Bicyclo[3.2.0]hepta-1,3,6-triene — UV photoelectron spectroscopic and computational studies

Pyrolysis of N,N-dimethylbicyclo[3.2.0]hepta-3,6-dien-1-amine oxide (5) at 120 °C in the source chamber of the UV photoelectron (PE) spectrometer yielded bicyclo[3.2.0]hepta-1,3,6-triene (6). The triene dimerizes to yield a mixture of two dimers in equal amounts. A PE spectrum of an authentic mixture of dimers was also recorded for the purpose of spectral subtraction. Pyrolysis of 5 also generates dimethylhydroxylamine (7), a spectrum of which was also recorded from an authentic sample. Spectral subtraction allowed the first two ionization potentials of triene 6 at 7.2 and 9.8 eV to be identified. The resultant PE spectrum was compared very favorably with a simulated spectrum obtained from DFT calculations. A QTAIM analysis and calculation of delocalization indexes showed that while homoconjugation between C4 and C6 is negligible there is a weak through-space interaction between the bridgehead carbon atom C1 and C6 of the four-membered ring.Key words: bicyclo[3.2.0]hepta-1,3,6-triene, UV photoelectron spectroscopy, DFT calculations, QTAIM, delocalization indexes.