Total Resonance Energy Research Articles

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

To probe the effect of external cyclopenta-fusion on a naphthalene core, ab initio valence bond (VB) calculations have been performed, using strictly atomic benzene p-orbitals and p-orbitals that are allowed to delocalize, on naphthalene (1), acenaphthylene (2), pyracylene (3), cyclopenta[b,c]acenaphthylene (4), fluoranthene (5), and cyclopenta[c,d]fluoranthene (6). For the related compounds 1-4 and 5,6 the total resonance energies (according to Pauling's definition) are similar. Partitioning of the total resonance energy in contributions from the possible 4n + 2 and 4n pi-electron conjugated circuits shows that only the 6pi-electron conjugated circuits (benzene-like) contribute to the resonance energy. The results show that cyclopenta-fusion does not extend the pi system in the ground state; the five-membered rings act as peri-substituents. As a consequence, the differences in (total) resonance energy do not coincide with the differences in thermodynamic stability. Notwithstanding, the relative energies of the Kekule structures can be estimated using Randic's conjugated circuits model.

Cyclic Conjugation Effects in Cyclacenes

Abstract Cyclic conjugation is studied in Hückel and Möbius cyclacenes as well as in a few other related open-chain and cyclic belt-shaped hydrocarbons (e.g., zig-zag cyclarenes). the main conclusions are that (a) the energy effect of the conjugation along the perimeter of cyclacenes is relatively large, (b) conjugation along the 6 -, 10 - and 14-membered cycles has a much greater effect, in spite of the fact that these cycles embrace bonds that formally are not involved in cyclic conjugation. Möbius cyclacenes are non-alternant hydrocarbons and their odd-cycles have a significant contribution to conjugation. the contribution of conjugation in individual cycles to the total resonance energy is briefly discussed.

Theoretical studies of the molecular second-order hyperpolarizabilities of polycyclic aromatics

Semiempirical-FF calculations were performed for the second-order hyperpolarizabilities 〈γ〉 of a series of polycyclic aromatics to examine the structure–optical–propertyrelationship. First, 〈γ〉 of the unsubstituted PAHs were presented. For linear polyacenes, 〈γ〉 and total resonance energy were found to be increased while the HOMO-LUMO gap and resonance energy per electron decreased as the number of six-membered rings increased. For nonlinear PAHs, the chain topology should be taken into consideration. An empirical formula was given to relate the computed 〈γ〉 and linear polarizabilities 〈α〉. 〈γ〉 of mono- and di-substituted PAHs were then presented to examine the substituent effects on 〈γ〉. © 1995 John Wiley & Sons, Inc.

Semi-empirical calculations of the nonlinear optical properties of polycyclic aromatic compounds

A systematic study of factors influencing nonlinear optical properties, in particular the second hyperpolarizability 〈γ〉, is conducted for a series of polycyclic aromatic hydrocarbons (PAHs). The HOMO-LUMO gap, polarizability 〈α〉, first hyperpolarizability 〈β〉, and second hyperpolarizability 〈γ〉 of polycyclic aromatic compounds are computed by using semi-empirical MNDO-FF calculation implemented in the MOPAC package. For linear polyacenes, the linear and nonlinear properties and total resonance energy are found to increase while the HOMO-LUMO gap and resonance energy per electron decrease as the number of six-membered rings increases. For nonlinear PAHs, the chain topology should be taken into account. An empirical formula is given to relate the computed 〈γ〉 and 〈α〉.

Semiempirical calculations of molecular polarizabilities and hyperpolarizabilities of polycyclic aromatic compounds

AbstractThe correlation of 2D conjugation and the nonlinear properties of a series of polycyclic aromatic compounds were studied by performing semiempirical calculations using the MOPAC package. For linear polyacenes, both linear and nonlinear properties are found to be increased as the number of six‐membered rings increases. In general, the polarizabilities, 〈α〉, and second hyperpolarizabilities, 〈γ〉, are found to correlate very well with total resonance energies. In the case of first hyperpolarizabilities, 〈β〉, symmetry is the dominating factor. © 1992 John Wiley & Sons, Inc.

A comparison of recent theoretical aromaticity indices

Abstract