Multistructural Anharmonicity Controls the Radical Generation Process in Biofuel Combustion

Abstract

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.

Simplified Protocol for the Calculation of Multiconformer Transition State Theory Rate Constants Applied to Tropospheric OH-Initiated Oxidation Reactions

“…The choice of DFT is motivated by its favorable compromise between accuracy and computational cost, a property of utmost importance in the study of reaction , which includes large molecular systems with many conformers. ,,,, Specifically, it is well-known that the accuracy of the barrier heights for hydrogen abstraction will depend on the choice of the exchange–correlation functional. The Minnesota family of functionals are known to have a good performance in describing the reactivity of VOCs. ,,,− ,,,− Although in this particular context M06-2X is generally used more often than M08-HX, the latter functional has recently been employed in accurate MS-VTST and MP-VTST rate constant calculations ,,− of atmospheric reactions. M08-HX has mean unsigned errors (MUEs) of 1.00 and 0.73 kcal mol –1 for the DBH76 and HTBH38 data sets involving hydrogen-transfer barrier heights, respectively .…”

“…In addition, for reactions at atmospheric temperatures, it is normally found that accounting for all conformers explicitly by summing their individual contributions, even in a simple RR-HO approximation, recovers most of the nonharmonicity introduced by the internal rotation, since the molecules will reside mostly in the thermally weighted lower-energy harmonic wells . Moreover, variational and recrossing effects may also have an important contribution, as they are known to decrease the rate contants sometimes significantly . In contrast, the simple and inexpensive one-dimensional Eckart tunneling correction is frequently able to reproduce expensive and robust multidimensional transmission coefficients, such as the small curvature tunneling (SCT) correction, at a very low computational cost.…”

“…40 Moreover, variational and recrossing effects may also have an important contribution, as they are known to decrease the rate contants sometimes significantly. 108 In contrast, the simple and inexpensive one-dimensional Eckart tunneling correction is frequently able to reproduce expensive and robust multidimensional transmission coefficients, such as the small curvature tunneling (SCT) correction, 139 at a very low computational cost. This often happens due to a favorable error cancellation in the Eckart approximation: the underestimation of the tunneling probability by neglecting cornercutting 140−142 is compensated by an overestimation of the tunneling probability caused by the rigidity of the Eckart function, which leads to a potential energy curve narrower than the actual PES.…”

Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory

“…Including these effects in ME calculations requires considerable extra effort . Multidimensional tunneling methods can be crucial to accurately describe certain chemical reactions even at relatively high temperatures (∼700 K), , and multistructural anharmonicity can be pronounced in reactions involving complex molecules in the high-temperature regime; , these effects can be efficiently incorporated in low-pressure rate constant calculations by the SS-QRRK theory, , justifying the need for further improvements of this methodology.…”

Predictive Combustion Kinetics of OH Radical Reactions with a C5 Unsaturated Alcohol: The Competitive H-Abstraction and OH-Addition Reactions of 2-Methyl-3-buten-2-ol

“…Although direct kinetic measurements are difficult under the elevated T – P conditions, the theoretical calculation has proven to be an alternative approach to predict the combustion kinetics in a wide range of temperatures and pressures. − A few theoretical studies on the MBO232 + OH reaction have been carried out. Baasandorj and Stevens investigated the stability of the two initial adducts of the MBO232 + OH reaction at the B3LYP/6-31G­(d,p) and B3LYP/6-311+G­(d) levels.…”