Vibrational spectroscopy as a probe of potential-dependent electrode-chemisorbate bonding: an assessment using finite-cluster density functional theory

Abstract

The application of Density Functional Theory (DFT) using variable-field finite-cluster models to the description of electrode-chemisorbate systems is considered, with the particular objective of assessing the relationships between potential-dependent metal-adsorbate vibrational frequencies and the chemical nature of the surface bond. The validity of employing finite-cluster models in variable homogeneous fields to describe potential-dependent electrode-chemisorbate bonding is discussed, including the choice of bulk-phase adsorbate reference states for ionic systems. The bond length-dependent charge polarization, as reflected in the so-called static ( μ S) and dynamic ( μ D) dipole moments, plays a central role in determining the field ( F)-dependence of the metal-adsorbate binding energies, E b, and vibrational force constants, K MA, respectively. A distinction between ‘ionic’ and ‘dative covalent’ surface bonding, based on whether the signs of μ S and μ D are the same or opposite, respectively, is demonstrated by means of DFT calculations for selected atomic and molecular adsorbates on Pt(111), and interpreted in terms of bond length-dependent charge polarization and orbital overlap. A basic consequence of these behavioral differences between μ S and μ D is to deny the occurrence of any uniform correlation between the E b– F and K MA– F behavior (i.e. between the potential-energy well depth and well ‘stiffness’, respectively). On the other hand, combined potential-dependent bond-energy and bond-frequency data should therefore be invaluable in ascertaining the nature of the surface coordination. A more uniform relationship, however, is observed between F-dependent force constants and equilibrium bond lengths. The applicability of these notions to the interpretation of Stark-tuning (i.e. frequency–potential) data for intramolecular as well as metal-adsorbate vibrations is also briefly discussed.