Herminio P. Diogo, Tetsu Kiyobayashi, Manuel E. Minas da Piedade, Natan Burlak, Donald W. Rogers, Daniel McMasters, Gabriele Persy, Jakob Wirz, Joel F. Liebman "The Aromaticity of Pyracylene: An Experimental and Computational Study of the Energetics of the Hydrogenation of Acenaphthylene and Pyracylene", J. Am. Chem. Soc., 2002, 124, 2065-2072.

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Abstract: In this work, the aromaticity of pyracylene was investigated from an energetic point of view. The standard enthalpy of hydrogenation of acenaphthylene (1) to acenaphthene (3), at 298.15 K, was determined to be -(114.5±4.2) kJ mol-1 in toluene solution and -(107.9±4.2) kJ mol-1 in the gas phase, by combining results of combustion and reaction-solution calorimetry. A direct calorimetric measurement of the standard enthalpy of hydrogenation of pyracylene (2) to pyracene (4) in toluene, at 298.15 K, gave -(249.9±4.6) kJ mol-1. The corresponding enthalpy of hydrogenation in the gas phase, computed from the (cr) and values obtained in this work for 2 and 4, was -(236.0±7.0) kJ mol-1. Molecular mechanics calculations (MM3) led to (1, g) = -110.9 kJ molŠ1 and (2, g) = -249.3 kJ mol-1, at 298.15 K. Density functional theory calculations [B3LYP/6-311+G(3d,2p)//B3LYP/6-31G(d)] for naphthalene and compounds 1-4 provided enthalpies for the isodesmic gas phase reactions 2 (acenaphthylene) --> pyracylene + naphthalene, (298.15 K, g) = 31.1 kJ mol-1, and 2 (acenaphthene) --> pyracene + naphthalene, (298.15 K, g) = 59.9 kJ mol-1, that were largely independent of the basis set. These data led to the enthalpies of formation of gaseous pyracylene and pyracene, at 298.15 K, from which the enthalpy of hydrogenation of pyracylene (2, 298.15 K, g) = -(244.6±8.9) kJ mol-1 was derived. The results are put in perspective with discussions concerning the "aromaticity" of pyracylene. It is concluded that on energetic grounds pyracylene is a borderline case in terms of aromaticity/ antiaromaticity character.