There are well known problems associated with many current attempts to rationalize the organic/metal interfaces. The charge transfer and electrostatic components do not have the usual physical meanings considered in the Mulliken and Morokuma analyses, respectively.1 Conduction mechanisms through molecular junctions have been based upon the orbital energetics without considering the spatial extent of the conduction channels.2 Besides, the descriptions of bonded interactions have been sought in non-invariant orbitals such as HOMO and LUMO depending upon particular basis sets.1 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 physically meaningful descriptions of electron behavior should be sought in density matrix (or related functions) and not in orbitals.3 Wave functions (or more generally density matrices) are therefore indispensable for the interpretation of electron behavior. At the one-electron level, the density matrix may be diagonalized by the natural orbitals. By writing the natural orbitals as linear combinations of the atomic orbitals, an invariant description of electron behavior can be related to a localized picture of the atomic orbitals involved in bonding.4 On one hand, natural charges obtained in this way were shown to be sufficiently reliable and stable to computational parameters.5 On the other hand, Bader recognized an alternative orbital-independent description of electron localization based on the electron density.6 A bond path, as defined in the quantum theory of atoms in molecules (AIM), is a universal indicator of bonded interactions.6 Thus, we investigate the electron distribution and nature of bonded interactions at the acrylonitrile-Cu9(100) interfaces by both the BVWN/DZVP density functional theory method and the MP2/6-31+G* strategy within the conceptual framework provided natural population analysis and AIM theory. By this approach, 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.
We have elucidated a natural link between the total electron density and the spatial extents (not energies) of the HOMO and LUMO.1 Therefore, the success of frontier molecular orbital theory to rationalize the organic/metal charge migration process may depend on 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.
Acknowledgments. This work was supported by the European Commission project SANEME (IST-1999-10323). The author gratefully acknowledges the Killam Trusts for financial support.
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