Vibrational spectroscopy of carbon monoxide, acetonitrile, and phenylalanine adsorbed on liquid ∣ electrode interfaces by sum frequency generation

This paper reviews recent work carried out in our laboratory on the application of sum frequency generation (SFG) to investigate liquid ∣ electrode interfaces in the presence of an external potential. First, we present a study of CO oxidation on a Pt(111) electrode. Although the CO stretching frequency observed by SFG agrees with that observed using IR spectroscopy, the intensity behaves differently from that observed by IR. While the intensity of IR absorption decreases due to CO oxidation, a 360% enhancement of SFG intensity was observed in the pre-ignition potential region. This suggests that the bonding state of CO is significantly perturbed in the pre-ignition potential region. Secondly, SFG was used to study the potential induced structural change of acetonitrile adsorbed on a Pt(111) electrode. SFG spectra indicate that acetonitrile reorients in response to the electrode potential with the CC bond perpendicular to the electrode surface. The orientation is predominately with the CH3 group toward the metal at 200 mV (vs. a reversible hydrogen electrode, RHE) and with the CN group toward the metal at 1200 mV. Finally, the adsorption of phenylalanine on a glassy carbon electrode was studied as a function of electrode potential. The SFG signal suggests that the phenylalanine is adsorbed on the electrode with the phenyl ring and the ammonium group nearly parallel to the surface. The dipole of the methylene group tilts toward the electrode surface when the applied potential increases.