Miklos Kertesz

Structure‐property predictions for new planar forms of carbon: Layered phases containing sp2 and sp atoms

Structure, thermodynamics, and electronic properties are predicted for a new low energy phase of carbon which contains planar sheets equally occupied by sp2 and sp carbon atoms. The isolated planar sheets have the same planar symmetry as do the layers in graphite (p6m) and can be formally viewed as resulting from the replacement of one‐third of the carbon–carbon bonds in graphite by –C 3/4 C– linkages. This material, called graphyne, is predicted to have a crystalline state formation energy of 12.4 kcal/mol carbon, which appears to be much lower than for any carbon phase which contains acetylenic groups as a major structural component. Based on the major structural reorganization required for graphitization and the observed high temperature stability of known model compounds, high temperature stability is predicted for graphyne. While graphyne will have similar mechanical properties as graphite, it is predicted to be a large bandgap semiconductor (Eg=1.2 eV) rather than a metal or semimetal. Based on this...

Electronic Structure of Polymers

Publisher Summary This chapter discusses the development of microscopic models of the electronic structure of polymers in general and of conjugated organic polymers in particular. The underlying physical model—namely, the one-particle picture, is being widely used in this field and is considered basically correct. On a practical level, this means that periodic models are usually constructed, which justify using the terminology of energy–band theory. With refinements of calculation tools, self-consistent calculations at even the ab initio level have become widespread. The chapter focuses on methodological problems related to the treatment of the electronic structure of polymers, such as the boundary conditions and problems related to the large size of these systems, ab initio techniques, and a symmetry dilemma connected with the quasidegeneracy of partially filled bands. The results of the application of these and semi-empirical techniques to the carbon atomic chain to (CH) x and a series of related polymers, polydiacetylene (PDA), and some further conjugated polymers, are compared with experimental results. Most of the discussion is based on one-dimensional (1D) model.

The effects of electron correlation on the degree of bond alternation and electronic structure of oligomers of polyacetylene

Full geometry optimizations on oligoenes have been performed with Hartree–Fock and density functional theory in combination with double zeta and triple zeta quality basis sets with primary focus on the degree of bond length alternation and on the energy gap. Monitoring the dependence of the computed properties on the oligomer size provides new insights into the reliability of the calculations, which are analyzed in terms of dynamical and nondynamical electron correlation. Our theoretical bond length alternation values for the oligomers of polyacetylene extrapolate to significantly smaller values than what has been established by experiments and earlier theoretical predictions. The exact exchange mixing to the exchange-correlation functional not only improves the agreement of the theoretical gap of oligoenes with experimental excitation energies but also increases the computed bond length alternations. Based on a newly proposed one parameter functional of Becke, the effect of the exact exchange mixing has ...

Performance of the Vienna ab initio simulation package (VASP) in chemical applications

Five different density functionals in combination with ultra-soft pseudopotentials and plane wave basis sets were used to optimize the geometries of common chemical systems using solid state program Vienna ab initio simulation package (VASP). These systems included diatomics, N2, O2, F2 and CO, and carbon based organic systems, ethane, ethylene, acetylene, 1,3-butadiene, 1,3,5-hexatriene, benzene, biphenyl, naphtalene graphene, polyethylene and all-trans-polyacetylene. The four functionals based on the generalized gradient approximation gave very good agreement on bond lengths and angles as compared with each other, with localized Gaussian basis set calculations and with experimental values. Reasonable results were also obtained for vibrational frequencies of selected normal modes of benzene and for torsional potentials of 1,3-butadiene and biphenyl. On the other hand, local density approximation tends to underestimate bond lengths. The performance of VASP for these properties is very similar to Gaussian type implementations of density functional theory explaining its successes in molecular, solid state, surface and polymer applications.

Is There a Lower Limit to the CC Bonding Distances in Neutral Radical π-Dimers? The Case of Phenalenyl Derivatives

Two-electron multicenter (2e/mc) bonding of phenalenyl (PHYL) pi-dimers was found to be significantly affected by the electron density on the bonding active sites. The computational analysis shows that, upon appropriate beta-substitutions, the newly introduced dimers have the shortest and strongest covalent bonding interactions seen in any neutral pi-dimer. The unusual strengthening of the bonding was attributed to the reduced lone pair bond weakening effect, LPBWE, upon substitutions with electron-withdrawing groups.

