Excitation of Chemiluminescence as a Reversal of Radiationless Transition: Semiempirical Simulation of Decomposition of Peroxides

Excitation of chemiluminescence is treated as a reversal of radiationless transition. The thermolysis of six 1,2-dioxetanes was examined by the semiempirical PM3 method. The major contribution to the reaction coordinate is made first by the stretching of the O–O bond and then by the cleavage of the C–C and O–O bonds. The energy profile is characterized by a shallow well (4–12 kJ/mol) in the region of the biradicaloid transition state. Here, the potential energy surfaces of the ground state and the triplet state are close to one other, and according to the Hund rule, the state T₁ lies lower than S₀. The well prolongs the lifetime . Therefore, the yield of chemiexcitation of the triplet product k _isc may be sufficiently high even at a low value of the rate constant k _isc of the spin-forbidden transition from S₀ to T₁. Methodological problems and the specifics of a quantum-chemical computer experiment (reliability of the Dewar semiempirical methods, reproducibility, and systematic and random errors) are considered.