,对纤维素降解或裂解初级可能生成的阿拉伯糖醇自由基的分子内氢迁移重排及其氧位自由基的β-消除反应、β-脱水反应以及形成呋喃环分子内取代消除反应的机理进行了理论研究。计算结果表明,糖醇自由基中不同碳位自由基的相对能量接近(<4.5 k J/mol),比氧位自由基相对能量低约38~65 kJ/mol。氧位自由基的β-消除反应有两种方式:α(C-H)键的断裂和α(C-C)键的断裂,前者生成五碳醛糖/酮糖,后者生成低碳醛糖和另一分子低碳糖醇自由基(或羟甲基自由基),但后者所需的活化能(约12~40 kJ/mol)比前者(约100~130 kJ/mol)小的多。而β-脱水反应和形成呋喃环反应所需活化能相当,比β-氢原子消除反应的活化能还大些。因此,在低温下糖醇烷氧自由基α(C-C)键断裂的β-消除反应是纤维素老化自由基降解机理的主要方式,该降解方式可产生羟甲基自由基,其夺取一个氢原子而生成绝缘纸老化的重要信号分子―甲醇。而在高温下则β-氢原子消除、β-脱水和形成呋喃环化合物等反应形成复杂竞争,但脱水反应是这些反应中明显的放热反应。

The H-migration rearrangement of arabitol radical initiated from the degradation of cellulose and the β-elimination, β-dehydration and intramoleclar substitution forming furan ring of its alkoxy-radical was investigated using density functional theory at B3LYP/6-311＋＋G＊＊//B3LYP/6-31G＊＊ level. The calculation results showed that the relative energies of arabitol alkyl radicals were lower than its alkoxyl radicals by ca 38-65 kJ/mol. There were two patterns of the β-elimination of arabitol alkoxy radical： cleavages of ct（C-H） bond and or（C-C） bond. The former pattern elimination yielded five-carbon aldose or ketose and the later pattern produced less-carbon aldose and less-carbon polyol alkyl radical or hydroxymethyl radical. The activation energy of the later elimination pattern （ca. 12-40 kJ/mol） was much lower than that of the former pattern （ca. 100- 130 kJ/mol）. The activation energy of the β-dehydration was close to that of the intramolecular substitution forming furan ring and slightly larger than that of the β-H elimination. At normal or lower temperature, the α（C-C）-bond-cleavage elimination of alkoxy radical was the main reaction path for the radical mechanism of the aging degradation of cellulose, which could produce hydroxymethyl radical capturing a hydrogen atom to yield methanol used as a new indictor of paper ageing. Whereas, at higher temperature, the β-dehydration, the β-H elimination and the formation of furan ring was complex competitive, but only the β-dehydration was explicitly exothermic.