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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Accurate Bond Energies of Biodiesel Methyl Esters from Multireference Averaged Coupled-Pair Functional Calculations
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Accurate Bond Energies of Biodiesel Methyl Esters from Multireference Averaged Coupled-Pair Functional Calculations

机译：从多参考平均偶对功能计算中准确估算生物柴油甲基酯的键能

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摘要

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.
机译：准确的键解离能（BDE）对于表征燃烧化学，特别是热解的初始阶段很重要。在这里，我们使用从头算的BDE来评估生物柴油甲酯分子的热化学作用，这些BDE源自基于多参考平均偶合对官能团（MRACPF2）的方案。先前已经针对烃和各种含氧化合物验证了该方法，在此我们对羧酸和甲酯中的键进行了进一步验证，发现我们的方案预测这些分子的BDE的化学准确度（即1 kcal / mol以内）。然后，通过巴豆酸甲酯分析甲酸甲酯的BDE趋势。我们发现，酯部分中的羰基仅对BDE具有局部作用。发现酯烷基链中的C ＝ C双键增加了与双键相邻的键的强度。一个重要的例外是β= C = C或C = O键的键，它们在解离时会产生类似烯丙基的自由基。观察到的趋势来自所产生自由基中不同程度的几何弛豫和共振稳定。我们还计算了各种小烷烃和烯烃中的BDE，作为实际生物柴油甲酯的长烃链的模型。我们再次表明，烯烃中的烯丙基键比小甲酯中的烯丙基键弱得多，这表明由于更多的π电子参与，氢在烷基链的烯丙基位点更可能发生夺氢，甚至在烷基的双烯丙基位点更可能夺氢。共振。最后，我们使用小替代物中的BDE来估算生物柴油燃料大甲酯中迄今未知的BDE。

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