NMR investigations of 13C labeled alkyl modified silica

Abstract

In liquid chromatography, a better understanding of the mechanisms involved in solute interaction with bonded alkyl groups is a topic which continues to generate interest. The most commonly employed stationary phases are octyl and octyldecyl modified silicas. Such surfaces typically are synthesized via monoor trireactive silanes. In the past, the bonded groups on these surfaces have been viewed as rather rigid hydrophobic “arms” (1, 2). During the last few years, a more dynamic nature has been suggested (37). The lack of nonchromatographic evidence to support either of these views is clearly evident. Although nuclear magnetic resonance and Fourier-transform infrared spectroscopy have been utilized to examine chemically modified phases (8, 9), such studies have been carried out under conditions which little resemble the chromatographic environment. In the case of NMR experiments, cross-polarization magic-angle spinning has been the technique used. Thus, the information obtained, although interesting, does not directly address solute-surface interaction where solvation plays a major role. Conventional 13C FT NMR has been used widely to investigate molecular dynamics and phase transitions. The application of this technique to chromatographic type surfaces has been limited due to problems of signal intensity and resonance width. Weak signals result from the low surface coverage typical of liquid chromatographic packings (5 to 10%) coupled with the low natural abundance of 13C nuclei. Broad resonances arise from hindered molecular mobility. These drawbacks may be overcome by selectively labeling individual methylene units in the attached hydrocarbon chains. Since sufficient enrichment produces a spectrum with a single resonance, an added advantage is that interpretation is simpler. Likewise, such experiments also may be carried out on low field instrumentation. Similar approaches have been reported by others to study the molecular motion of macromolecules and adsorbed species (10, II). The labeled alkylchlorosilanes given in the first column of Table 1 were synthesized as previously reported (12, 13). Chemical shift and percent enrichment information for the labeled carbon in the synthesized monomer also are included. The chemical shift values relative to TMS were collected in CDCl,. Chromatographic surfaces were prepared by reacting each of the labeled chlorosilane monomers with LiChrosorb SI-60 10 P silica using conditions described elsewhere