ELECTRONIC-STRUCTURE AND GAS-PHASE THERMOLYSIS OF 4-SUBSTITUTED 3,3,5,5-TETRAMETHYL-3,5-DIHYDRO-4H-PYRAZOLES STUDIED BY PHOTOELECTRON-SPECTROSCOPY - FIRST EVIDENCE FOR AN ALKYLIDENESELENIRANE

The electronic structures of the tetramethyldihydropyrazoles 1 - 5 hav e been studied by photoelectron spectroscopy and interpreted with the aid of semi-empirical SCF MO calculations (PM3). Gas-phase pyrolyses o f 1 - 5 have been carried out and analysed by photoelectron spectrosco py. Both 1 and 2 and in part 3 eliminate molecular nitrogen to form re active species which cyclize to three-membered rings, i.e. the novel a lkylideneselenirane 16, the alkylidenethiirane S-oxide 17, and the cyc lopropanone hydrazone 19, respectively, which decompose at higher temp eratures. The elusive alkylideneselenirane 16 was studied by IR spectr oscopy in an argon matrix at 10 K.