Theoretical Investigation on the Gas Phase Thermolysis Mechanism and Rate Constant of Acetic Anhydride

The gas-phase thermolpsis mechanism of acetic anhydride (CH₃C(O)OC(O)CH₃) has been investigated by quantum chemistry methods at AM1 and HF/6-31G(d, p) levels of theory. A concerted mechanism with a six-membered cyclic transition state was found at these levels of theory. The refined barrier at MP2/6-31G(d, p)//HF/6-31G (d, p) level of theory is about 180kJ/mol.Microcanonical transition state theory has been used to calculate rate constants for the reaction. The calculated rate constants are in good agreement with experimental value. The activation entropy is important for calculating the rate constants.