Gas‐Phase Thermolysis of Pyrazolines, 2[1]. Electronic Structure and Gas‐Phase Thermolysis of 4,5‐Dihydro‐3H‐pyrazoles Studied by Photoelectron Spectroscopy, Semiempirical Quantum‐Chemical Calculations, and Flash Vacuum Pyrolysis

The PE spectra of the 4,5‐dihydro‐3 H ‐pyrazoles 1, 2 , of 2,5‐dihydro‐1,3,4‐oxadiazole 3 , and 2,5‐dihydro‐1,3,4‐thiadiazole 4 have been recorded. Based on HAM/3, MNDO, AM1, and PM3 calculations, the ionization potentials have been assigned to molecular orbitals. The gas‐phase thermolyses of 1–4 have been studied by PE‐controlled gas analysis. Extrusion of molecular nitrogen leads to reactive species which cyclize to three‐membered rings of different stability. At higher temperatures and in flash vacuum pyrolysis, consecutive reactions may lead to smaller acyclic molecules. The cyclo‐propanol 7 , obtained by denitrogenation of 2 , is thermally rather stable. Its PE spectrum has been recorded and analyzed.