Bruce L. Hawkins

Characterization of silanol reactivity and acidity on octadecyl-bonded chromatographic supports by29Si solidstate nuclear magnetic resonance spectroscopy and surface titration

Abstract Surface analysis and chromatographic data are used to examine the structure of surface groups in a reversed-phase chromatographic environment involving an octadecyl-derivatized silica substrate. The support is characterized by 29 Si solid-state nuclear magnetic resonance spectroscopy and surface titration, and results are compared to other spectroscopic methods for the quantitation of surface species. Quantitative nuclear magnetic resonance data permits examination of the number and reactivity of single versus geminal hydroxyl sites as a function of octadecyldimethylsilyl coverage. It is postulated that an acidic, reactive subset of surface silanols exists that includes a large proportion of the geminal silanol sites, and a subset of the single silanol sites including hydrogen-bonded species such as the vicinal silanols. Knowledge of surface structure helps to explain various types of chromatographic behavior including the inaccessibility of some surface region silanols at moderate silane coverages, the partition-like behavior of octadecyl substrates, and the pH-dependent retention of non-polar solutes in reversed-phase chromatography.

Cross polarization, magic-angle spinning 13C nuclear magnetic resonance spectroscopy of soil humic fractions

Abstract Cross polarization, magic-angle spinning 13 C nuclear magnetic resonance spectroscopy was used to characterize humic fractions isolated from different soils. The acid fractions are more aromatic than the humin fractions, probably due to the higher polyssaccharide content of humins. However, fulvic acid fractions are more aromatic than the corresponding humic acid and humin fractions. These results can be interpreted in terms of the isolation procedure, because the high affinity of Polyclar AT for phenols results in higher aromaticities as compared with other isolation methods (e.g. charcoal).

Surface characterization of systems with diphenylphosphinoethylsilane immobilized on silica gel

Abstract Diphenylphosphinoethyltriethoxysilane and alkylsilylating reagents have been reacted with silica gel to produce a surface with anchored ligands within a hydrophobic matrix. The extent of surface coverage has been determined using elemental analysis data. Cross-polarization/magic angle spinning, nuclear magnetic resonance has contributed toward a description of the diphenylphos-phinoethylsilyl anchored on the surface. Using high-performance liquid chromatography, the extent of substrate coverage and ligand stability were evaluated. The experimental data provide some insight into the synthetic procedure which is most effective for the preparation of stable diphenyl-phosphine moieties immobilized on silica gel. Since diphenylphosphinoethylsilyl is often used to immobilize metal complexes, the information should be useful for optimizing systems for use in heterogeneous catalysis.

Synthesis, solid-state nuclear magnetic resonance characterization and chromatographic evaluation of a diphenylphosphine-octadecylsilane hybrid bonded phase

Abstract Mixed-mode bonded phases consisting of diphenylphosphinoethylsilane concurrently immobilized with octadecyldimethylsilane were synthesized. Elemental analysis and solid-state nuclear magnetic resonance (NMR) spectroscopy were employed in order to characterize the surface structure. Using the techniques of cross-polarization/magic angle spinning, 13C, 31P and 29Si NMR spectra were obtained with a resolution approaching that usually obtained for liquids. The novel diphenylphosphine-octadecylsilane (DPP-ODS) bonded phase was then evaluated for its chromatographic potential in the separation of a series of aromatic compounds. Preliminary results indicate that the column prepared from this material is a ”hybrid“, exhibiting reversed-phase character under hydroorganic eluent conditions, and polar character when the eluent is non-polar.

Off resonance cross-polarization: A technique to reduce rf power requirements for magnetization transfer experiments in solids

Cross-polarization of nuclei (I) with a low magnetogyric ratio is one of the pillars of many modern solid state NMR experiments, enhancing the sensitivity often by an order of magnitude. Cross-polarization is based upon cross-relaxation in the rotating frame between different nuclides, and occurs when the Hartmann-Hahn condition (2) is met. For magnetization transfer from spin Z to spin S, this condition requires radiofrequency fields, HI and Hs, applied simultaneously close to resonance for spin Z and spin S, respectively. H, and Hs must have the relation r&Jr = rsfk