In the last several years there has been a considerable growth of interest in the study of chemically modified silica gels due to their regular use in liquid chromatography and solid phase extraction processes. By far the most commonly used liquid chromatographic stationary phases are the n-alkyl modified silica gels. Silica gels have been reliably used as the stationary phase for separations in adsorption chromatography as they are very stable at high pressure and they yield reproducible separation efficiencies. Reversed phase chromatographic systems are current materials of choice for chromatographic separations and are prepared via surface modification of silica gels with n-alkylsilanes of varying chain lengths and functional groups. An understanding of the dynamical and conformational features of the bound n-alkyl ligands is useful for continuing optimisation of new chromatographic stationary phases.
The aim of the current work was to investigate the effect of n-alkyl chain length, sample temperature and segment position on the chain dynamics and its conformational state in silica gels. In the samples consisting of undeuterated n-alkyl chains the length of the chains covered C8, C9, C10, C12, C16, C18, C22 and C30 systems. These samples were primarily used for conformational studies by examination of specific infrared vibrational modes (i.e., methylene stretching and methylene wagging modes). Via these vibrational modes it was shown that the conformational disorder in such systems is highly dependent on the length of the n-alkyl moiety and was expressed by the number of various 2- or 3-bond gauche conformer sequences present in the n-alkyl chains. Generally there is a great tendency toward an increase in conformational order with increasing chain length, which can be attributed to a better chain packing of the longer chains. Likewise, a temperature increase typically gives rise to a larger conformational disorder, which holds for all systems examined here except for the C30 modified system where the temperature effect is negligible. To obtain site-specific conformational information the selectively deuterated n-nonyl, n-octadecyl and n-docosyl modified systems (labelled at positions C-4, C-6 and C-12) were used. With the help of the CD2 rocking and stretching vibrational modes, it was shown that the chain position nearer to the silica surface (i.e. the C-4 position) was conformationally more disordered than the other two positions in the longer chains of the C18 and C22 modified samples. Likewise, the C22 modified silica gels were more ordered than the corresponding C18 samples. The shorter chains in the C9 modified samples were much more disordered than the longer chains with similar gauche populations at the C-4 and C-6 positions with the conformational order almost independent of the temperature. On the basis of these IR studies, it is proposed that the better chain packing is responsible for the higher conformational order in longer chains. At low temperatures, the C18 and C22 chains possess conformationally ordered region in the middle parts of the chains whereas the ends are more disordered. With temperature increase a kind of "melting" process sets in from the free end towards the regions, which are closer to the silica surface. As a result, chains become randomly disordered at higher temperatures. With the help of the 2H NMR spectroscopy information about the motional processes, i.e., type and timescale was obtained. Here the same selectively deuterated compounds (silica gels modified with n-nonyl, n-octadecyl and n-docosyl chains) were used as discussed before in connection with the FT IR studies. The differences in the dynamics as a function of the n-alkyl chain lengths have been investigated by dynamic 2H NMR techniques, comprising variable-temperature lineshapes, spin-spin (T2) and spin-lattice (T1) relaxation experiments. The temperature-dependent lineshapes exhibit characteristic spectral effects due to the onset of molecular processes that are mainly conformational and overall types of chain motions. At low temperatures, most of the overall-type of motions are frozen and only local motions like trans-gauche isomerisation are taking place. With increasing temperature, the n-alkyl chain gain more mobility, and the motionally averaged NMR spectra at room temperature and above show that the nature of the molecular motions is almost isotropic. In addition, the shorter chains of the C9 modified silica gels are found to be more mobile at lower temperature than their longer counterparts. The effect of the chain position is less pronounced, with similar lineshapes almost independent of the actual labelled position. The spin-spin and spin-lattice relaxation times for these systems further support the observations based on the lineshape changes. The T1 and T2 minima for the C9 samples lie at much lower temperatures than the respective minima for the longer chains in the C18 and C22 modified silica gels. With the help of 2H NMR lineshape simulations it is shown that the motional processes taking place in the n-alkyl chains are the trans-gauche isomerisation and overall chain wobble motion. The simulations of the experimental lineshapes substantiate the statement that the shorter chains are more mobile than their longer counterparts. The derived activation energies reflect the local (trans-gauche isomerisation) as well as the overall nature (wobble motion) of these processes. The absolute values are consistent with the values reported from other studies of such motional processes. The conformational disorder is found to be highest in the shorter chains, which is also seen in the FT IR studies. The gauche populations present in these systems obtained from the lineshape simulations are found to be in similar order as those derived from the FT IR investigations.
The second group of systems examined here are polysiloxane carrier matrices that are basically highly cross-linked inorganic-organic hybrid polymers. They are used as stationary phase components on which the reactive centres like transitions metal complexes or organic molecules with functional groups are attached via long chain spacers. These interphases are utilised mainly as the polymeric backbone for carbonylation reactions like hydroformylation, stereoselective hydrogenation and regioselective partial hydrogenation. The co-condensation agents employed during the synthesis of the carrier matrix play an important role in controlling the density by determining the optimised cross-linkage within the matrix as well as the distance of the reactive centres and in avoiding leaching. Siloxane monomers are useful as co-condensing agents to heterogenise transition metal complexes and ligands. Hence, in the present work polymers based on monomers RSi(OMe)2(CH2)3C6D4(CH2)3(OMe)2SiR [R = Me (4), R = OMe (5)], bearing deuterated phenylene ring, were studied via 2H NMR techniques in the dry state. The difference in the resulting polysiloxane carrier matrices is based on their molecular structures. The matrices prepared from the monomer 4 (denoted as X4) have a lower cross-linking density than those obtained from 5 (denoted as X5) due to different number of methoxy groups in the monomers. In addition, each polysiloxane matrix was prepared in two different solvent media, THF and methanol. Hence, the main aim was to study the effect of the molecular structure as well as the synthetic route (i.e. the reaction media) on the dynamic features of these systems. The temperature dependent 2H NMR lineshapes showed that even at low temperatures the aromatic rings undergo 180° flips. The lineshape changes further demonstrate that the X4 matrices are more mobile than the X5 matrices. At higher temperatures, a further narrowing of the lineshapes was observed for the former systems, indicating the presence of additional motional processes. The spin-spin relaxation times of these polysiloxane matrices provide further support on this topic. The X5 series showed a typical T2 relaxation curve with one minima. For the X4 matrices a second minimum was seen at higher temperatures with very small absolute T2 values. 2H NMR lineshape simulations were further used to quantify the underlying motional processes, that were ascribed to 180° ring flip motions as well as overall wobble motions of larger molecular segments. The 180° ring flip process of the phenylene rings is preserved even at lower temperatures and has activation energies comparable to those reported for related systems. The overall wobble motions which involves larger molecular units are more dominant at higher temperatures and in general are at least one decade slower than the local ring flip motions. It has been found that the actual structure of the polysiloxane carrier matrices has a significant impact on the molecular mobility. The derived data clearly demonstrate that the higher cross-linking restricts the mobility of the polymeric backbone of the polysiloxane carrier matrices. In particular this holds true for the overall (reorientational) chain motions. The choice of the solvent used during the sol-gel process has only a minor influence on the matrix dynamics. The polysiloxane carrier matrices synthesised in methanol are slightly more mobile than those prepared in THF.
In the present work, it has been shown that the 2H NMR and FT IR spectroscopic techniques are powerful tools to get information on molecular structure and interactions in these interphase systems.
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