Higher order Peierls distortion of one-dimensional carbon skeletons

Abstract In some one-dimensional chains with metallic band structures the energetically most advantageous distortion opens an energy gap non-linear in the distortion. The absence of a linear Peierls effect is due to the symmetry of the degenerate pair of orbitals at the Fermi level: they do not interact directly in the distorted system. The effect provides a natural bridging of the one-dimensional polyacetylene (linearly Peierls-distorted) to the two-dimensional graphite (not distorted) carbon skeletons. The simple explanation bears some generality.

Ab initio Hartree–Fock crystal orbital studies. Energy bands in polyene reconsidered

Ab initio ’’exact exchange’’ LCAO Hartee–Fock (HF) energy bands are given for a polyene model with equal bond lengths. In addition to the symmetric wavefunction a broken‐symmetry solution of charge density wave (CDW) type with lower total energy occurs. This instability of the metallic‐type wavefunction with respect to the charge‐density wave was overlooked by Andre and Leroy in their ab initio HF treatment of polyene. We wish to discuss the nature of the CDW and the screening effect of the σ electrons, which tend to diminish the amplitude of the CDW of the π electrons. Comparison with a model with alternant bond lengths shows the latter to be energetically more stable.

Validation of intermolecular transfer integral and bandwidth calculations for organic molecular materials.

We present an interpretation of the intermolecular transfer integral that is independent from the origin of the energy scale allowing convergence studies of this important parameter of organic molecular materials. We present extensive numerical studies by using an ethylene pi dimer to investigate the dependence of transfer integrals on the level of theory and intermolecular packing. Transfer integrals obtained from semiempirical calculations differ substantially from one another and from ab initio results. The ab initio results are consistent across all the levels used including Hartree-Fock, outer valence Greens function, and various forms of density functional theory (DFT). Validation of transfer integrals and bandwidths is performed by comparing the calculated values with the experimental values of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), bis[1,2,5]thiadiazolo-p-quinobis(1,3-dithiole), (BTQBT) K-TCNQ, and hexagonal graphite. DFT in one of its presently popular forms, such as Perdew-Wang functionals (PW91), in combination with sufficient basis sets provides reliable transfer integrals, and therefore can serve as a basis for energy band calculations for soft organic materials with van der Waals gaps.

Evidence of σ- and π-dimerization in a series of phenalenyls.

Phenalenyl and a wide variety of its derivatives form stable radicals, which often associate in various aggregates with interesting properties that include magnetism and high electrical conductivity. The two main modes of aggregation involve π-stacking pancake multicenter bond formation and σ-bond formation. We explore the energetics of the various σ- and π-dimers for six phenalenyl derivatives with both computational and experimental methods. A modern density functional theory (M05-2X) is used to survey the potential energy surface revealing the mechanism of the aggregation. In order to enrich experimental data, the triphenyl and trimethyl derivatives are newly prepared and their aggregation behaviors are investigated by various analytical methods including ESR, (1)H NMR, UV-vis, and single-crystal X-ray diffraction. The agreement between computations and experiments are very good forming the basis of describing trends in this series. We find that π-dimer formation can proceed via an asynchronous concerted path from the monomers or in a stepwise process via σ-dimers. The strength of the π-stacking pancake interaction depends strongly on substituents and covers a wide range both in terms of binding energies and contact distances. The spin densities in the π-stacking dimers reflect these trends and display a wide range of diradicaloid characters. Many σ-dimer configurations compete some of which are separated by small barriers leading to fluxional structures between σ-bonded configurations or σ- and π-bonded configurations.

Rotational barrier in phenalenyl neutral radical dimer: separating pancake and van der Waals interactions.

Pancake π-stacking produces shorter contacts than van der Waals bonding but it has strongly preferred configurations. By high-level multireference average quadratic coupled cluster theory for the singlet and triplet, we identify the specific orbital component and the nonspecific vdW contributions in the prototypical pancake-bonded dimer of phenalenyl thereby explaining the configurational preferences.