Abstract
13C-NMR measurements of t-butylamine and benzylamine adsorbed on silica gel and octadecylsilyl (ODS)-silica added in various solutions were carried out. In the presence of silica gel, two well-resolved peaks were observed ascribable to species dissolved in solution and adsorbed on silica gel, respectively. An increase in salt concentration in aqueous solutions, however, provided no chemical shift difference between two states but only a narrowing of the 13C-NMR signals. Solvent shifts of the methyl and quaternary carbons were observed in the adsorption state of amines. These solvent shifts correlate closely with the values of the solvent strength for silica gel in the adsorption chromatography. The 13C-NMR shifts of amines in the adsorption state can be explained in terms of the protonation or quasi-protonation of amines with the silanol groups on the solid surface. Pretreatments of silica gel caused some changes of relative intensities between two signals but no changes in chemical shifts of these carbon resonances. On the other hand, in the solution containing ODS-silica the chemical shift difference between free and adsorbed amines was not observed. The above observations directly show that the adsorption of amine molecules on solid surface highly depends on the solubility of amines into solutions containing solid silicas.
