Multichannel Gas-Phase Unimolecular Decomposition of Acetone: Theoretical Kinetic Studies

The multichannel thermal decomposition of acetone is studied theoretically. The isomerization of acetone molecule to its enol form, 1-propene-2-ol, is of especial interest in this research. Steady-state approximation is applied to the thermally activated species CH₃COCH₃* and CH₂C(CH₃)OH*, and by performing some statistical mechanical manipulations, integral expressions for the rate constants for the formation of different products are derived. The geometries of the reactant, intermediates, transition states, and products of the reaction are optimized at the MP2(full)/6-311++G(2d,2p) level of theory. More accurate energies are evaluated by single-point energy calculations at the CBS-Q, G4, and CCSD(T,full)/augh-cc-pVTZ+2df levels of theory. In order to account correctly for vibrational anharmonicities and tunneling effects, microcanonical rate constants for various channels are computed by using semiclassical transition state theory. It is found that the isomerization of CH₃COCH₃ to the enol form CH₂C(CH₃)OH plays an important role in the unimolecular decomposition reaction of CH₃COCH₃. The possible products originating from unimolecular decomposition of CH₃COCH₃ and CH₂C(CH₃)OH are investigated. It is revealed from present computed rate coefficients that the dominant product channel is the formation of CH₂C(CH₃)OH at low temperatures and high pressures due to the low barrier height for the isomerization process CH₃COCH₃ → CH₂C(CH₃)OH. However, at high temperatures and low pressures, the product channel CH₃ + CH₃CO becomes dominant. Also, the roaming product channels CH₂CO + CH₄ and C₂H₆ + CO could be important at high temperatures.