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C(18) phases prepared by different synthetic pathways are examined by solid-state NMR spectroscopy. Silane functionality is clearly indicated by (29)Si CP/MAS NMR spectroscopy. Order and mobility of the alkyl chains are investigated with high-speed (1)H MAS and (13)C CP/MAS NMR spectroscopy. Differences in coverage are monitored by (1)H line widths,( 13)C chemical shifts, (13)C cross-polarization constants, and (1)H relaxation times in the rotating frame. It is shown that C(18) phases prepared by the surface polymerization technique exhibit a more regular surface coverage than sorbents prepared by conventional polymeric synthesis. The findings from solid-state NMR investigations are discussed in the context of liquid chromatography (LC) separations of linear and bulky polycyclic aromatic hydrocarbon (PAH) solutes.

The relationship between alkyl phase structure and chromatographic performance is investigated for a series of docosyl (C22)-modified silica surfaces. The results of solid-state nuclear magnetic resonance (NMR) spectroscopy and fluorescence spectroscopy are evaluated and correlated with liquid chromatographic retention for relevant shape-selective separations. A set of four different stationary phases was prepared by solution and surface polymerization approaches, yielding materials with surface coverages ranging from 3.6 to 7.0 μmol/m2. 13C cross polarization magic angle spinning (CP/MAS) NMR spectra indicate that a predominance of trans conformations exists for high-coverage C22 phases (>4.0 μmol/m2). Two-dimensional solid-state NMR spectroscopy (wide line separation, WISE) was utilized to evaluate the mobility of the trans and gauche alkyl chain conformations. Temperature-dependent 13C CP/MAS NMR measurements of the bonded phases exhibit large differences in the dynamic behavior of the immobilized C22 chains. Unusually high chain rigidity was found for the self-assembled monolayer C22 phase (7.0 μmol/m2). Fluorescence lifetime measurements of 1,6-diphenylhexatriene (DPH) exhibit two different lifetimes of τF ≈ 1 and 7 ns, which are ascribed to probe molecule populations in the mobile and bonded phases, respectively. Quantitative evaluation of the fluorescence decay curves shows that the partitioning of DPH into the alkyl phase is favored at higher surface coverages, reaching a maximum at a ligand density of 4.9 μmol/m2. Time-resolved fluorescence anisotropy measurements also revealed that probe mobility was minimized at this surface coverage. With increasing temperature, the mobility of DPH was found to increase and the fraction of sorbed molecules to decrease. A shape selectivity test mixture containing five polycyclic aromatic hydrocarbons including DPH was employed for temperature-dependent chromatographic studies. In accord with the spectroscopic results, shape selectivity is enhanced at low temperatures and at high surface coverages. The combination of these spectroscopic and chromatographic tools provides a wealth of information on the surface morphology of systematically prepared C22 sorbents and greater insight on the molecular recognition process in liquid chromatography.

Figure 10. Model for the shape recognition of the C30 phase at high (left) and subambient (right) temperatures.