Interfacial Lewis Acid-Base Adduct Formation Probed by Vibrational Spectroscopy.

Understanding Lewis pair (LP) interactions at heterogeneous environments is important for controlling surface reactions. We report the formation of interfacial Lewis adducts with tris(pentafluorophenyl)borane as the Lewis acid and 4-mercaptobenzonitrile attached to gold as the Lewis base. We use the nitrile vibrational frequency as a probe of adduct strength, with stronger adducts leading to larger frequencies. The vibrational frequency shifts of the surface adducts were measured via sum frequency generation spectroscopy and compared to the frequency shifts of bulk adducts. Our results show a distinctly smaller frequency shift for the surface adducts compared to the bulk, indicating a weaker Lewis acid-base interaction near the surface. We explore three possible origins of this difference: interfacial frustration, surface electric fields, and electronic energy level alignment. We highlight the relevance of each and note that likely more than one of them affect the observed surface LP interactions.