Computational kinetics of the hydrogen abstraction reactions of n-propanol and iso-propanol by OH radical†
Propanol (n-propanol or iso-propanol (i-propanol)) is a promising clean-burning oxygenated fuel component and fuel additive. Understanding its reactions with OH radical is of great significance in both combustion and atmospheric chemistry. In this work, we calculate the rate constants and branching ratios of the hydrogen abstraction reactions of n-propanol and i-propanol by OH radical in a broad temperature range of 63–2000 K using the competitive canonical unified statistical (CCUS) model. For both n-propanol and i-propanol, in both the high-pressure and low-pressure limits, the total reaction rate constants show a significant negative dependence on temperature in the low temperature regime and approach the capture rate for the formation of the pre-reactive complex when temperature is down to the ultracold regime. Several factors, tunneling, remarkable anharmonicity of high-frequency modes of transition states, and the presence of reaction channels with a negative free energy barrier, contribute to this phenomenon. We observe pressure-dependent branching fractions at T < ∼400 K for n-propanol or T < 200 K for i-propanol, and at higher temperatures, the branching fractions are independent of the pressure. The alpha-hydrogen (Hα) abstraction with a lower barrier is not always dominant as traditionally expected. The H-abstraction from the terminal carbon (Ht) of i-propanol, with a higher barrier, is dominant above 1000 K because of the remarkably larger effect of multi-structural and torsional (MS-T) anharmonicity. In the pressure-dependent ultra-low temperature regime and high-pressure limit, the beta-hydrogen (Hβ) abstraction and the hydrogen abstraction from the hydroxyl group (HO) become dominant for n-propanol and i-propanol, respectively, mainly due to the tunneling effect.