Pentalene:  Formation, Electronic, and Vibrational Structure

Pentalene (1) is generated for the first time in argon matrices by photocleavage of the corresponding dimer (2). It is found that the cleavage occurs in two distinct steps, the first of which leads presumably to a diradical. 1 is characterized by its electronic and vibrational absorption 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 which had been ignored in previous discussion of the electronic structure. Due to the antiaromatic nature of pentalene, the distortive force of the π-electrons which favor a C_2h structure with localized single and double bonds predominates over the effect of the σ-electrons which drive the molecule to a D_2h structure.