The thermal and photochemical decomposition of maleic anhydride in the gas phase

Abstract

The thermal decomposition of maleic anhydride has been studied in the gas phase in a static system at temperatures from 645 to 760 K and pressures from 0.7 to 20 Torr. The first-order rate constant for the homogeneous unimolecular reaction,[Formula: see text]is described by the Arrhenius parameters log A (s−1) = 14.33 (±0.3), and E = 60.9 (± 1) kcal/mol. The reaction appears to proceed through a concerted mechanism rather than a biradical one.The photochemical decomposition, studied at wavelengths from 220 to 350 nm, yielded the same products. At 300 nm and below, the decomposition was unaffected by pressure, but at longer wavelengths collisional quenching was observed. Weak light emission was observed on excitation between 350 and 380 nm. The absorption spectrum was measured from 250 to 400 nm, and three overlapping transitions, π*←π, π*←n+, and π*←n−, can be distinguished. The mechanism of the photolysis is discussed and it is concluded that it probably proceeds through internal conversion to a vibrationally excited ground state.