Stability and aromaticity of the cyclopenta-fused pyrene congeners.
The aromaticity of all possible cyclopenta-fused pyrene congeners has been investigated at various levels of theory. On the basis of the calculated resonance energies and magnetic properties (delta(1)H data, magnetic susceptibility anisotropies, and NICS values), the overall aromaticity of these compounds is found to decrease gradually with increasing number of externally fused five-membered rings. The relatively small differences (<5 kcal/mol) in thermodynamic stability of the isomeric dicyclopentapyrenes (E(tot): dicyclopenta[cd,fg]- > dicyclopenta[cd,jk]- > dicyclopenta[cd,mn]pyrene), which differs from the aromaticity order based on the magnetic criteria (dicyclopenta[cd,mn]- > dicyclopenta[cd,fg]- > dicyclopenta[cd,jk]pyrene), is shown by model calculations to be dominated by sigma-strain imposed on the pyrene skeleton by sequential cyclopenta-fusion. This is supported by the computed homodesmotic reaction energies and aromatic stabilization energy (ASE(isom)) from isodesmic aromatic-nonaromatic isomerization, and by the model calculations on "distorted" cyclopenta[cd]pyrenes. The elusive tetracyclopenta[cd,fg,jk,mn]pyrene is computed to be bowl-shaped; its corresponding planar geometry is the transition state for bowl-bowl interconversion.
Havenith
Remco W A
RW
Department of Physical Organic Chemistry, Debye Institute, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
Jiao
Haijun
H
Jenneskens
Leonardus W
LW
van Lenthe
Joop H
JH
Sarobe
Martin
M
Schleyer
Paul von Ragué
Pv
Kataoka
Masahiro
M
Necula
Atena
A
Scott
Lawrence T
LT