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.

An ab Initio Valence Bond Study on Cyclopenta-Fused Naphthalenes and Fluoranthenes

“…39 These results confirm, in line with our VB results, that there is no significant aromatic interaction between the cyclopenteno-bridges of 2 and 3 and the naphthalene core. A detailed analysis of the redox potentials and in particular their reductive behavior of cyclopenta-fused aromatic hydrocarbons 40 led to the same conclusions.…”

Thermochemistry of benzoquinones

“…Isodesmic stabilization energy is also a useful criteria to know the aromatic character of a molecule. This energy of stabilization is associated with the change of enthalpy accompanying hypothetical reactions, known as isodesmic reactions, where the relationship between bonds change in the process, but keeping the number of each specific formal bond [13][14][15][16][17]. In the case of chemical structures with alternate double bonds, isodesmic reaction can be designed to determine the energy corresponding to the total stabilization energy due to p electron delocalization [15], as is shown in equation (9) for the benzene.…”

“…From results of enthalpy of formation in the gas phase derived from calorimetric measurements, and applying the scheme of Klages [12], the energy of resonance was deduced, both molecules featuring it. However, we have also made use of isodesmic and homodesmotic stabilization energy criteria [13][14][15][16][17] in order to clarify the aromatic character of both quinones. Results show that the p-benzoquinone is an antiaromatic compound, whereas the duroquinone is marginally aromatic.…”

Perchloropyracylene and its Fusion with C60 by Chlorine‐Assisted Radio‐Frequency Furnace Synthesis

“…[60][61][62][63][64] The perchlorinated pyracylene (C 14 Cl 8 ) is attached to the [6,6] bond of C 60 through a formal [2+2] addition resulting in a strained cyclobutane ring. The lengths of both the sp 3 -sp 3 [6,6] carbon bond (1.601 (9) ) and the four neighboring sp 2 -sp 3 [5,6] carbon bonds (1.518 (7) ) of the C 60 fragment are similar to the corresponding bond lengths of C 60 DielsAlder adducts (d 6:6 = 1.602(4), [65] 1.62(4) ; [66] d 5:6 = 1.529(4), [65] 1.52(4) [66] ) and the C 60 fullerene dimer (d 6:6 = www.chemeurj.org 1.581 (7); [67] d 5:6 = 1.530(8) [67] ). As expected, the remaining fullerene part remains practically unchanged and the [6,6] and [5,6] [68] ).…”

“…The lengths of both the sp 3 -sp 3 [6,6] carbon bond (1.601 (9) ) and the four neighboring sp 2 -sp 3 [5,6] carbon bonds (1.518 (7) ) of the C 60 fragment are similar to the corresponding bond lengths of C 60 DielsAlder adducts (d 6:6 = 1.602(4), [65] 1.62(4) ; [66] d 5:6 = 1.529(4), [65] 1.52(4) [66] ) and the C 60 fullerene dimer (d 6:6 = www.chemeurj.org 1.581 (7); [67] d 5:6 = 1.530(8) [67] ). As expected, the remaining fullerene part remains practically unchanged and the [6,6] and [5,6] [68] ). The bond lengths of pyracylene will be discussed in the following section.…”