Jw Jan de Haan

The molecular basis of aging of aqueous silica gel

Abstract The influence of aging on the local structure of aqueous silica gels has been investigated using small angle X-ray scattering (SAXS), solid state nuclear magnetic resonance (NMR), and nitrogen sorption techniques (BET). From SAXS experiments it is deduced that aging of aqueous silica aggregates is a process of migration of active, dissolved silicate species (e.g., monomeric silicic acid) from the more soluble and less dense places within silica aggregates (i.e., the peripheral primary particles) to the less soluble and denser parts within the aggregates (i.e., the core of the aggregates). This process results not only in an increase in size of the scattering primary particles but also in an increase in the gradient of mass density, as such corresponding to a decrease in fractal dimensionality of the silica aggregates from D = 2.25 to D = 2.0. The rate at which aging processes occur is strongly effected by the pH value of the solution. Addition of fluorine anions to the polymerizing silicate solutions causes an increase in rate of restructuring as well. Nitrogen sorption measurements on freeze-dried samples of the gels show that a porous silica structure is only obtained in case the silica gel network has been reinforced through reorganization of the fragile aggregates. The aged silica gel consists of an ensemble of densified aggregates formed by primary particles with sizes in the nanometer range.

Gel transformations in silicas: A combined NMR and SAXS study

Abstract 1H 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 with1H 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 concentrations.

13C MAS n.m.r, study of solid poly(vinyl alcohol) and of ethylene-vinyl alcohol copolymers

Abstract High resolution 13 C n.m.r. spectra of solid ethylene-vinyl alcohol (EVOH) copolymers have been observed, exhibiting a splitting of the methine carbon resonance. This splitting could be explained taking into account a rather unique combination of both tacticity effects and sequence effects. An explanation is offered for the values of the 13 C n.m.r. chemical shifts which is based on substituent induced shifts taken from rigid compounds rather than hydrogen bonding.

Intramolecular base–backbone association in 8-bromo-2′,3′-O-isopropylidene-adenosine. Detection of an O(5′)–H ⋯ N(3) hydrogen bond via a long range H(5″)–N(3) spin–spin coupling

A completely rigid conformation is found for the syn-nucleoside 8-bromo-2′,3′-O-isopropylidene-adenosine in apolar solvents, which is due to an effective O(5′)–H ⋯ N(3) hydrogen bond, as is demonstrated by a substantial N(3) quadrupolar broadening of the H(5″) resonances in the 1H n.m.r. spectra.

13C nuclear magnetic resonance chemical shifts of small hydrocarbons in the gas phase

Gas phase 13C n.m.r. chemical shifts of small hydrocarbons are given and discussed briefly.

INDO calculations on the sigmatropic [1,5] H-shift in 1,3-cyclohexadiene and 1,3,5-cycloheptatriene : a homo-cyclopentadienyl transition state model

INDO calculations have been performed for the activated complex of the [1, 5] H.-shift in 1,3-cyclohexadiene and 1,3,5-cycloheptatriene. During the migration in the cyclohexadiene system a homoconjugation was calculated between the carbon atoms C1 and C5.For cycloheptatriene it could be demonstrated that one double bond does not participate in the reaction.Activation enthalpies are related to (homo)conjugation in the transition state of the reaction for cyclic conjugated dienes and trienes. The electron density on the migrating hydrogen can be related to the electron affinity of the ring system in the transition state.[/p]

α-Substituent effects in 13C NMR of hydrocarbons : Quantum chemical calculations

Abstract The 13C NMR α-substituent effects within the series methane to neopentane and ethene to isobutene have been calculated using Hartree—Fock perturbation theory with MINDO/3. The sign and order of magnitude of the calculated α-effects are in agreement with new experimental values, including effects of multiple substitution.

CNDO/2 and INDO (intermediate neglect of differential overlap) calculations of a reaction pathway for the sigmatropic [1,5] hydrogen shift in cyclopentadiene

CNDO/2 and INDO calculations have been carried out in order to construct a suitable model for the activated complex during the reaction. In this reaction model the migrating hydrogen atom moves along an edge of the cyclopentadiene ring. An analysis of this situation suggests a partial electron transfer from the migrating hydrogen to the nascent cyclopentadienyl system. This charge transfer is discussed in terms of aromaticity. The calculated activation enthalpies are 10 kcal/mole (CNDO/2) and 17 kcal/mole (INDO), whereas the experimental value is ca. 24 kcal/mole [1].

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

Spectral simulations of 27Al MAS and double rotation (DOR) NMR spectra measured at 7.0 T prove the preferential hydration of one tetrahedral Al site in AlPO-11, transforming this site reversibly into octahedral aluminium.

Cope interconversions of the 9-methoxy-endo-tricyclo[4.2.1.02,5]nona-3,7-dienyl cation and its [4.3.0.02,5] tautomer. Evidence of enhanced ionic stability by delocalization of neighboring sigma bonds

Dissolution of 9-chloro-9-methoxy-endo-tricyclo[4.2.1.02.5]nona-3,7-diene (7b, c) in liquid SO2 gives rise to a dissociation equilibrium which involves the rearranged 7-methoxy-endo-tricyclo[4.3.0.02.5]nona-3,9-dienyl cation (11). This allylic ion was formed quantitatively by dissolution of (7b, c) in FSO3H–SO2 or AlCl3–CH2Cl2. Its structure was established by 1H and 13C n.m.r. spectroscopy. Quenching of the ionic solutions with methanol afforded the starting acetal (7a). In contrast, reaction of (11) with pyridine produced its corresponding pyridinium salt (17a). Ionization of (7b, c) under short-life conditions, i.e. reaction with pyridine in CH2Cl2, proceeds without skeletal rearrangement. The structure of the reaction products of the reference compounds (8)–(10) appeared to be independent of the conditions used. In all cases the unrearranged structures were observed. The ready skeletal rearrangement of (7b, c) under acidic conditions is ascribed to the involvement of a homoaromatic Cope transition state. From the enhanced reaction rate of (7b, c) relative to (8b, c)–(10b, c) it is concluded that the σ bond involved in the Cope rearrangement provides stabilization of the incipient cation in (7). This kind of stabilization is also present in the rearranged cation (11), as indicated by its enhanced stability relative to the corresponding allylic ion (3) and by comparison of their 1H and 13C n.m.r. data.