How long are the ends of polyene chains?

In this work we study conjugation in all-trans polyene chains H[Single Bond](HC[Double Bond]CH)(n)[Single Bond]H with a view to establishing the length scale for the interaction between conjugated double bonds. As a polyene oligomer is made longer, bond length alternation between formal carbon-carbon single and double bonds diminishes toward the middle of the chain, eventually reaching a constant value characteristic of an "infinite" chain. However those bonds near the end of the chain continue to be influenced by the end, even in the long-chain limit. We have determined optimized geometries for polyene oligomers with up to n=11 repeat units at the MP2/cc-pVTZ level. At this length the central-most bonds are almost converged to the long chain limit, for which we estimate R(C[Double Bond]C)=1.3652 A and R(C[Single Bond]C)=1.4238 A. In contrast, the endmost double bond has a length of 1.3442 A and the endmost single bond has a length of 1.4425 A. We find that a given bond is significantly influenced by conjugation paths through up to six neighboring conjugated double bonds. End effects can also be monitored by examining the energy increment per added monomer as the oligomer length is increased. This analysis also indicates that significant conjugation effects extend out through approximately six neighboring double bonds. From the energy per monomer of the longest chains we extract a value of about 8 kcal/mol for the extra stabilization energy per monomer due to conjugation in long chains.

Clar theory for radical benzenoids.

Eric Clar's ideas concerning "aromatic sextets" are extended from closed-shell benzenoids to the case of radical benzenoids, particularly those where the unpaired electrons are largely localized on sites of one "type" (starred or unstarred). A quantitative format in terms of a new Clar polynomial is introduced to make quantitative correlations with a selection of numerical data, including delocalization energies, spin densities, and energy gaps between states of different spin multiplicities. The correlations are generally quite good, thereby further validating Clar's ideas and our extension and quantification of them.

Curvature matching and strain relief in bucky-tori: usage of sp3-hybridization and nonhexagonal rings.

The relief of different types of curvature strain in bucky-tori of elemental carbon is considered in a general formal framework. This theory then is used to aid in the design of several structures, which are treated via molecular mechanics. Novel illustrations of the remnant strain are made, and some modest conclusions as to the nature of the structure of the experimentally observed bucky-tori are suggested.

Chemical sub-structural cluster expansions for molecular properties.

Abstract The correlation of different molecular sub-structures with various molecular properties has a long history, of over a century. And currently such structural characterizations still remain of central interest in chemistry. Thus a general formalism to analyze a property or activity in terms of sub-structural contributions is of interest, and is pursued here. The approach may indeed be viewed as a formalization and extension of standard bond-energy ideas as arise even in introductory chemistry courses. The present formalism allows for: • a more complete and comprehensive formulation, with higher-order corrections to achieve greater accuracy; • a more general form for the class of sub-structures appearing in the expansion, thereby allowing more general (e.g., "multiplicative") properties to be expanded; and • a more general form for the expansion functions, thereby allowing more rapid convergence rates for the expansions. An illustrative example for the structure/property correlation of conjugated-hydrocarbon π-energy is made. Some comments on the use for describing bio-activities, and in particular toxicities, are made.