Aqueous alkali-metal silicate anions containing fully condensed four-coordinate sites.

Conspicuous absence: (29)SiNMR studies show that highly condensed, four-coordinate Si-containing anions are common in concentrated alkali-metal silicate solutions, but reveal no evidence for the existence of specialized zeolite building units. The results add to the mounting evidence that silicate polymerization simply proceeds through stepwise condensation of monosilicate tetrahedra.

Silicon-29 NMR Studies of Tetraalkylammonium Silicate Solutions. 1. Equilibria, (29)Si Chemical Shifts, and (29)Si Relaxation.

The addition of tetraalkylammonium cations to aqueous silicate solutions enhances the abundance of symmetric, cagelike, polysilicate anions including the cubic octamer, Si(8)O(20)(8)(-). The equilibrium ratio of tetramethylammonium (TMA) cations to the octameric silicate anion is 8:1 for solutions with a concentration ratio [OH(-)]:[Si] >/= 1:1. Evidence indicates that organocations directly associate with cagelike polyanions to form a protective shell of hydrophobic hydration that impedes hydrolysis of the central anion.

Silicon-29 NMR Studies of Tetraalkylammonium Silicate Solutions. 2. Polymerization Kinetics.

The kinetics of formation of the silicate cubic octamer, Q(3)(8), in aqueous tetramethylammonium (TMA) silicate solutions was investigated by (29)Si NMR. The rate equation for solutions at pH 13.2-13.6 is d[Q(3)(8)]/dt = k(f) [H(+)](1.6)(+/-)(0.1)[TMA(+)](0.36)(+/-)(0.08)[Si](0.8)(+/-)(0.3) where k(f) = (2.2 +/- 0.8) x 10(16) mol(-)(1.8) kg(1.8) s(-)(1) at 296 K. The findings prove unequivocally that alkylammonium cations participate directly in the formation and subsequent stabilization of cagelike polysilicate anions. This implies a radically different mechanistic role than "templating" for alkylammonium cations in the synthesis of molecular sieves.

Biosynthesis and characterization of 14C-enriched flavonoid fractions from plant cell suspension cultures.

A range of radiolabeled anthocyanins, proanthocyanidins, and other flavonoids were accumulated by cell suspension cultures of two plant species, ohelo (Vaccinium pahalae) and grape (a Vitis hybrid, Bailey Alicant A), after providing uniformly labeled [(14)C]sucrose to the medium. Approximately 15% of administered label was recovered in a series of flavonoid-rich fractions varying in composition. Anthocyanins, and monomers to oligomers of proanthocyanidins, were labeled effectively and characterized from both species. Most of the proanthocyanidin oligomers were based on the flavan-3-ols (+)-catechin and (-)-epicatechin. Cyanidin and peonidin glycosides were the dominant forms of anthocyanins in both species. Whereas the predominant form of flavonoids identified from ohelo cell cultures was proanthocyanidins, grape cell cultures produced mostly anthocyanins. The labeled phytochemicals were produced for use in subsequent in vivo animal feeding studies to gauge their bioavailability and accumulation in target organs.

Do zeolite precursor species really exist in aqueous synthesis media?

The authors of a recent report in this journal (Houssin, et al., Phys. Chem. Chem. Phys., 2003, 5, 3518, ) claim that a tetrapropylammonium (TPA) silicate mixture with molar composition 4.41 TPAOH : 10 SiO(2) : 117 H(2)O contains high concentrations of two silicate oligomers, the prismatic double five-ring and a novel pentacyclic dodecamer. The latter species is derivative of the framework structure of silicalite-1, a MFI-type zeolite that spontaneously crystallizes from this system, and, indeed, the authors declare it to be a "nanoprecursor" in the TPA-mediated growth of silicalite-1. Using two-dimensional (29)Si COSY NMR spectroscopy to examine an equivalent mixture enriched in the (29)Si isotope, we show that the published peak assignments are erroneous. The signal assigned to the double five-ring corresponds, in reality, to the well-characterized pentacyclic octamer, while the three signals assigned to the pentacyclic dodecamer arise from three completely separate species. We find no evidence, therefore, to support the suggestion that silicalite-1 is formed by sequential condensation of precursor species any more complex than the simple orthosilicate anion.

The structure of aqueous pentaoxo silicon complexes with cis-1,2-dihydroxycyclopentane and furanoidic vicinal cis-diols.

Addition of cis-1,2-dihydroxycyclopentane to aqueous alkaline silicate solutions results in the spontaneous formation of three organosilicate species, each with a 2:1 ligand to Si ratio and a pentacoordinated silicon centre. By using a mixture of both cis-1,2-dihydroxycyclopentane and 1,4-anhydroerythritol we show unambiguously that all three species are diastereomers of the monomeric bis(diolato)-hydroxo complex, [(L=)(2)SiOH](-)(where L represents the cis-diol ligand), thus clarifying the general assignment of (29)Si NMR spectra reported for silicate solutions containing furanoidic sugars with vicinal cis-diol functionality, such as ribose.