NMR and IR Study of Fluorobenzene and Hexafluorobenzene Adsorbed on Alumina

The adsorption of fluorobenzene (C₆H₅F) and hexafluorobenzene (C₆F₆) onto the surface of neutral alumina is investigated by reflectance IR spectroscopy, near-IR spectroscopy, and measurement of ¹⁹F NMR chemical shift values. Chemical shifts are dependent on surface coverage and reveal multiple peaks where different adsorption environments occur. C₆H₅F appears to be adsorbed through polar interactions with surface hydroxyls, whereas C₆F₆ shows separate resonances for the first layer of coverage and outer layers. Available surface areas are estimated, and reorientation of adsorbed hexafluorobenzene is proposed to account for chemical shift behavior and differences between calculated and measured monolayer coverage.