Theoretical Study Of The Reactions Of OH Radicals With Alkanes Over A Wide Temperature Range

Worldwide, about one third of the fossil energy is from petroleum, which mainly consists of hydrocarbons, including alkanes, cycloalkanes, and aromatic hydrocarbons. Automobile is a main source of fossil energy consumption and pollution emissions, and precise control of the combustion process of gasoline and diesel engines can greatly improve the fuel combustion efficiency, reduce pollution emissions, and so on. Understanding the detailed chemical kinetics of practical fuels is important for improving the fuel efficiency and reducing the pollution emissions of engines. It is still a challenge to develop detailed chemical kinetic models for practical fuels because modeling the chemical kinetics of each fuel component is computationally expensive. One way of reducing the complexity is to group fuel compounds together into representative classes, and construct a compact "surrogate fuel" model that represents the chemical and physical characteristics of the practical fuel. The surrogate model usually includes n-alkanes, branched and iso-alkanes, aromatics, polycyclic alkanes, olefins, naphthenes, and oxygenated hydrocarbons for gasoline and diesel fuel, among which the straight-chain n-alkane species are the most important components.At present, the research methods of the reaction rate constants can be divided into the experimental measurements and theoretical prediction. It is usually difficult to conduct the quantitative experimental measurements of the reaction rates because of the limitations of the experimental technology and operating conditions. With the rapid development of computing technology and theoretical chemistry, it becomes possible to predict reaction rate constant using theoretical methods.In this paper, all the reactions were studied extensively at the BH&HLYP/6-311G (d, p) level with the Gaussian 09 program package. The geometries of all species (reactants, intermediates, transition states, and products) were optimized. All reaction channels were fully investigated with the vibrational mode analysis to confirm the transition states and with electron population analysis to discuss the electron redistribution, and to elucidate the reaction mechanism. The rate constants of the Alkane+OH reaction were calculated by using the improved canonical transition-state theory with small-curvature tunneling correction (ICVT/SCT).Firstly, the primary oxidation and the dominant pathways of a series of alkanes, i.e., hydrogen abstraction reactions from alkanes by hydroxyl radicals (OH+RH? R+H2O), from methane (C1) to n-tetradecane (C14) over the temperature range 298-2000 K were studied. Based on the calculated transition state and its reaction energy, the energetic profiles of these reactions were obtained, so were the activation energy. Then, the ICVT/SCT transition-state theoretical method was used to calculate the rate constants. A brief survey of the NIST Chemical Kinetics Database indicates the lack of the measurements of the rate constants for the reactions of OH with even the smaller alkanes (C3-C5) at high temperatures (>1000 K). Accurate estimations of the rate constants of the abstraction of H-atoms from series alkanes (C1-C14) by OH radicals over a very wide temperature range is crucial for understanding the oxidation mechanism of practical gasoline and diesel.