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 (ν M - CO ) as well as the intramolecular (ν CO ) 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 ν 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 π-back-donation, although offset by progressively weaker σ-donation along with greater surface−chemisorbate steric repulsion. Although these individual orbital and steric interactions exert similar effects on the ν M - CO frequencies, the overall ν 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 ν 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 ν M - CO and r M - CO behavior into individual interactions exhibits close parallels, with π-bonding acting to markedly blue-shift ν M - CO and decrease r M - CO, being offset increasingly toward more negative fields by the effects of σ-bonding and steric repulsion. In contrast, the monotonically red-shifted ν CO frequencies and the correspondingly elongated C−O bond lengths, r CO, found toward negative fields arise chiefly from the well-known effects of dπ−2π* back-donation. A common correlation is observed between the field-dependent ν CO and r CO values for each of the metal−CO systems and even uncoordinated CO. The likely role of electrostatic factors in the ν M - CO − F dependencies is also considered: the increasing M → CO charge polarization seen toward negative fields can account qualitatively for the ν M - CO − F maxima. A semiquantitative agreement is evident with electrode potential-dependent ν M - CO and ν CO vibrational data, although ν M - CO − F maxima have yet to be observed experimentally.