Field-dependent electrode-chemisorbate bonding: sensitivity of vibrational stark effect and binding energetics to nature of surface coordination

Illustrative quantum-chemical calculations for selected atomic and molecular chemisorbates on Pt(111) (modeled as a finite cluster) are undertaken as a function of external field, F, by using Density Functional Theory (DFT) with the aim of ascertaining the sensitivity of the field-dependent metal-adsorbate binding energetics and vibrational frequencies (i.e., the vibrational Stark effect) to the nature of the surface coordination in electrochemical systems. The adsorbates selected - Cl, I, O, N, Na, NH3, and CO - include chemically important examples featuring both electron-withdrawing and -donating characteristics. The direction of metal-adsorbate charge polarization, characterized by the static dipole moment, µS, determines the binding energy-field (Eb-F) slopes, while the corresponding Stark-tuning behavior is controlled primarily by the dynamic dipole moment, µD. Significantly, analysis of the F-dependent sensitivity of µS and µD leads to a general adsorbate classification. For electronegative adsorbates, such as O and Cl, both µS and µD are negative, the opposite being the case for electropositive adsorbates. However, for systems forming dative-covalent rather than ionic bonds, as exemplified here by NH3 and CO, µS and µD have opposite signs. The latter behavior, including electron-donating and -withdrawing categories, arises from diminishing metal-chemisorbate orbital overlap, and hence the extent of charge polarization, as the bond is stretched. A clear-cut distinction between these different types of surface bonding is therefore obtainable by combining vibrational Stark-tuning and Eb-F slopes, as extracted from experimental data and/or DFT calculations. The former behavior is illustrated by means of potential-dependent Raman spectral data obtained in our laboratory.