Reprint of "Indenone and cyclopentadienone energetics via mass spectrometry and computations: Are these species antiaromatic or "merely" nonaromatic?"

Abstract The heat of hydrogenation of indenone was measured via two partially independent thermodynamic cycles by carrying out energetic measurements (i.e., electron affinities, proton affinities and ionization potentials) on both negative and positive ions (ΔH^o _H2 = 17.8 ± 5.5 and 17.5 ± 5.7 kcal mol^-1, respectively). High level G3 computations were also carried out to provide the heats of formation of indenone (16.8 kcal mol^-1) and cyclopentadienone (14.0 kcal mol^-1). These 4n π electron systems are found to be nonaromatic in contrast to previous views. A recent report on cyclopropenyl anion (J. Org. Chem. 2013, 78, 7370-7372) indicates that this ion is also nonaromatic, and suggests that NMR ring currents and nucleus independent chemical shift (NICS) calculations do not correlate with the energetic criterion for antiaromatic compounds.