Accurate Bond Energies of Biodiesel Methyl Esters from Multireference Averaged Coupled-Pair Functional Calculations

Abstract

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.

Mechanism and kinetics of low-temperature oxidation of a biodiesel surrogate−methyl acetate radicals with molecular oxygen

“…[21,37] which are too many to include this study; therefore, we limit ourselves on the main fragments having the same carbon-oxygen backbone of MA due to the breaking of C-H bond, namely CH 3-C(=O)OC•H 2 and • CH 2 C(=O)OCH 3 . These two radicals can isomerize to each other through the hydrogen migration reactions via five-membered ring transition states (cf.…”

Bond Dissociation Energies of C10and C18Methyl Esters from Local Multireference Averaged-Coupled Pair Functional Theory

“…Vinylic C−H bonds α to CC have high BDEs of 108.8−109.9 kcal/mol because the resulting radical is sp 2 hybridized and the geometry does not relax much. 37 Allylic C−H bonds β to CC have dramatically lower BDEs of 84.0− 87.7 kcal/mol when only one CC is involved, and ∼74 kcal/ mol (in cis-methyl-4,7-decadienoate) or ∼79 kcal/mol (in transmethyl-4,7-decadienoate) for bis-allylic C−H bonds due to resonance stabilization of the resulting radicals. Oxyallylic C−H bonds β to the carbonyl groups also produce resonancestabilized radicals, which weaken the incipient bonds, leading to BDEs of 91.7−92.5 kcal/mol−about 8 kcal/mol lower than energies of other secondary C−H bonds.…”

Dissociation of methyl formate (HCOOCH3) molecules upon low-energy electron attachment

“…Methyl formate contains an ester group and therefore is important in chemical evolution of molecules of organic acids, amines and other compounds. Moreover, as a small ester, HCOOCH 3 is also a model compound for large biodiesel esters (Oyeyemi et al 2014). Ions formed from methyl formate can also be reagents in the formation of larger molecules.…”