Multipath VTST rate constants for D + methyl formate reactions: Importance of torsional anharmonicity and conformational flexibility for combustion chemistry

Pyrolysis of Methyl Formate and the Reaction of Methyl Formate with H Atoms: Shock Tube Experiments and Statistical Rate Theory

Methyl formate (MF) is the smallest carboxylic ester and currently considered a promising alternative fuel. It can also serve as a model compound to study the combustion chemistry of the ester group, which is a typical structural feature in many biodiesel components. In the present work, the pyrolysis of MF was investigated behind reflected shock waves at temperatures between 1430 and 2070 K at a nominal pressure of 1.1 bar. Both time-resolved hydrogen atom resonance absorption spectroscopy (H-ARAS) and time-resolved time-of-flight mass spectrometry (TOF-MS) were used for species detection. Additionally, the reaction of MF and perdeuterated MF-d₄ with H atoms was investigated at temperatures between 1000 and 1300 K at nominal pressures of 0.4 and 1.1 bar with H-ARAS. In the latter experiments, ethyl iodide served as precursor for H atoms. Rate coefficients of seven parallel unimolecular decomposition channels of MF and five parallel reaction channels of the MF + H reaction were calculated from statistical rate theory on the basis of molecular and transition state data from quantum chemical calculations. These calculated rate coefficients were implemented into an MF pyrolysis/oxidation mechanism from the literature, and the experimental concentration-time profiles of H (from ARAS) as well as MF, CH₃OH, HCHO, and CO (from TOF-MS) were modeled. It turned out that the literature mechanism, which was originally validated against flow-reactor experiments, ignition delay times, and laminar burning velocities, was generally able to fit also the concentration-time profiles from the shock tube experiments reasonably well. The agreement could still be improved by substituting the original rate coefficients, which were estimated from structure-reactivity relationships, by the values calculated from statistical rate theory in the present work. Details of the channel branching are discussed, and the updated mechanism is given, also in machine-readable form.

Identification of Torsional Modes in Complex Molecules Using Redundant Internal Coordinates: The Multistructural Method with Torsional Anharmonicity with a Coupled Torsional Potential and Delocalized Torsions

Identification of internal-rotation modes in the normal-mode analysis of complex molecules is important for accurately describing the thermodynamic properties and kinetics of complex molecules when it is necessary to treat the anharmonicity of torsions and the multiconformer anharmonicity caused by the internal rotations. However, identifying and distinguishing torsional modes are very challenging because they are coupled to one another. In this work, we present a new strategy to automatically identify torsional vibrations and separate them from the other vibrational modes. By combining a redundant-internal-coordinate auto-generation procedure with torsional projection techniques, we automate the procedure of identifying and separating the coupled torsions, and we show that we can obtain robust and consistent results with various reasonable definitions of redundant-internal-coordinate sets. This model has been implemented in a new development version of the MSTor program to reduce the user input needed for multistructural and torsional anharmonicity (MS-T) calculations. The new method is called multistructural and torsional anharmonicity with a coupled torsional potential and delocalized torsions ([MS-T(CD)]. As example applications, we consider MS-T(CD) calculations on three molecules (2-hexyl radical, n-propylbenzene, and 5-hydroperoxy-6-oxohexanoylperoxy radical) that have multiple rotors and that provide challenges to choosing good sets of nonredundant-internal coordinates, and we compare the performance of the new strategy to five other torsion identification methods. The new strategy is demonstrated to be efficient in separating the torsional and nontorsional elements in the Hessian matrix, as well as in providing reasonable projected nontorsional frequencies to be used for calculations of partition function and thermochemistry.

Variational transition state theory rate constants and H/D kinetic isotope effects for CH3 + CH3 OCOH reactions

The rate constants and H/D kinetic isotope effect for hydrogen abstraction reactions involving isotopomers of methyl formate by methyl radical are computed employing methods of the variational transition state theory (VTST) with multidimensional tunneling corrections. The energy paths were built with a dual-level method using the moller plesset second-order perturbation theory (MP2) method as the low-level and complete basis set (CBS) extrapolation as the high-level energy method. Benchmark calculations with the CBS_D-T approach give an enthalpy of reaction at 0 K for R1 (-4.5 kcal/mol) and R2 (-4.2 kcal/mol) which are in good agreement with the experiment, that is, -4.0 and - 4.8 kcal/mol. For the reactional paths involving the isotopomers CH₃ + CH₃ OCOH → CH₄ + CH₃ OCO and CH₃ + CH₃ OCOD → CH₃ D + CH₃ OCO, the value of k^H /k^D (T = 455 K) using the canonical VTST/small-curvature tunneling approximation method is 6.7 in close agreement with experimental value (6.2). © 2019 Wiley Periodicals, Inc.

Multistructural Anharmonicity Controls the Radical Generation Process in Biofuel Combustion

The OH radical plays an important role in combustion, and isopentanol (3-methylbutan-1-ol) is a promising sustainable fuel additive and second-generation biofuel. The abstractions of H atoms from fuel molecules are key initiation steps for chain branching in combustion chemistry. In comparison with the more frequently studied ethanol, isopentanol has a longer carbon chain that allows a greater number of products, and experimental work is unavailable for the branching fractions to the various products. However, the site-dependent kinetics of isopentanol with OH radicals are usually experimentally unavailable. Alcohol oxidation by OH is also important in the atmosphere, and in the present study we calculate the rate constants and branching fractions of the hydrogen abstraction reaction of isopentanol by OH radical in a broad temperature range of 298-2400 K, covering temperatures important for atmospheric chemistry and those important for combustion. The calculations are done by multipath variational transition state theory (MP-VTST). With a combination of electronic structure calculations, we determine previously missing thermochemical data. With MP-VTST, a multidimensional tunneling approximation, multiple-structure anharmonicity, and torsional potential anharmonicity, we carried out more realistic rate constant calculations than can be computed by conventional single-structure harmonic transition state theory or by the empirical relations that are currently used in atmospheric and combustion modeling. The roles of various factors in determining the rates are elucidated, and we show that recrossing, tunneling, and multiple structures are all essential for accurate work. We conclude that the multiple structure anharmonicity is the most important correction to conventional transition state theory for this reaction, although recrossing effects and tunneling are by no means insignificant and the tunneling depends significantly on the path. The thermodynamic and kinetics data determined in this work are indispensable for the gas-phase degradation of alcohols in the atmosphere and for the detailed understanding and prediction of ignition mechanisms of biofuels in combustion.