GAS-PHASE THERMOLYSIS OF PYRAZOLINES .2. ELECTRONIC-STRUCTURE AND GAS-PHASE THERMOLYSIS OF 4,5-DIHYDRO-3H-PYRAZOLES STUDIED BY PHOTOELECTRON-SPECTROSCOPY, SEMIEMPIRICAL QUANTUM-CHEMICAL CALCULATIONS, AND FLASHVACUUM PYROLYSIS

The PE spectra of the 4,5-dihydro-3H-pyrazoles 1, 2, of 2,5-dihydro-1, 3,4-oxadiazole 3, and 2,4-dihydro-1,3,4-thiadiazole 4 have been record ed. Based on HAM/3, MNDO, AM1, and PM3 calculations, the ionization po tentials have been assigned to molecular orbitals. The gas-phase therm olyses of 1-4 have been studied by PE-controlled gas analysis. Extrusi on of molecular nitrogen leads to reactive species which cyclize to th ree-membered rings of different stability. At higher temperatures and in flash vacuum pyrolysis, consecutive reactions may lead to smaller a cyclic molecules. The cyclopropanol 7, obtained by denitrogenation of 2, is thermally rather stable. Its PE spectrum has been recorded and a nalyzed.