HPLC and (2)H NMR studies of the interfacial properties of silica immobilized alkylamine and alkylamide ligands

In the present study, selectively deuterated alkylamine ligands were synthesized and subsequently used to modify the surface of porous silica. The alkylamine modified silica was then reacted with alkyl acid chlorides with varying chain lengths to produce a series of alkylamide surfaces. Subsequently, wide-line {dollar}sp2{dollar}H NMR spectra of these surfaces were collected in the presence of protic solvents such as methanol and acetonitrile and aprotic solvents like hexane, as well as in the dry state. The resulting line shapes have been found to correlate directly with the degree of interfacial hydrogen bonding between the attached ligands and the silica surface. Although the degree of hydrogen bonding increased with decreasing solvent polarity, the extent of this increase varied between surfaces. In addition, in all of the solvents studies the amine stationary phase exhibited a higher degree of hydrogen bonding as determined via {dollar}sp2{dollar}H NMR measurements than any of the amide stationary phases.; In addition, {dollar}sp2{dollar}H NMR experiments were carried out under binary solvent conditions (i.e., varying concentrations of methanol and water) similar to mobile phase compositions commonly used in high performance liquid chromatography (HPLC). Subsequently, wide-line {dollar}sp2{dollar}H NMR spectra of these surfaces were collected under binary solvent conditions (e.g., varying concentrations of methanol and water) similar to mobile phase compositions commonly used in high performance liquid chromatography (HPLC). Chromatographic peak asymmetry measurements obtained under binary mobile phase solvent conditions identical to the {dollar}sp2{dollar}H NMR studies using aniline as a test solute were performed on these phases as well as on an unmodified surface. Comparison of the wide-line NMR and HPLC results indicate that as the degree of hydrogen bonding determined via {dollar}sp2{dollar}H NMR increases, the relative degree of chromatographic peak asymmetry decreases, thereby substantiating the internal masking theory of the chromatographic behavior of these stationary phases.; Finally, in an attempt to spectrally separate the {dollar}sp2{dollar}H NMR signal arising from the hydrogen bonded component, the spin-lattice relaxation experiments were performed with the amine and amide stationary phases in contact with both polar and apolar solvents. The results indicate the presence of two distinct species, one narrower component (3-10 kHz) and one broader component (44 kHz that relax at significantly different rates). The broader component which had a shorter T{dollar}sb1{dollar} time arises from restricted motion (i.e., hydrogen bonding) while the narrower component correlates to the less hindered nearly isotropic motions. This component was determined to be independent of solvent polarity, solvent viscosity and bonded ligand type for the phases studied. The narrow spectral feature which had a longer spin-lattice relaxation time correlates to the less hindered nearly isotropic motions from bonded ligands which do not undergo internal masking with the silica support. The T{dollar}sb1{dollar} times of the narrow component varied with solvent viscosity, solvent polarity, and bonded ligand type. (Abstract shortened by UMI.)...