Role of π stabilization in cyclic polyenes

The Huckel model with bond-length-dependent coupling (beta), contrary to the original Huckel model with a constant beta, may be parametrized to give C-C bond lengths and some other properties in good agreement with accurate ab initio results. The paradox in seemingly contradictory conclusions of some recent works on whether the pi electrons promote equal bond lengths or bond alternation in benzene is discussed for the more general group of cyclic polyenes. We find that pi electrons promote bond alternation but that the interaction between pi electrons of different bonds always leads to a decrease of the bond length alternation in cyclic systems (except C4H4) compared to the difference in bond length between pure single and double bonds. (C) 1998 John Wiley & Sons, Inc.