Surface Enhanced Infrared Absorption Spectroscopy Studies of DMAP Adsorption on Gold Surfaces

Attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements have been employed to study the adsorption of dimethylaminopyridine (DMAP)and its conjugate acid (DMAPH(+)) on gold surfaces as a function of applied potential and solution pH. Based on our transmission measurements, we have been able to demonstrate that the acid/base forms of this pyridine derivative can be readily differentiated due to their distinct IR signals. When the solution pH is equal to the pK(a) of DMAPH(+), we demonstrate that the adsorbing species is DMAP, oriented with its, heterocyclic ring perpendicular to the electrode surface. In acidic electrolytes, our SEIRAS data provide direct spectroscopic evidence of DMAP monolayer formation even though the pH is 5 units below the pK(a) of the conjugate acid. Our data support a potential induced deprotonation of the endocyclic nitrogen and resulting coordination of the nitrogen lone pair to the gold surface. Both of these results confirm our existing model of DMAP adsorption previously based solely on electrochemical measurements. However, the present SEIRAS study also indicates that, at low pH, DMAPH(+) can electrostatically coordinate to very negatively charged surfaces. This mode of adsorption was previously unobserved, illustrating the ability of in situ spectroscopic techniques to reveal new information that is not apparent from tiaditional electrochemical techniques such as differential capacity and chronocoulometry.