Gas phase thermolysis of pyrazolines .5. Electronic structure and gas phase thermolysis of tetrazole derivatives studied by photoelectron spectroscopy

The photoelectron spectra of substituted tetrazoles 1-3, 1,4-dihydro-5H-tetrazol-5-ones 4-9, and 1,4-dihydro-5H-tetrazole-5-thiones 10-15 have been recorded. Based on PM3 and some ab initio calculations, the ionization potentials have been assigned to molecular orbitals. Gas-phase thermolyses of 1-15 have been studied by real-time gas analysis controlled by photoelectron spectroscopy. Compounds 1 and 2 lose formaldehyde and thioformaldehyde, respectively, leaving unsubstituted tetrazole (16), which decomposes mainly through extrusion of a nitrogen molecule and formation of cyanamide. Thiirane is split off from 3, and the remaining molecule decomposes into smaller products. Compounds 4-9 are cleaved by [3+2] cycloreversion to isocyanates and azides. Some of the unsymmetrically substituted compounds exhibit a marked selectivity in this reaction. For thiones 10-15 [3+2] cycloreversion is the main way of decomposition affording isothiocyanates and azides. In addition, the sulfur atom can split off and dimerize or abstract hydrogen atoms to form hydrogen sulfide. Some products like thiirene, formaldehyde, thioformaldehyde and acetaldehyde are generated solely from substituents. Photoelectron spectroscopy proved to be an excellent method for such thermolysis studies.