Variable temperature 2H NMR and FT IR studies of n-alkyl modified silica gels

The conformational behavior of silica gels, chemically modified with selectively deuterated n-nonyl, n-octadecyl, and n-docosyl chains (at positions C-4, C-6, and G 12) in the dry state, were examined over a broad temperature range (123-353 K) via H-2 NMR and FT IR spectroscopic techniques. The aim of this study is to investigate the influence of temperature, chain length, and position on the conformational disorder and the chain dynamics in the attached n-alkyl chains. For this purpose the conformation- sensitive symmetric and antisymmetric stretching as well as rocking modes of selectively deuterated methylene groups were exploited. The CD2 rocking band provided absolute amounts of gauche conformers at a specific site in the tethered n-alkyl chains. Likewise, a comprehensive H-2 NMR line shape analysis was performed which focused on the dynamical features of these systems. In general, it could be shown that the conformational state of the n-alkyl chains in fact depends on the aforementioned external parameters. Thus, the percentage of gauche conformers is found to increase with temperature, while shorter n-alkyl chains (i.e., n-nonyl systems) are more disordered than their longer counterparts (i.e., n-octadecyl and n-docosyl chains). Likewise, the effect of chain position on the order of the system is more pronounced in the longer n-docosyl and n-octadecyl chains than in the n-nonyl chains. The variable temperature H-2 NMR data were analyzed with a theoretical motional model which includes local (trans-gauche isomerization) and overall types of motion (chain reorientations). It was shown that the n-alkyl chain dynamics also depends on the actual n-alkyl chain length; that is, the n-nonyl chains are more mobile than the longer n-octadecyl and n-docosyl chains. The derived kinetic parameters were found to be consistent with the underlying motional models.