An 27Al NMR study of the interaction of water with AlPO4-11

Exptl. and spectral simulations of 27Al MAS and double rotation NMR spectra measured at 7.0 T proved the preferential hydration of 1 tetrahedral Al site in AlPO-11, transforming this site reversibly into octahedral Al. [on SciFinder (R)

29Si N.M.R longitudinal relaxation times in ZSM-5 zeolites

The presence of Pr4N+ templates causes an increase of Tl for 29Si nuclei in ZSM-5 zeolites of more than one order of magnitude, and relatively small changes in zeolite structure are important; due care is required in extracting quantitative data from 29Si magic angle spinning n.m.r. spectra of zeolites

Conformational equilibria of (E)-6-bromo-7-hydroxy-3,7-dimethyl-2-octene

The 1H NMR spectral parameters of the title compound have been determined in cyclohexane-d12 and p-dioxane-d8 solutions by means of high-frequency NMR. The results are interpreted in terms of conformational equilibria. Linear Electric Field (LEF) effects may play a major role in explaining the differences between the 13C NMR chemical shifts of the title compound and those of the model (E)2,6-dimethyl-2,6-octadiene. Some preliminary calculations of the LEF-induced shifts are presented, as well as the problems involved in the recognition of the LEF contributions to the experimental shift differences

29Si NMR Model Dissolution Study of the Degradation of Reversed Phases for High-Performance Liquid Chromatography

To simulate aging under chromatog. conditions, mono-, di-, and trifunctional octyl and octadecyl derivatized reversed-phase high-performance liq. chromatog. silicas were exposed to very aggressive mobile phases like the medium of 6 M NaOH in MeOH/H2O (50/50 vol./vol.). 29Si NMR was used to study the dissoln. characteristics of these phases by monitoring the concns. of the dissolving silane and silica structures. Dissoln. products of alkylchlorosilanes were used as model compds. for assignment purposes. Octadecyl phases appeared to degrade by dissoln. of the silica substrate; octadecylsilane structures were insol. under the exptl. conditions. Monofunctional octyl phases were shown to deteriorate through initial dissoln. of monomeric ligand silane particles, wheres for difunctional octyl phases, the silica backbone appeared to dissolve with the silane ligands still attached. The latter mechanism was also obsd. for the trifunctional octyl phases, but these phases resemble octadecyl phases, probably because the free ligand silane particles are almost insol. and the major cause of phase degrdn. is dissoln. of the silica substrat

Gel transfromations in silicas - a combined NMR and SAXS study

H-1 NMR transverse spin-spin relaxation times (T2) of water have been measured to monitor the aggregation and aging processes of low concentration silica gels. Along with H-1 NMR, small-angle X-ray scattering (SAXS) and physisorption measurements; were used to obtain additional information on aging. After acidification of water-glass (silica solution) the primary building units combine to form aggregates with fractal dimensionalities (decreasing T2). Gelation occurs when these growing aggregates form a percolating network, resulting in a minimum value of the T2 relaxation time. During and after these processes, rearrangements of silica take place (aging), a crucial step in the formation of pores. The aging of silica gel can be monitored by changing the relaxation behaviour (increasing T2). With SAXS a densification of the structure is indicated by an observed decrease in fractal dimensionalities and an increase in the radius of the primary building units, indicating a migration of silica from the tips of the aggregates towards the centre of the aggregates. Aging is a slow process compared with the aggregation of the primary particles, but it can be accelerated by adding catalytic quantities of fluorine or by preparation at higher pH or at higher temperatures. By using spin-spin relaxation measurements, reacting aqueous silica systems with a low solid content could be studied in situ. It is shown that hydroxyl and fluoride ions have different impacts on the silica systems, leading to different mechanisms. For aggregation and aging we observed (different) optimal fluorine concentration

Patching in reversed-phase high-performance liquid chromatographic materials studied by solid-state NMR spectrometry

The distribution of silane chains over the silica gel surface in reversed-phase high-performance liq. chromatog. (RP-HPLC) phases was studied with special attention being paid to surface homogeneity: there might be areas with high coverage and areas with low coverage of silane chains, hence clustering or patching of the silane chains can occur. Two RP phases were studied before and after well conditioned aging, together with 4 silylated silica gels (serving as models). Two solid-state NMR techniques (e.g.,1H-29Si dipolar dephasing 29Si cross-polarization magic angle spinning NMR and 13C spin-lattice relaxation in the rotating frame) were used. For the 4 model compds. with varying degrees of coverage, only differences in the NMR time consts. were obsd. between the maximally covered phase and the 3 less densely covered silica gels. This proves that silane chains on an RP phase with max. coverage are restricted in their mobilities with respect to the less densely packed materials. For the 2 non-aged RP phases, the silane chains are probably homogeneously distributed over the silica gel surface. Further, the non-aged RP phases were compared with their counterparts, aged under well defined exptl. conditions. After aging, no differences were found between the original and the aged phases, now indicating however, that patching had developed upon agein