Radialenes are minimally conjugated cyclic π-systems

Abstract

ABSTRACTConjugation energy (CE) in benzene is larger than its aromatic stabilisation energy (ASE). A far-reaching conclusion offered by this work is that per π-electron, CE is energetically larger than aromaticity. If a diene has a doubly degenerate HOMO, then its Diels–Alder reaction will be kinetically faster than a similar diene with a nondegenerate HOMO. The topological conjugation energy (TCE) for the radialene, monocyclic, dendralene, and linear polyene series has quite different trends. Radialenes are minimally conjugated cyclic systems with the TCE/No. π-bond = 0.432 β; the members of the dendralene series approach this same value from smaller values with increasing size. With increasing size, the members of the monocyclic and linear polyene series have, respectively, decreasing and increasing TCE/No. π-bond values approaching 0.547 β. Topological resonance energy (TRE) for radialenes, dendralenes, and linear polyenes all have TRE = 0, and the TRE/π-electron for monocyclic polyenes has alternating declining values between antiaromatic (−0.3066 β, −0.07435 β, −0.03287 β, …) and aromatic (0.04543 β, 0.01594 β, 0.00807 β, …). For benzene, TRE/No. π-bond = 0.0909 β and TCE/No. π-bond = 0.576 β.