IN-SITU SURFACE VIBRATIONAL SPECTROSCOPY OF THE VAPOR SOLID AND LIQUID-SOLID INTERFACES OF ACETONITRILE ON ZRO2

Infrared-visible sum-frequency generation (SFG) is employed in an internal reflection geometry to investigate, in situ, the adsorption of acetonitrile on ZrO2 from liquid and vapor. This configuration induces large field enhancements near the critical angle that increase signal levels by more than 100-fold compared with reflection from the liquid or vapor side of the interface. Acetonitrile is a widely used, highly polar, chromatographic solvent that has simple structure and strong stretching modes with large frequency separation, making it an ideal liquid for SFG studies. The C=N and C=H stretching modes are observed at submonolayer and neat liquid coverages on ZrO2. The relative polarization dependent SFG oscillator strengths are determined providing estimates of the binding energy, 4.7+/-0.2 kcal/mol, and the mean orientation of the molecular axis to the surface, 22+/-2-degrees. Spectra of CD3CN show resolvable structure in the C=N and C-D stretching regions, suggesting the presence of aggregates on the surface. SFG measurements as a function of CH3CN concentration in CCl4, at equilibrium, is shown to be consistent with the formation of dimers and higher order surface species.