Field-dependent chemisorption of carbon monoxide on platinum-group (111) surfaces: relationships between binding energetics, geometries, and vibrational properties as assessed by density functional theory

The field-dependent frequency behavior of the metal-adsorbate (¿M-CO) as well as the intramolecular (¿CO) vibration of carbon monoxide chemisorbed in atop and threefold-hollow sites on three platinum-group (111) metal surfacesPt, Ir, and Pdis explored in relation to the metal-chemisorbate (M-CO) binding energetics and geometries by means of Density Functional Theory (DFT) calculations for finite clusters. This overall objectivehaving particular importance in electrochemical systemsof linking field-dependent vibrational, energetic, and geometric properties of the M-CO bond, prompted by the availability of potential-dependent ¿M-CO data at Pt-group electrodes from Raman spectroscopy, provides an opportunity to assess in quantum-chemical terms these surface-adsorbate binding parameters in relation to the extensively studied intramolecular CO vibration. The binding energies (-Eb) tend to increase toward negative fields (F), especially for hollow-site binding. An energy decomposition into specific orbital and steric interactions shows that this effect is driven primarily by enhanced p-back-donation, although offset by progressively weaker s-donation along with greater surface-chemisorbate steric repulsion. Although these individual orbital and steric interactions exert similar effects on the ¿M-CO frequencies, the overall ¿M-CO-F dependencies are notably different, typically displaying a broad maximum at moderate/large negative fields (ca. -0.3 to -0.5 V Å-1). Unlike the binding-energy behavior, these nonmonotonic ¿M-CO-F dependencies correlate roughly with the corresponding F-dependent M-CO equilibrium bond lengths, rM-CO. A decomposition of the field-dependent ¿M-CO and rM-CO behavior into individual interactions exhibits close parallels, with p-bonding acting to markedly blue-shift ¿M-CO and decrease rM-CO, being offset increasingly toward more negative fields by the effects of s-bonding and steric repulsion. In contrast, the monotonically red-shifted ¿CO frequencies and the correspondingly elongated C-O bond lengths, rCO, found toward negative fields arise chiefly from the well-known effects of dp-2p* back-donation. A common correlation is observed between the field-dependent ¿CO and rCO values for each of the metal-CO systems and even uncoordinated CO. The likely role of electrostatic factors in the ¿M-CO-F dependencies is also considered: the increasing M ¿ CO charge polarization seen toward negative fields can account qualitatively for the ¿M-CO-F maxima. A semiquantitative agreement is evident with electrode potential-dependent ¿M-CO and ¿CO vibrational data, although ¿M-CO-F maxima have yet to be observed experimentally.