Ab initio cluster model study of electric field effects for terminal and bridge bonded CO on Pt(100)

cis-{Pt(NH3)2(L)}2+/+ (L = Cl, H2O, NH3) Binding to Purines and CO: Does π-Back-Donation Play a Role?

The ability of cis-[Pt(NH(3))(2)(L)](2+/+), a molecular fragment of the anticancer drug cisplatin, to bind to purines and CO by pi-back-donation from Pt to the ligand was examined computationally. Optimized geometries and computed vibrational frequencies suggest that cis-[Pt(NH(3))(2)(L)](2+/+) (L = Cl, H(2)O, NH(3)) is a poor pi-donor and that pi-back-donation does not play an important role for Pt(II)-ligand interactions in general.

What can we learn about electrode-chemisorbate bonding energetics from vibrational spectroscopy? An assessment from density functional theory

An analysis is presented of the manner and extent to which the metal surface-chemisorbate bond energetics and geometries as functions of the metal and the applied field can be correlated with vibrational frequencies, with specific reference to electrochemical systems. Emphasis is placed on metal-adsorbate stretching frequencies, vM-A, using oxygen and carbon monoxide chemisorption as illustrative examples; the intramolecular stretch (vCO) of the latter adsorbate is also examined in view of the extensive experimental utilization of this vibrational mode. Results based on Density Functional Theory (DFT) are presented for finite-cluster models of Pt-group and coinage-metal (111) surfaces. The DFT calculations enable a separation between steric repulsion and orbital contributions to the potential-energy surface (PES), and additionally, in the case of CO chemisorption, between the 5sigma and 27pi* orbital components. While rough metal-dependent correlations between vM-A and the surface binding energy, -Eb, are observed, such a relationship is not expected in general. Thus for CO chemisorption, the variations in -Eb are affected more by changes in the 5sigma rather than 2pi* orbital energies, whereas these components influence the M-CO stretching frequency, vM-CO, to a comparable extent. Moreover, the metal-dependent vCO frequencies do not correlate even qualitatively with -Eb; this is because the former are dominated by 2pi*, rather than 5sigma, interactions. The factors influencing the field (F) (and hence electrode potential) dependence of Eb versus VM-CO and vCO mirror somewhat this pattern. While the field-dependent influence of the 5sigma and 2pi* interactions are offsetting, the latter affects the vM CO-F, and especially the vCO-F, behavior to a greater extent than the -Eb-F dependence. Generally, then, the lack of broad-based correlations between chemisorbate vibrational frequencies and binding energetics can be understood in terms of the differing influence of the individual interaction components on the PES well shape and depth. The description of such bonding contributions in terms of dipole-moment parameters is illustrated. Also considered are relations between vibrational frequencies and bond lengths.

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, NH(3), 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, mu(S), determines the binding energy-field (E(b-F) slopes, while the corresponding Stark-tuning behavior is controlled primarily by the dynamic dipole moment, mu(D). Significantly, analysis of the F-dependent sensitivity of mu(S) and mu(D) leads to a general adsorbate classification. For electronegative adsorbates, such as O and Cl, both mu(S) and mu(D) are negative, the opposite being the case for electropositive adsorbates. However, for systems forming dative-covalent rather than ionic bonds, as exemplified here by NH(3) and CO, mu(S) and mu(D) 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 E(b)-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.