Accurate bond energies of biodiesel methyl esters from multireference averaged coupled-pair functional calculations.
Accurate bond energies of biodiesel methyl esters from multireference averaged coupled-pair functional calculations.
复制标题
DOI：
10.1021/jp412727w
复制
发表时间：
2014-03
期刊：
影响因子：
--
通讯作者：
Victor B. Oyeyemi;J. Keith;E. Carter
中科院分区：
文献类型：
--
作者：
Victor B. Oyeyemi;J. Keith;E. Carter
精确的键离解能（BDE）对于表征燃烧化学，特别是热解的初始阶段是重要的。在这里，我们有助于评估生物柴油甲酯分子的热化学使用从头算BDE来自多参考平均耦合对功能（MRACPF 2）为基础的计划。在先前对烃和各种含氧化合物的这种方法进行了验证之后，我们在此对羧酸和甲酯中的键进行了进一步的验证，发现我们的方案在化学准确度范围内预测了BDE（即，1 kcal/mol）。然后通过巴豆酸甲酯分析甲酸甲酯的溴化二苯醚趋势与酯大小。我们发现，酯部分中的羰基基团对溴化二苯醚只有局部影响。发现酯烷基链中的C → C双键增加邻近双键的键的强度。一个重要的例外是C C或C O键的β键，它们在解离时产生烯丙基样自由基。所观察到的趋势产生不同程度的几何松弛和共振稳定的自由基。我们还计算了各种小烷烃和烯烃中的溴化二苯醚，作为实际生物柴油甲酯长烃链的模型。我们再次表明，烯丙基键的烯烃是弱得多的小甲酯，表明氢的提取更可能在烯丙基网站，甚至更可能在双烯丙基网站的烷基链由于更多的电子参与π-共振。最后，我们使用的溴化二苯醚在小的替代品，以估计迄今未知的溴化二苯醚在大型甲酯的生物柴油燃料。
Accurate bond dissociation energies (BDEs) are important for characterizing combustion chemistry, particularly the initial stages of pyrolysis. Here we contribute to evaluating the thermochemistry of biodiesel methyl ester molecules using ab initio BDEs derived from a multireference averaged coupled-pair functional (MRACPF2)-based scheme. Having previously validated this approach for hydrocarbons and a variety of oxygenates, herein we provide further validation for bonds within carboxylic acids and methyl esters, finding our scheme predicts BDEs within chemical accuracy (i.e., within 1 kcal/mol) for these molecules. Insights into BDE trends with ester size are then analyzed for methyl formate through methyl crotonate. We find that the carbonyl group in the ester moiety has only a local effect on BDEs. C═C double bonds in ester alkyl chains are found to increase the strengths of bonds adjacent to the double bond. An important exception are bonds beta to C═C or C═O bonds, which produce allylic-like radicals upon dissociation. The observed trends arise from different degrees of geometric relaxation and resonance stabilization in the radicals produced. We also compute BDEs in various small alkanes and alkenes as models for the long hydrocarbon chain of actual biodiesel methyl esters. We again show that allylic bonds in the alkenes are much weaker than those in the small methyl esters, indicating that hydrogen abstractions are more likely at the allylic site and even more likely at bis-allylic sites of alkyl chains due to more electrons involved in π-resonance in the latter. Lastly, we use the BDEs in small surrogates to estimate heretofore unknown BDEs in large methyl esters of biodiesel fuels.