thermal reaction paths in the valenceisomerization 2,7-dihydro-2,2,7,4-tetra-methylpyrene (1)⇄2,2,7,7-tetramethyldicyclopropa [a,g]pyracene (2) were traced by flash photolysis, optical and ESR spectroscopy, thermal reaction kinetics, and oxygen trapping experiments. The heat evolved in the reaction 2→1 was determined by differentialscanning and photoacoustic calorimetry. All reactions connecting 1