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 (v(M)-co) as well as the intramolecular (vco) vibration of carbon monoxide chemisorbed in atop and threefold-hollow sites on three platinum-group (111) metal surfaces-Pt, Ir, and Pd-is 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 objective-having particular importance in electrochemical systems-of linking field-dependent vibrational, energetic, and geometric properties of the M-CO bond, prompted by the availability of potential-dependent V-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 (-E-b) 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 pi -back-donation, although offset by progressively weaker a-donation along with greater surface-chemisorbate steric repulsion. Although these individual. orbital and steric interactions exert similar effects on the v(M)-co frequencies, the overall v(M-CO)-F dependencies are notably different, typically displaying a broad maximum at moderate/large negative fields (ca. -0.3 to -0.5 V Angstrom (-1)). Unlike the binding-energy behavior, these nonmonotonic v(M-CO)-F dependencies correlate roughly with the corresponding F-dependent M-CO equilibrium bond lengths, r(M-CO) A decomposition of the field-dependent v(M-CO) and r(M-CO) behavior into individual interactions exhibits close parallels, with rr-bonding acting to markedly blueshift v(M-CO) and decrease r(M-CO), being offset increasingly toward more negative fields by the effects of a-bonding and steric repulsion. In contrast, the monotonically red-shifted V-CO frequencies and the correspondingly elongated C-O band lengths, r(CO), found toward negative fields arise chiefly from the well-known effects of d pi -2 pi* back-donation. A common correlation is observed between the field-dependent vco and vco values for each of the metal-CO systems and even uncoordinated CO. The likely role of electrostatic factors in the v(M-CO)-F dependencies is also considered: the increasing M --> CO charge polarization seen toward negative fields can account qualitatively for the v(M-CO)-F maxima. A semiquantitative agreement is evident with electrode potential-dependent y(M-CO) and v(CO) vibrational data, although YM-CO-F maxima have yet to be observed experimentally.