The electron-pair origin of antiaromaticity: Spectroscopic manifestations

It is shown that the antiaromatic character of certain conjugated cyclic hy drocarbons is due to the presence of an even number of distinct electron pa irs in the system (such as, but not necessarily pi electrons). In these sys tems, the ground state is constructed from an out-of-phase combination of t wo valence bond (VB) structures, and its equilibrium geometry is necessaril y distorted along the coordinate that interchanges these structures. If a n ew symmetry element appears during the transition between the two structure s, the ground electronic state at the symmetric point transforms as one of the nontotally symmetric irreducible representations of the point group. Th e conjugate excited state, formed from the in-phase combination of the same two structures, transforms as the totally symmetric representation of the group and is strongly bound. Its structure is similar to that of the ground state at the symmetric point, and the energy separation between the two st ates is small compared to that of conjugated cyclic hydrocarbons having an odd number of distinct electron pairs. Motion along the "Kekule-type" vibra tional mode on the excited-state potential surface is very similar to motio n along the reaction coordinate connecting the two distorted structures on the ground-state surface. It is characterized by a significantly higher vib rational frequency compared to frequencies of similar modes in ground-state molecules. These qualitative predictions are supported by quantum chemical calculations on cyclobutadiene, cyclooctatetraene, and pentalene. (C) 1999 John Wiley & Sons, Inc.