POTENTIAL-DEPENDENT METAL-ADSORBATE STRETCHING FREQUENCIES FOR CARBON-MONOXIDE ON TRANSITION-METAL ELECTRODES - CHEMICAL BONDING VERSUS ELECTROSTATIC-FIELD EFFECTS

The dependence of the metal-carbon vibrational frequency (nu(M-C)) upo n electrode potential (E) for saturated CO adlayers on palladium, plat inum, rhodium, and iridium film electrodes is examined in comparison w ith that for the well-studied intramolecular (C-O) vibration (nu(CO)) by means of surface-enhanced Raman spectroscopy (SERS) in order to eva luate the likely roles of chemical bonding versus the electrostatic fi eld in the electrochemical Stark effect. In each case, the d nu(M-C)/d E values are negative, from ca. -10 to -20. cm(-1) V-1, contrasting th e positive d nu(CO)/dE values, ca, 30 to 60 cm(-1) V(-)1, observed for adsorbed CO on these Pt group metals. The findings are compared with the predictions of theoretical treatments which account variously for the roles of the interfacial electrostatic field (i.e., the classical vibrational Stark effect) and potential-dependent chemical bonding (i. e., metal-adsorbate orbital overlap). It is necessary to invoke that t he latter factor is exerting a major role in the surface-adsorbate int eractions in order to account for the observed nu(M-C)-E dependences.