Wide-line NMR studies of the influence of solvent polarity on the conformation and dynamics of silica-immobilized alkylamine and alkylamide ligands

In the present study, selectively deuterated alkylamine ligands were prepar ed and subsequently used to modify the surface of porous silica. The result ing material was then reacted with either hexanoyl chloride or heptanoyl ch loride to produce the corresponding alkylamide-modified surfaces. The wide- line H-2 NMR spectra of the intermediate and final materials were measured in the dry state and in the presence of both protic and aprotic solvents. T he shapes of the resulting resonances are complex and are made up of spectr ally broad and narrow components with widths in the restricted motion range (i.e., about 40 kHz) and widths characteristic of more motionally dynamic species. The relative contribution of the broad component is dominant in th e dry state and in aprotic solvent spectra and is significantly decreased o r absent in the spectra collected in polar solvents. These results can be e xplained via changes in the degree of interfacial hydrogen bonding between the amine and amide groups in the attached ligands and residual surface sil anols.