Periodic trends in electrode-chemisorbate bonding: benzonitrile on platinum-group and other noble metals as probed by surface-enhanced Raman spectroscopy combined with density functional theory.

Abstract

Detailed intramolecular vibrational spectra obtained by means of surface-enhanced Raman scattering (SERS) for benzonitrile adsorbed on seven electrode surfaces-four Pt-group metals (platinum, palladium, rhodium, and iridium) and the Group IB metals (copper, silver, and gold)-are reported with the aim of exploring the metal-dependent nature of surface-chemisorbate interactions. The Pt-group surfaces were prepared as ultrathin electrodeposited films on gold, enabling the SERS activity inherent to the substrate to be imparted to the overlayer material. Benzonitrile was selected as a "model" organic adsorbate since it displays a rich array of coupled aromatic ring as well as substituent modes which collectively can provide insight into the various molecular perturbations induced by surface coordination via the nitrile substituent. The experimental spectra are compared with ab initio calculations of vibrational frequencies, bond geometries, and charge distributions obtained by means of Density Functional Theory (DFT), which yields valuable insight into the underlying structural reasons for the sensitivity of the experimental coordination-induced frequency shifts to the nature of the intramolecular mode and the metal surface. The DFT results also form an invaluable aid in making SER spectral assignments, along with providing detailed information on the coupled atomic displacements involved in each vibrational mode. Benzonitrile surface coordination was modeled in the DFT calculations by binding the nitrile group to metal atoms and small metal clusters. While the majority of the aromatic-ring SER frequencies are altered only slightly (approximately < 5 cm(-1)) upon surface coordination, several modes (especially nu(1), nu(6a)) are blue-shifted substantially (by up to 50 cm(-1)). These shifts were identified by DFT as arising from mode coupling to the nitrile substituent, especially involving the C-CN bond that is compressed upon nitrile coordination, associated with metal-adsorbate back-donation. The small (<5 cm(-1)) red-shifts seen for ring vibrations not involving coupled substituent motion apparently arise from increased antibonding aromatic electron density. The metal-dependent frequency shifts seen for these coupled aromatic vibrations as well as for the more localized C-N nitrile stretching mode are consistent with increased back-donation anticipated in the sequence d(10) < d(9) < d(8) within a given Periodic row. Overall, the findings provide a benchmark illustration of the virtues of DFT in interpreting complex vibrational spectra for larger polyatomic adsorbates.