Nuclear Magnetic Resonance Measurements of Line Width, Chemical Shift, and Intensity for Molecules Adsorbed on Silica

Abstract

High-resolution NMR spectra were taken of molecules adsorbed on silica to determine the effects of specific surface interactions on line widths, chemical shifts, equilibria, and freezing points. Line widths were measured for cyclohexane, benzene, acetone, and methanol adsorbed at monolayer coverage. Hydrogen bonding to the surface was evident for the latter two compounds from the decrease in line width when the silica was silanized. In addition, the hydroxyl hydrogen of methanol was found to undergo exchange with the surface. After silanization, chemical-shift changes were noted for cyclohexane, p-xylene, mesitylene, acetone, and methyl acetate. Attempts were made to explain the magnitude of the changes by comparison with data from infrared spectroscopy and gas-solid chromatography. Chemical shifts of mixtures of adsorbates were studied to gain additional information on relative adsorption strengths. The tautomeric equilibrium of adsorbed acetylacetone was measured from the relative integrated intensities of the keto and enol resonances. The enol form was favored more on silanized silica than on hydrated silica. Relative intensities were also used to study the freezing of adsorbates.