NMR and IR Study of Fluorobenzene and Hexafluorobenzene Adsorbed on Alumina

The adsorption of fluorobenzene (C<sub>6</sub>H<sub>5</sub>F) and hexafluorobenzene (C<sub>6</sub>F<sub>6</sub>) onto the surface of neutral alumina is investigated by reflectance IR spectroscopy, near-IR spectroscopy, and measurement of <sup>19</sup>F NMR chemical shift values. Chemical shifts are dependent on surface coverage and reveal multiple peaks where different adsorption environments occur. C<sub>6</sub>H<sub>5</sub>F appears to be adsorbed through polar interactions with surface hydroxyls, whereas C<sub>6</sub>F<sub>6</sub> 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.