Paweł Szarek

Electronic stress tensor description of chemical bonds using nonclassical bond order concept

The stress tensors are used widely for description of internal forces of matter. For some time it is also applied in quantum theory in studies of molecular properties in chemical systems. Electronic stress tensors measure effects caused by internal forces acting on electrons in molecules and particularly those between bonded atoms. Utilized here stress tensor originated bond orders express bond strengths in terms of these internal forces. The unique concept of energy density and electronic chemical potential based bond orders gives natural evaluation of interaction strength compared with classical definition, considering delocalized nature of electrons. In addition to other causes, the relation to electronic energy may be used to predict relative stabilities of geometrical isomers or even conformers.

Noble gas monoxides stabilized in a dipolar cavity: a theoretical study.

Encouraged by our previous theoretical results that indicated the stabilization of the HeO unit inside the ferroelectric cavity composed of two parallel LiF dipoles, we have now undertaken the theoretical study for the related noble gas systems, (NgO)(MF)2, Ng = Ar, Kr, Xe, M = Li, Na, K. The computational results indicate that all such molecules constitute local minima, which are protected by sizable energy barriers especially for M = Li, Na, and thus these systems might constitute interesting synthetic targets at low temperatures.

Physical nature of intermolecular interactions within cAMP-dependent protein kinase active site: differential transition state stabilization in phosphoryl transfer reaction.

The origin of enzyme catalytic activity may be effectively explored within the nonempirical theory of intermolecular interactions. The knowledge of electrostatic, exchange, delocalization, and correlation components of the transition state and substrates stabilization energy arising from each enzyme active site residue allows to examine the most essential physical effects involved in enzymatic catalysis. Consequently, one can build approximate models of the catalytic activity in a systematic and legitimate manner. Whenever the dominant role of electrostatic interactions is recognized or assumed, the properties of an optimal catalytic environment could be simply generalized and visualized by means of catalytic fields that, in turn, aids the design of new catalysts. Differential transition state stabilization (DTSS) methodology has been applied herein to the phosphoryl transfer reaction catalyzed by cAMP-dependent protein kinase (PKA). The MP2 results correlate well with the available experimental data and theoretical findings indicating that Lys72, Asp166, and the two magnesium ions contribute -22.7, -13.3, -32.4, and -15.2 kcal/mol to differential transition state stabilization, respectively. Although all interaction energy components except that of electron correlation contribution are meaningful, the first-order electrostatic term correlates perfectly with MP2 catalytic activity. Catalytic field technique was also employed to visualize crucial electrostatic features of an ideal catalyst and to compare the latter with the environment provided by PKA active site. The map of regional electronic chemical potential was used to analyze the unfavorable catalytic effect of Lys168. It was found that locally induced polarization of TS atoms thermodynamically destabilizes electrons, pulling them to regions displaying higher electronic chemical potential.

On reversible bonding of hydrogen molecules on platinum clusters

The local reactivity of hydrogenated platinum clusters (Pt clusters) has been studied using the regional density functional theory method. We observed that antibond orbitals constitute the preferable binding site for hydrogen molecules H(2). Those sites are characterized by lowered electronic chemical potential and strong directionality and exhibit electrophilic nature. The platinum-dihydrogen (Pt-H(2)) sigma complexes were formed only by occupation of the lowest electronic chemical potential sites associated with Pt-H antibonds (sigma(PtH) ( *)) in saturated platinum clusters. The formation of sigma complexes caused mutual stabilization with the trans Pt-H bond. Such activated H(2) molecules on Pt clusters in a sense resemble heme-oxygen (heme-O(2)) complex with interaction strength greater than physisorption or hydrogen bonding but below chemisorption strength.

Polarization justified Fukui functions

New Fukui functions have been derived within the conceptual density functional theory by the analysis of the polarization effect of a system in static electric field. Resulting Fukui functions accurately reproduce the global softness and electronic dipolar polarizability; they meet the condition integral[f(r)/r]dr = -(partial differential mu/partial differential Z)(N) and lead to very reasonable values of the global hardness for atoms for the group of 29 main group elements. Computational clarity makes the new Fukui functions a promising tool in studies of molecular reactivity.

Electronic Structure Study of Local Dielectric Properties of Lanthanoid Oxide Clusters

Density functional calculations are performed for lanthanum-oxide clusters in order to study the local dielectric properties of such clusters using the dielectric constant defined at local points. An increase in coordination number brings about an increase in electron population on the central lanthanum atom, leading to an increase in the local dielectric constant.

Polarization justified Fukui functions: The theory and applications for molecules

The Fukui functions based on the computable local polarizability vector have been presented for a group of simple molecules. The necessary approximation for the density functional theory softness kernel has been supported by a theoretical analysis unifying and generalizing early concepts produced by the several authors. The exact relation between local polarizability vector and the derivative of the nonlocal part of the electronic potential over the electric field has been demonstrated. The resulting Fukui functions are unique and represent a reasonable refinement when compared to the classical ones that are calculated as the finite difference of the density in molecular ions. The new Fukui functions are strongly validated by their direct link to electron dipole polarizabilities that are reported experimentally and by other computational methods.

Reactivity Patterns of Imidazole, Oxazole, and Thiazole As Reflected by the Polarization Justified Fukui Functions

The concept of the polarization justified Fukui functions has been tested for the set of model molecules: imidazole, oxazole, and thiazole. Calculations of the Fukui functions have been based on the molecular polarizability analysis, which makes them a potentially more sensitive analytical tool as compared to the classical density functional theory proposals, typically built on electron density only. Three selected molecules show distinct differences in their reactivity patterns, despite very close geometry and electronic structure. The maps of the polarization justified Fukui functions on the molecular plane correctly identify important features of the molecules: the site for the preferential electrophilic attack in imidazole (-NH, see the TOC image) and oxazole (5-C), as well as uniquely aromatic character of the thiazole molecule and the acidic forms XH(+) of all three species.

Modeling the electron density kernels

Existing approximation to the softness kernel, successfully explored in earlier work, has been extended; the normal Gauss distribution function has been used instead of the Dirac delta. The softness kernel becomes continuous functions in space and may be used to calculate the linear response function of the electron density. Three‐dimensional visualization of the softness kernel and the linear response function are presented for a nitrogen atom as a working example. By using a single parameter of the spatial Gauss distribution, the novel softness kernel has been adjusted to be consistent with the standard form of the hardness kernel, representing the leading fraction of the electronic interactions in the system.

Theoretical study of hydrogenated tetrahedral aluminum clusters

We report on the structures of aluminum hydrides derived from a tetrahedral aluminum Al4 cluster using ab initio quantum chemical calculation. Our calculation of binding energies of the aluminum hydrides reveals that stability of these hydrides increases as more hydrogen atoms are adsorbed, while stability of Al-H bonds decreases. We also analyze and discuss the chemical bonds of those clusters by using recently developed method based on the electronic stress tensor.