Restricted geometry optimization for estimating stabilization energies of polycyclic aromatic hydrocarbons

At the B3LYP/6‐31G* level, our program of geometry optimization under the restrictions of π‐orbital interactions was applied to five homologous series such as acene‐, phenanthrene, triphenylene, chrysene, benzopyrene series, and seven additional molecules such as tri‐ and tetra‐benzanthracene, benzo‐ and naphtho‐pentaphene, dibenzopentacene and tribenzotetracene, providing each of the 35 polycyclic aromatic hydrocarbons (PAHs) with two types of localized reference geometries: (i) the ‘GL’ geometry where all double bonds are localized and (ii) m different ‘GE‐m’ geometries, where only a specific pair of double bonds are permitted to conjugate and all other bonds are localized.. Each GE‐m resulted from the local π interaction between a pair of double bonds in the GL geometry. The total sum of molecular energy differences, ([E(Ground) ? E(GL)] ? Σ[E(GE‐m) ? E(GL)]), is defined as the extra stabilization energy (ESE) for a PAH. The corrected ESE (CESE) then resulted from considering π interactions between pairs of nonbonded double bonds. In this work, the position, energy and GL sextet rules were developed to ensure that the GL geometry chosen from its candidates as well as the GE‐m geometries is no longer arbitrary. Therefore, it is the GL and GE‐m geometries of the PAH itself, rather than nonaromatic compounds, that serve as reference structures for calculating CESE. As a result, for each of the four homologous series (acene, phenanthrene, triphenylene, and benzopyrene), CESE/π can be fitted as a linear function of the p band frequency . For each of the four isomeric series with molecular formulas of C₁₈H₁₂, C₂₂H₁₄, C₂₆H₁₆, and C₃₀H₁₈, CESE/π can also be fitted as a second‐order polynomial function of the p band frequency as well as of the number of GL sextets. Copyright © 2009 John Wiley & Sons, Ltd.     

Conjugation,number of Dewar resonance structures (DSs) in homologous polyzethrene and related conjugated polycyclic hydrocarbon series,and kinked versus straight

Kinked polyzethrenes are more stable than linear polyzethrenes making them better candidates as materials for organic electronic devices (e.g., organic field effect transistors, nonlinear optics, and semiconductors) because of their greater singlet biradical properties. For series of molecules constructed by successive attachment of a given aufbau unit, we are able to derive analytical or recursion expressions relating certain properties. For example, starting with a few known number of Dewar resonance structures (DSs) for such a series, one is often able to derive analytical or recursion expressions for these DS values by our method of successive differences which then lead to either constant or Fibonacci numbers, respectively. The increasing order of π-electronic stability of isomers with the same number of Kekulé structures (K) is determined by their increasing number of DSs. Kinked polycyclic conjugated polyenes with a single classical structure (i.e., K = 1) are more conjugated and stable than their straight polycyclic isomers with a single classical structure.