Thermal unimolecular decomposition of acetic anhydride in the gas phase

The thermal decomposition of acetic anhydride has been investigated behind incident and reflected shock waves at temperatures between 750 K and 980 K. The decomposition was monitored by vacuum-UV absorption and IR emission. We confirmed that the decomposition proceeds to yield ketene and acetic acid with the first order reaction rate. Arrhenius parameters for the rate constant were log (A/s -1 ) = 11.8 and E a 138 kJ/mol in our temperature region. Since the rate data evaluated in this work coincided with an extrapolation of the previous data, an Arrhenius expression which covers a wide temperature range from 470K to 980K was re-evaluated as k = 10 12,2 exp(-145kJ/mol -1 /RT) s -1 An ab initio MO calculation showed that the most probable reaction path is the concerted elimination through a 6-center transition state. The potential barrier height for this path was evaluated to be 156 kJ/mol at the MP4(SDTQ) level.