Stable π-electron systems and new aromatic structures

Aromatic character, as expressed in the reactions of azulene and its derivatives, is correlated with the development, in the appropriate transition state, of a stable π-electron system (sextet) in the five- or the seven-membered ring.

The recognition that considerable stability is associated with the ions and radical derived from perinaphthene leads to the formulation of several new types of hydrocarbons which may be expected to show aromatic character. A brief theoretical consideration of these hydrocarbons is given.

The synthesis of carbocyclic and heterocyclic derivatives of one of these hydrocarbons, cyclopenta[a]perinaphthene, is outlined, and a detailed account of their properties is given.     

Semiempirical Hamiltonians for spatially confined π-electron systems

A study of π-electron systems confined by impenetrable surfaces is presented. The study results in a nonempirical-based approach to obtain confinement-adapted semiempirical π-Hamiltonians including repulsive terms (PPP or Hubbard). The impenetrable surface confinement of a physical system involves changes in the boundary conditions that the eigenvectors of its differential Hamiltonian operator have to fulfill, while the Hamiltonian itself remains unchanged. However, if this Hamiltonian is written in second quantization language, then confinement only involves changes of the Hamiltonian scalar factors (integrals). Semiempirical Hamiltonian integrals are replaced by parameters; therefore, confinement involves only changes of these parameters. It is shown that confinement changes Coulomb (α_i) and exchange (β_ij), while repulsion (γ_ij) parameters remain unaffected. Next, the influence of confinement upon the electron correlation of (i) π-electron molecular systems, (ii) atoms, and (iii) an electron gas is discussed. The behaviour of the correlation energy vs. the confinement size is found to be different for each type of system. A neat explanation of this variety is given in terms of the Coulomb attractive fields of the systems. Some chemical confinement effects such as an increase in the reactivity of π-electron systems is also outlined. © 1996 John Wiley & Sons, Inc.     

Spin-projected EHF method. III. Applications to π-electron systems

The surely convergent procedure to obtain extended Hartree–Fock (EHF ) solutions of the spin-projected scheme, for which the equations are given in Part II, is applied to eleven π-electronic systems with 2–10 electrons at the PPP level of integral approximations. The method takes into account a considerable part of the correlation energy. The symmetry properties of the EHF wave functions obtained are discussed together with some computational details. A simplified algorithm is also described.     

Symmetry distortion of extended 1-D π-electron systems

It has been shown that a symmetry lowering from a state with higher symmetry to a state with lower symmetry (D_(2m)d → C_(2m)) occurs in 1-D polymers with polymethine fragments as elementary units when electron–vibration interaction is taken into account. The investigations are carried out using an extended Su–Schrieffer–Heeger method, where the electron–electron interaction is taken into account.     

Hydrogen bonding in simple π-electron systems II. Pyridine–pyrrol

Previous work in this laboratory concerning the properties of hydrogen bonds in the base pairs of DNA [1–6] has led to considerable interest in the properties of hydrogen bonds in π-electron systems. The first paper in this series [7] has investigated the usefulness of the LCAO –MO –SCF method and the Pariser–Parr–Pople approximation as applied to this problem, by calculation on the ideal pyridine–pyridinium complex. In this paper, a relation with experiment will be established by comparison of the results obtained from this method of calculation with the properties of the experimentally observable pyridine–pyrrol system.     

Hydrogen bonding in simple π-electron systems I. Pyridinium–pyridine

In this work, the application of the Pariser–Parr–Pople scheme to hydrogen-bonded systems containing π electrons has been examined. The potential energy curves for the movement of the hydrogen atom in the bond have been calculated, and the reliability of the results obtained from this method, applied in this manner, have been discussed.     

Crystallographic and Spectroscopic Evidence for an Intramolecular (OH … π)-Interaction

Crystallographic and spectroscopic evidence for an intramolecular (OH … π)-interaction in anti^9,10-10endo-hydroxy, 10exo-butyltricyclo [4.2.1.1^2.5]deca-3, 7-dien-9-one ( 1 ) is presented.     

Bounds for total π-electron energy of polymethines

The introduction of a -CH=CH- fragment into an arbitrary alternant ,ω-disubstituted polymethine increases the total Hückel π-electron energy at least by 2(√5 − 1) = 2.472 β and at most by 2(√3 + 1 − √2) = 2.636 β. These bounds are the best possible. With increasing length of the polymethine chain, the energy difference tends to 8/π = 2.546 β.     

First π-electron transitions in finite polymethine chains

In the present article, polymethine and annulene electronic spectra are described in a unique way by means of the Pariser-Parr-Pople method. The analytical expressions were derived for the first π-electron transitions energies. The character of the annulene spectra, like the character of the open-chain spectra, was shown to be dependent on the ratio between electron and site numbers. The Dahne's triad theory and the existence of finite energy gap in polymethines are discussed. © 1996 John Wiley & Sons, Inc.     

Semiempirical determination of Dyson's states in conjugated systems within a full-CI π-electron scheme

The Dyson states as orbitals and geminals describing one- and two-electron detachment processes in molecules are treated within a spin-free formulation of the full configuration interaction (CI) theory. The ensuing computational scheme is presented and some semiempirical calculations on π-shells are given for unsaturated hydrocarbons such as medium-sized polyenes and aromatics. It is shown that in these systems the correlation effects turn out to be more important than the relaxation effects, leading to a marked reduction in the calculated full-CI photoionization crosss ections relative to the Hartree-Fock ones.