ROLE OF THE DOUBLE-LAYER CATION ON THE POTENTIAL-DEPENDENT STRETCHING FREQUENCIES AND BINDING GEOMETRIES OF CARBON-MONOXIDE AT PLATINUM NONAQUEOUS INTERFACES

The influences of the double-layer cation upon the electrode potential-dependent infrared spectral properties of saturated CO adlayers on polycrystalline platinum have been examined in acetonitrile, methanol, tetrahydrofuran (THF), and dichloromethane. These solvents were chosen so to yield a range of dielectric and solvating environments. Two classes of electrolytes were examined, involving tetraalkyl-ammonium and alkali-metal cations. For each solvent containing the former electrolytes, near-exclusive terminal CO coordination was observed throughout the accessible potential range (from ca. -2.5 to 1 V vs ferrocenium-ferrocene), as evidenced by a single potential-dependent C-O stretching band at ca. 2040-2090 cm-1. The nu(CO) frequency-potential slopes depend significantly on the size of the tetraalkyl-ammonium cation. This dependence is approximately consistent with the expectations of a simple double-layer model featuring a linear potential drop throughout the inner layer, with the position of the outer Helmholtz plane being determined by the CO adlayer thickness plus the unsolvated cation radius. The infrared spectra in alkali-metal (Li+, Na+, K+) electrolytes yielded a similar terminal nu(CO) feature, which is, however, replaced entirely at relatively negative potentials by a band at ca. 1730-1800 cm-1. This effect is ascribed to a potential-induced conversion from terminal to multifold CO coordination geometries, driven by Lewis acid-base interactions between the partly desolvated alkali cations and the CO adlayer. Analogies are noted with cation-induced coordination shifts in polynuclear carbonyl complexes.