title: The aromaticity of pyracylene: an experimental and computational study of the energetics of the hydrogenation of acenaphthylene and pyracylene.
abstract: In this work, the aromaticity of pyracylene (2) 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 minus sign(114.5 +/- 4.2) kJ x mol(-1) in toluene solution and minus sign(107.9 +/- 4.2) kJ x 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 plus minus 4.6) kJ x mol(-1). The corresponding enthalpy of hydrogenation in the gas phase, computed from the Delta(f)H(o)m(cr) and DeltaH(o)m(sub) values obtained in this work for 2 and 4, was -(236.0 +/- 7.0) kJ x mol(-1). Molecular mechanics calculations (MM3) led to Delta(hyd)H(o)m(1,g) = -110.9 kJ x mol(-1) and Delta(hyd)H(o)m(2,g) = -249.3 kJ x mol(-1) at 298.15 K. Density functional theory calculations [B3LYP/6-311+G(3d,2p)//B3LYP/6-31G(d)] provided Delta(hyd)H(o)m(2,g) = -(244.6 +/- 8.9) kJ x mol(-1) at 298.15 K. 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.
authors.affiliation: Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, 1049-001 Lisboa, Portugal.
authors.forename: Hermínio P
authors.initials: HP
authors.lastname: Diogo
authors.forename: Tetsu
authors.initials: T
authors.lastname: Kiyobayashi
authors.forename: Manuel E
authors.initials: ME
authors.lastname: Minas da Piedade
authors.forename: Natan
authors.initials: N
authors.lastname: Burlak
authors.forename: Donald W
authors.initials: DW
authors.lastname: Rogers
authors.forename: Daniel
authors.initials: D
authors.lastname: McMasters
authors.forename: Gabriele
authors.initials: G
authors.lastname: Persy
authors.forename: Jakob
authors.initials: J
authors.lastname: Wirz
authors.forename: Joel F
authors.initials: JF
authors.lastname: Liebman