An ab Initio/rice-ramsperger-kassel-marcus study of the reactions of propenols with OH. mechanism and kinetics of H abstraction channels

Abstract

Propenols have been found to be common intermediates in the hydrocarbon combustion and they are present in substantial concentrations in a wide range of flames. However, the kinetics properties of these species in combustion flames have not received much attention. In this work, the mechanism and kinetics of the OH hydrogen abstraction from propenols are investigated. Three stable conformations of propenols, (E)-1-propenol, (Z)-1-propenol, and syn-propen-2-ol, are taken into consideration. The potential energy profiles for the three reaction systems have been first investigated by the CCSD(T) method. The geometric parameters and relative energies of the reactants, reactant complexes, transition states, product complexes, and products have been investigated theoretically. The rate constants are calculated in the temperature range of 200-3000 K by the Variflex code based on the weak collision master equation/microcanonical variational RRKM theory. For all considered reactions, our results support a stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel. In the reaction of OH with (£)-l-propenol, the hydrogen abstractions from the -CH3 and -OH sites are dominant and competitive with each other in the temperature range from 500 to 2000 K. Above 2000 K, the hydrogen abstraction from the -CH group bonded to O atom becomes dominant with a relative yield of 51.1% at 3000 K. In the reaction of OH with (Z)-1-propenol, the hydrogen abstractions from -CH3, -CH bonded to O atom, and -OH are preferable in the temperature range from 500 to 1800 K, with the first two channels being competitive with each other. Above 1800 K, the hydrogen abstraction reaction from the CH group bonded to the CH3 group becomes dominant with the branching ratio of 90.3% at 3000 K. In the reaction of OH with syn-propen-2-ol, the abstractions from the -CH3 and -OH sites are competitive with each other when the temperature is higher than 500 K, and they become dominant above 800 K with the relative yields of 70.5% and 29.5% at 3000 K, respectively. The predicted total rate constants at the pressure of 1 atm fitted by modified three-parameter Arrhenius expressions in two different temperature ranges are also provided. © 2009 American Chemical Society.