Computational Surface Chemistry

Chemical Processes at Organic/Metal Interfaces

Many experimental studies have been addressing the question of identifying the functional abilities of single molecules or molecular self-assemblies to behave as wires, diodes, transistors, and rectifiers. Although useful, there are well-known problems associated with many current theoretical attempts to rationalize experimental results characterizing the organic/metal interfaces. First, conduction mechanisms through molecular junctions have been based upon the orbital energetics without considering the spatial extent of the conduction channels. Second, the divisions of the molecular interaction energy into classically interpretable segments are arbitrary and do not distinguish different aspects of electron behavior. Third, the charge transfer and electrostatic components do not have their usual physical meanings in the Mulliken and Morokuma analyses, respectively. Finally, the descriptions of bonded interactions have been sought in the non-invariant orbitals, such as HOMO and LUMO depending upon the particular basis set. Hence, there is a fundamental interest in the investigation of the interfacial interactions and charge migration processes between organic molecules and metallic surfaces from theoretical standpoints.

The study - P.M. Mitrasinovic, Can. J. Chem. 81, 542-554, 2003 has indicated that the determination of electron behavior in terms of the electron distribution and nature of the organic/metal-bonded interactions is crucial. The conceptual framework provided by natural population analysis and the quantum theory of atoms in molecules (QTAIM) is able to achieve the goal at the one-electron level. The beauty of the strategy is that the interfacial interactions are given physical definitions free of any assumptions and can be visualized by using the topological features of the total electron density. In addition, it is possible to judge on the extent of success to which the frontier molecular orbital argument is capable of rationalizing the organic/metal charge migration process. The possibility is contained in the question of whether the spatial extents of the HOMO and LUMO resemble those of the negative (charge locally concentrated) and positive (charge locally depleted) Laplacian of the total electron density, which determines the reactivity. Consequently, Dr. Mitrasinovic is interested in investigating the essential origin of electron distribution by analyzing the density of states and band structures in various organic/metal complexes. Sophisticated density functional theory analyses could show that the origin of electron distribution in many systems is more complicated than that based solely on classical interpretations. The question is fundamentally profound and worth being pursued using experimental and theoretical investigations converging to the same answer.