| The application of Density Functional Theory (DFT) to electrochemical vibrational spectroscopy, using variable-field finite-cluster and periodic-slab models of surface-chemisorbate bonding, is considered, with the particular objective of developing a more complete understanding of how the experimentally accessible electrode-, adsorbate- and potential-dependent (i.e., Stark-tuning) vibrational frequency is a measure of specific chemical properties not accessible to experiment in electrochemical systems, such as the nature of the surface-adsorbate orbital and electron-transfer interactions, adlayer structure, bond geometry and binding energy. The field dependence of the bonding and vibrational properties of several archetypical adsorbates are examined, including the surface-adsorbate and intramolecular vibrations of carbon monoxide on Pt-group (111) surfaces and the surface-adsorbate vibrations of halogens, other monoatomic adsorbates and ammonia. A general classification between the nature of the surface-adsorbate interaction (i.e., electron donation or withdrawal and covalent or ionic) and the sign of the field-dependent binding energy and vibrational frequency slopes is presented so that interpretation or misinterpretation of potential-dependent vibrational frequencies in terms of bond lengths can be recognized. The relationships (or lack thereof) between vibrational frequency, binding energy and bond length for electrode-adsorbate interactions are also established. In addition to bonding properties of surface-adsorbate interactions, other aspects of electrochemical vibrational spectroscopy are explored, including vibrational band assignments and interpretation of the complex spectra of benzonitrile bound to Pt-group and coinage-metal surfaces, infrared and Raman vibrational band intensity predictions for various surface-adsorbate bonds, and influences of solvent coadsorption on the vibrational behavior of carbon monoxide on Pt(111) so that spectral differences in electrochemical and ultrahigh vacuum environments can be appreciated. |
