PENTALENE - FORMATION, ELECTRONIC, AND VIBRATIONAL STRUCTURE

Pentalene (1) is generated for the first time in argon matrices by pho tocleavage of the corresponding dimer (2). It is found that the cleava ge occurs in two distinct steps, the first of which leads presumably t o a diradical. 1 is characterized by its electronic and vibrational ab sorption spectra which are assigned and interpreted with reference to different quantum chemical calculations. These show that the first two excited states of pentalene involve a doubly excited configuration wh ich had been ignored in previous discussion of the electronic structur e. Due to the antiaromatic nature of pentalene, the distortive force o f the pi-electrons which favor a C-2h structure With localized single and double bonds predominates over the effect of the alpha-electrons w hich drive the molecule to a D-2h structure.