INFRARED-SPECTROSCOPY OF MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - IONIC VERSUS INTERFACIAL SOLVATION BY ACETONE AND ACETONITRILE ON PT(111)

Infrared reflection-absorption spectroscopic (IRAS) measurements are r eported for acetone and acetonitrile dosed onto Pt(lll) in ultrahigh v acuum (uhv) both in the presence and absence of adsorbed potassium ato ms at 90 K with the objective of elucidating the nature of cation solv ation and its influence on surface-solvent interactions at these model electrochemical interfaces. Corresponding variations in the metal-uhv work function (Phi) evaluated with a Kelvin probe yield additional in sight into the interfacial electrostatic environment as a function of the alkali and solvent exposure. Acetone and acetonitrile are particul arly suitable solvents with which to evaluate the involvement of the s urface on interfacial ion solvation, since both interact specifically with Pt(lll), In both cases, chemisorption gives rise to vibrational s ignatures which are clearly distinguishable from those of the ''bulk-l ike'' multilayers, associated in particular with carbonyl (nu(C=O)) an d C=N stretching (nu(C=N)) vibrations. Acetone and acetonitrile intera ct with the Pt(lll) in two distinct fashions: the former binds via the oxygen lone pair of the carbonyl group, while the latter interacts vi a the nitrile pi orbitals. In addition, the nu(C=O) and nu(C=N) bands are sensitive to the coordination environment upon interaction with th e cation. The present spectral evidence indicates that both solvents, despite the disparate nature of their surface interactions, reorient e xtensively to solvate K+ in preference to the Pt surface, even for low K+ fractional coverages (theta(K) similar to 0.02). The initial catio n solvation by acetone yields the complete disappearance of the nu(C=O ) band at 1642 cm(-1), associated with chemisorbed acetone, being repl aced by a weaker feature at 1678 cm(-1) indicative of predominant inte ractions with K+ instead. In the acetonitrile case, an IRAS band at 22 43-9 cm(-1) indicates coordination to K+ via the nitrogen lone pair, I n addition, the significant (ca. 0.3 eV) Phi increases observed upon i on solvation support the presence of negative-outward C-delta+=O-delta - and C-delta+=N-delta- dipole orientations, Evidence of a substantial modifying influence of the metal surface on solvent-ion interactions, however, is provided by significant downshifts in the nu(C=O) and nu( C=N) vibrational frequencies with respect to those observed in acetone and acetonitrile solutions of alkali cations. The present results cal l into question the validity of conventional models of electrochemical interfaces which treat the metal surface-solvent and ion-solvent inte ractions as separate problems: a substantial synergy between these phe nomena is clearly evident.