The PE spectra of title compds. I (X = O, NMe, CH2; R = Me, CH2CMe3) have been recorded. The ionization potentials have been assigned to MOs on the basis of MNDO, AM1, and PM3 calcns. The most important occupied MOs are characterized as pNNN, pC:X, nN-3, nNN+, nNN- and nX. The gas-phase thermolyses of I have been studied by PE-controlled real-time gas anal. After extrusion of mol. nitrogen, the remaining reactive species cyclize to three-membered rings. At higher temps. and in flash vacuum pyrolysis, subsequent reactions lead to smaller acyclic compds.