Theoretical Study On The Chemical Reaction Kinetics Of Alkyl Hydroperoxides

As is widely known, organic peroxides are important atmospheric trace species that serve as temporary reservoirs for HO_x and RO_x radicals. Alkyl hydroperoxides are found to be important intermediates in combustion and low-temperature oxidation processes of hydrocarbons, which are directly involved in both formation and destruction of ozonosphere. Moreover, they can reduce soot formation and promote the burnout process in hydrocarbon combustion, which have aroused wide concern in recent years. Extensive studies on CH3OOH have been previously accomplished theoretically and experimentally, while studies of ethyl hydroperoxide (CH₃CH₂OOH) are relatively limited.In this work, kinetics and mechanisms for unimolecular decomposition of CH₃CH₂OOH have been investigated. Potential energy surface of decomposition reactions have been first predicted at CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level, from which we have obtained fifteen possible reaction pathways, including direct C-C, C-O, C-H, O-H bond dissociation processes and small molecule elimination reactions. The intrinsic reaction coordinate (IRC) calculations were used to confirm the connection between the designated transition states and the reactant or products. The results show that the formation of CH₃CH₂O + OH via the O-O bond direct dissociation is dominant with the lowest dissociation energy.The rate constants for the main product channels have been computed with variable reaction coordinate-transition state theory (VRC-TST) and Rice-Ramsperger-Kassel-Marcus (RRKM) theory based on the PES using the VARIFLEX code. In order to achieve convergence in the integration over the energy range, an energy grain size of 100 cm?1 is used, and the energy span ranges from -13955.2 to 65944.8 cm^-1. We also calculated with two more energy size of 50 and 200 cm^-1, which shows they have little influence on the results. Therefore our option of 100 cm^-1 is reasonable. Reactions with tight transition states are treated with the rigid-rotor harmonic-oscillator (RRHO) assumption. For the barrierless pathways, we employed the multireference methods CASSCF and CASPT2 to scan the Potential Energy Surface (PES), in which the active space was chosen as a bonding orbitalσand an anti-bonding orbitalσ^* for the breaking bond. Moreover, we have evaluated the Morse and Varshni potential fuctions and calculated the rate constants with VTST. The results show that the branching ratio of the reaction CH₃CH₂OOH→CH₃CH₂O + OH is over 99% in the whole temperature range from 300 to 1000 K, and its rate constant can be expressed as k1 = 9.26×10⁵²T^-11.91exp(-26879/T) s^-1 at 1 atm.Moreover, we studied O-O bond decomposition energy of several special hydroperoxides such as branched ones, ketohydroperoxides and so on, which will exhibit structure-reactivity relationships and help to establish more detailed kinetic models in further combustion and oxidation studies.