Combined Ab Initio, Kinetic Modeling, and Shock Tube Study of the Thermal Decomposition of Ethyl Formate

Abstract

The potential energy surfaces (PESs) and reaction rate constants of the unimolecular decomposition of ethyl formate (EF) were investigated using high-precision theoretical methods at the CCSD(T)/CBS(T-Q)//M06-2X/6-311++G(d,p) level of theory. The calculated PESs of EF dissociation and molecular decomposition reactions indicate that the intramolecular H-shift to produce formic acid and ethylene is the dominant decomposition pathway. A detailed chemical kinetic mechanism for EF pyrolysis was constructed by incorporating the important reactions of EF and its radicals into an existing mechanism previously developed for small methyl esters. The updated mechanism was first used to reproduce CO, CO, and HO concentration time histories during EF pyrolysis in the shock tube reported by Ren et al. [ Ren , W. , Mitchell Spearrin , R. , Davidson , D. F. , and Hanson , R. K. J. Phys. Chem. A 2014 , 118 , 1785 - 1798 ]. The rate of production and sensitivity analyses show that the competing dehydration and decarboxylation channels of the intermediate formic acid control the final product yields of EF pyrolysis. The EF mechanism was further validated against the shock tube data of OH, CO, CO, and HO time histories measured during EF oxidation (equivalence ratio Φ = 1.0) at 1331-1615 K and 1.52-1.74 atm. This revised EF mechanism captured all of the species' time histories over the entire temperature range. Such modeling capability was due to the more accurate rate constants of EF reactions determined by high-precision theoretical calculations and a high-fidelity C-C basis mechanism.

Ethyl lactate: a sinister molecule exhibiting high chemical diversity with potential as a “green” solvent

“…This value was then used to calculate kJ mol −1 from isodesmic reaction (1). Note that a Thermo MultiWell calculation for ethyl formate using scaled M06‐2X/6‐311++G(d,p) frequencies, rotational constants, and treating three vibrational modes as hindered rotors translates our 0 K value to kJ mol −1 in very good agreement with Ning et al 30 …”

“…Unfortunately, the experimental literature is conflicted 28,29 over the correct value for ethyl formate with numbers of −398 and −361.7 kJ mol −1 (these apply at 298.15 K but similar confusion reigns at 0 K). A recent theoretical calculation 30 at CCSD(T)/CBS(T‐Q)//M06‐2X/6‐311++G(d,p) does not resolve the situation reporting −390.4 kJ mol −1 at 298.15 K.…”

“…One of the six scans is problematic-namely, rotation about the OCCO bond, which corresponds to the lowest frequency mode and hence makes the largest individual contribution, 21%, to the vibrational entropy which in turn comprises 29% of the total entropy. 30 A formation enthalpy obtained directly from multicomposite atomization calculations is useful, in the sense that a measure of uncertainty can be gleaned which relates directly to ethyl lactate itself and is not imported from elsewhere, and in those situations where suitable chaperons do not exist; here, averaging the results yields −606.1 ± 6.2 kJ mol −1 . This is in good agreement with the recent, more advanced, composite method, WMS, 20 which yields −600.2 ± 1.5 kJ mol −1 where the uncertainty here represents the impact that using two different geometries, namely M06-2X/6-311++G(d,p) and B3LYP/cc-pVTZ+d has on the computed atomization energy of ethyl lactate.…”

“…Unfortunately, the experimental literature is conflicted28,29 over the correct value for ethyl formate with numbers of −398 and −361.7 kJ mol −1 (these apply at 298.15 K but similar confusion reigns at 0 K). A recent theoretical calculation30 atCCSD(T)/CBS(T-Q)//M06-2X/6-311++G(d,p) does not resolve the situation reporting −390.4 kJ mol −1 at 298.15 K. Hence we have determined ethyl formate de novo via the isodesmic: HC(O)OC 2 H 5 + CH 3 OH = HC(O)OCH 3 + C 2 H 5 OH (2) featuring reference 26 values for methanol (−189.95 ± 0.15) and ethanol (−216.92 ± 0.21) and a corrected value27 for methyl formate of −347.0 ± 1.5, which supplants the Active Thermochemical Tables (ATcT) of −344.73 ± 0.59) which in conjunction with a computed Δ of 0.35 ± 0.29 kJ mol −1 , pleasingly near thermoneutrality, yields a formation enthalpy of −374.0 ± 0.7 kJ mol −1 . This value was then used to calculate Δ • − (0 K) = −600.2 ± 2.2 kJ mol −1 from isodesmic reaction(1).…”

Infrared Emission from Photodissociation of Methyl Formate [HC(O)OCH3] at 248 and 193 nm: Absence of Roaming Signature

“…The parent molecule cis -HC­(O)­OCH 3 can also sever its C–O or C–H bond directly to form radical products CH 3 + HC­(O)­O, H + CH 3 OCO, H + HC­(O)­OCH 2 , and HCO + CH 3 O, similar to those for direct bond fission of HC­(O)­OCH 2 CH 3 to form HC­(O)O + C 2 H 5 , H + C 2 H 5 OCO, H + HC­(O)­OCHCH 3 , and HCO + C 2 H 5 O, as reported by Ning et al These radical channels are summarized below: …”