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

Abstract

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.