Steven M. Risser

DNA: insulator or wire?

DNA-based electron transfer reactions are seen in processes such as biosynthesis and radiation damage/repair, but are poorly understood. What kinds of experiments might tell us how far and how fast electrons can travel in DNA? What does modern theory predict?

Bridge‐mediated electronic interactions: Differences between Hamiltonian and Green function partitioning in a non‐orthogonal basis

An analysis of the partitioning (projection) technique is given with emphasis on non‐orthogonal basis sets. The general expression for the effective Hamiltonian obtained via Lowdin partitioning of the Schrodinger equation is discussed in the context of semi‐empirical theories and electron transfer matrix elements. Numerous pitfalls in calculations of matrix elements are pointed out. More importantly, it is shown that contrary to the case of an orthogonal basis, for a non‐orthogonal basis Lowdin partitioning of the Schrodinger equation and partitioning of the Green function equation are not equivalent. The latter method provides a more general prescription for deriving effective Hamiltonians. Such Hamiltonians reproduce the full propagation in the partitioned subspace.

DNA-mediated electron transfer

Abstract Electron transfer in DNA has been investigated for decades, but recent experiments highlight our limited fundamental understanding of these processes. Modern electron transfer theory may help to address some of the open mechanistic issues. We summarize and analyze the results of recent experiments from a theoretical perspective. Future research directions are suggested that might help to establish the molecular mechanism(s) for long-range DNA electron transfer.

DNA double-helix-mediated long-range electron transfer

A theoretical analysis based upon large-scale self-consistent Hartree-Fock calculations at a semiempirical quantum theory level (CNDO/S) is performed to investigate long-range electron transfer in a donor-DNA-acceptor molecule, where the donor and acceptor moieties are tethered to the DNA. The π-stacked base pairs are found to dominate the long-range electronic coupling. Despite the π-electron mediated coupling, the exponential distance decay constant of the electron transfer rate is ∼ 1.2–1.6 A−1, values typical of electron transfer proteins. The calculated long-range electron transfer rate of the order of 106 s−1 for a metal-to-metal distance of 21 A is found to be in agreement with kinetic measurements by Meade and Kayyem. Based on the current analysis, the π-electrons dominate the long-range electronic coupling interactions in DNA, but they do not lead to one-dimensional molecular wire-like properties.

Effects of surface defects on the local electric field in inhomogeneous media

Surface defects both focus the local electric field and enhance its intensity in the vicinity of the defect Using a finite element model of an inhomogeneous dielectric film, we have examined the relationship between the defect shape and local electric field in the film and defect. We find that raised defect regions that are peaked have the largest local fields while the slope of removed regions has little effect on the field in raised defects. Both peaked and flat surface defects can lead to large enhancements of the local electric field above that predicted by effective medium approximation (EMA) methods.

SURFACE DEFECT ENHANCEMENT OF LOCAL ELECTRIC FIELDS IN DIELECTRIC MEDIA

Abstract Surface localized defects represent a special class of microstructure in dielectric films. Whereas inhomogeneous media are often classified by volume fractions of their constituents, determinations of the local electric field require both microstructural definition and specific electric interaction among defect sites. Herein, we have used a finite element method to determine the local electric field for surface defects in dielectric films. The local electric field is both enhanced and focused in regions about the surface localized defect site. The resulting surface intensities exhibit a much broader range of magnitudes than accomodated by effective medium approximation and random resistor methods.

N-State Interpretations of the First and Second Hyperpolarizabilities of Cyanobiphenyl-Based Liquid Crystal Molecules

Electronic structure calculations on the frequency-dependent hyperpolarizabilities of 5-alkyl cyanobiphenyl have been performed using the AM1 Hamiltonian and time dependent Hartree-Fock theory. Both the average first and second hyperpolarizabilities decrease with increasing twist angle, o, between the two phenyl groups, and increase with increasing incident frequency. While the nonlinear optical properties of materials are frequently interpreted within the framework of two and three state models for the electronic spectrum, systems with a more complex electronic structure require an approach which can denote state importance. The linear optical properties of 5CB are dominated by two low-lying charge transfer transitions from the ground state, which would seem suitable to a three state model. However, large state dipole moments offset of low oscillator strengths for other transitions allowing them to also become significant contributors to the nonlinear optical properties.

Microstructural and dielectric susceptibility effects on predictions of dielectric properties

In modeling the dielectric properties of inhomogeneous materials, the treatment of the electric field interaction s differentiate the usual modeling formalism and their accuracy. In this paper, we show that the performance of effective medium methods is dependent upon a number of variables - defect concentration, alignment, and the dielectric constant of the material itself. Using our previously developed finite element model of an inhomogeneous dielectric, we have developed models for a number of dielectric films of varying dielectric constant and microstructures. Alignment to of defects parallel to the applied field and the larger defect aspect ratios increase the overall dielectric constant. The extent of these effects is dependent on the dielectric constant of the bulk component.

Inclusion and overlayer effects on the electric fields in dielectric films

The surface and bulk microstructures inherent to the fabrication process of dielectric films, affects both the distribution of the local electric field intensities and its associated dielectric properties. In this paper, we have used a finite element electric field model to examine the effects of a low dielectric surface layer, alignment of a low dielectric component within the dielectric film, and the interaction of these two influences on the local electric fields and dielectric constants. Columnar microstructure and alignment of dielectric components perpendicular to the surface norm are shown to enhance electric intensities and dielectric constants. Predicted dielectric constants are compared against conventional effective medium approximation results.

Enhancement factors for local electric fields in inhomogeneous media

The microstructure of dielectric films provides a significant influence on the electric field distribution in these materials. In this paper, we focus on the relationship between the electric field distribution and organization of film constituents. Using our self- consistent determination of the local electric field in inhomogeneous media, we have shown that enhanced fields can result from columnar microstructures such as typically generated by CVD-type fabrication processes, and low dielectric components in optical coatings. In addition to the microstructural enhancement, a surface specific enhancement due to presence of low dielectric components is observed.