Electron donor and acceptor properties of alkyl substituents

The polar effects of alkyl substituents in electrophilic and nucleophilic chemical and electronic transitions is discussed. The question of the importance of hyperconjugation in the electron donor properties of alkyl substituents is raised. In view of the cogent arguments of Dewar, it is doubtful whether quantum mechanical calculations embodying hyperconjugation constitute proof of this effect. That the art of quantum mechanics may not yet be sufficiently developed to be used as proof for or against secondary resonance effects also is evidenced by the calculations of Simpson, who found that an internal dispersion force model (in which conjugation was neglected) reproduced the properties of butadiene just as satisfactorily as the models embodying conjugation. The experimental facts do not unequivocally support the hyperconjugation hypothesis and indeed are, at least in part, contradictory to it. In particular, the demonstration that the Baker-Nathan Effect² may be due to the influence of alkyl substituents on the differential solvation of ground and transition states casts doubt on the interpretation that this experimental effect is due to a dominant role of C-H hyperconjugation.

In nucleophilic chemical reactions, rate or equilibrium constants for para (or meta) alkyl derivatives are somewhat smaller than those of the corresponding hydrogen compounds. A number of authors have interpreted this in terms of a permanent electron donor role of alkyl substituents (e.g. by hyperconjugation) relative to the hydrogen substituent. However, this static viewpoint of substituent effects fails to account for the finding that p-alkyl substituents function as apparent electron acceptors (relative to the p-hydrogen substituent) in appreciably lowering the energy of the nucleophilic principal electronic transition of phenol, anisole, aniline and N,N-dimethylaniline. These results are qualitatively rationalized in terms of ‘substituent-polarizability” and electronegativity.

The p-neopentyl substituent lowers the energy of both electrophilic and nucleophilic electronic transitions to an appreciably greater extent than either the p-methyl or p-t-butyl substituent. This extra stabilizing effect of the neopentyl substituent on both electron deficient and electron rich centers may be due to an internal dispersion force interaction, since the geometry of the neopentyl compounds is particularly favorable for such an interaction.     

Coating of silica sand with aluminosilicate clay

The objective of this work was to coat aluminosilicate clays on an inert silica support, and to characterize the properties and stability of the clay-silica coating. Two polymers, polyacrylamide (PAM) and polyvinyl alcohol (PVA), were used to bind kaolinite, illite, and smectite onto silica grains. The clay-polymer composites were studied by X-ray diffraction, FTIR, and electrophoretic mobility. Clay coatings on silica grains were characterized by mass coverage, scanning electron microscopy, specific surface area, and pH stability. Silica sand was successfully coated with clays by using the two polymers, but with PVA, the clay coating had a greater mass coverage and was more stable against pH variations. Less polymer was needed for the clay coating using PVA as compared to using PAM. Clay-polymer complexes and pure clay minerals had similar cation exchange capacities and electrophoretic mobilities, indicating that overall surface charge of the clays was little affected by the polymers. Some decrease in hydrophilicity was observed for illite and smectite when clays where coated with the polymers. The methodology reported here allows the generation of a clay-based porous matrix, with hydraulic properties that can be varied by adjusting the grain size of the inert silica support.     

Glycine oligomerization on silica and alumina

Oligomerization of glycine (gly) and diglycine (gly₂) on silica and alumina was observed in experiments simulating wetting-drying cycles at 80°C. Glycine produces less than 1% total yield of gly₂ and diketopiperazine (DKP) after one week. In experiments starting from gly₂, more than 10% DKP is formed. Formation of higher oligomers (gly₃–gly₆) proceeded as well, with 3.8% and 5.1% total yields on silica and alumina surfaces respectively.     

Phenothiazine cation radical formation on silica, silica-alumina and alumina

The solution of phenothiazine in benzene produces phenothiazine cation radicals on the surface of silica, alumina and silica-alumina catalysts.

---

- SiO₂, Al₂O₃ .

     

Studies of mixed silica/alumina and silica/boric oxide materials

Multinuclear low temperature solution NMR and FTIR has been used to monitor the hydrolysis and polymerisation chemistry involved in the preparation of multicomponent silica/alumina and silica/boric oxide monolithic gels. Pre-hydrolysis of the silicon component (tetraethylorthosilicate) in the presence of low levels of water is an important factor in obtaining transparent materials. In order to obtain high homogeneity and minimise the subsequent precipitation of the fastest hydrolysing component, ethylacetonate (etac) has been used to modify the aluminum alkoxides. Solid state nmr and FTIR studies show that the borosilicate system contains Si-O-B bonds during the early stages of polymerisation but they are absent on final gelation. Thermal treatment to around 500°C is required to generate mixed Si-O-B bonds. Length of prehydrolysis has little affect on the nature of the Si-O-B gel materials but has a significant effect on the chemical nature of Si-O-Al materials. Longer silicon alkoxide prehydrolysis times lead to better defined materials.formerly Carole C. Perry.formerly at Chemistry Department, Brunel University, Uxbridge, Middlesex. UB8 3PH UK.     

NMR investigation of concentrated alumina and silica slurries

The longitudinal relaxation times (T₁) of water in concentrated silica and alumina slurries were measured as a function of solids content. It was shown that the results could be fit very well with a two-phase fast-exchange model between free and surface-bound water. As expected, values of T₁ for bound water were in the order of 20–2000 times lower than that for free water, indicating a higher effective viscosity of the surface-bound water. The strength of the interaction depended on the particular surface, and all of the aluminas examined interacted more strongly with water than the two silicas studied, which themselves differed considerably. The chemical mechanical polishing (CMP) removal rate of tantalum by silica slurries was shown to be directly correlated with the interaction parameters, derived from the NMR relation times rather than with total surface hydroxyl group concentration.     

Electron donating properties and catalytic activity of samarium oxide and its mixed oxides with alumina

The catalytic activity of Sm₂O₃ and its mixed oxides with alumina in the Oppenauer oxidation of a secondary alcohol and Meerwein-Ponndorf-Verley reduction of ketones is reported. The data were correlated with their surface electron donor properties. The electron donating properties of the oxides have been determined from the adsorption of electron acceptors of various electron affinity on the surface of these oxides.     

Anodic alumina supported dual-layer microporous silica membranes

We present a new processing scheme for the deposition of microporous, sol–gel derived silica membranes on inexpensive, commercially available anodic alumina (Anodisk™) supports. In a first step, a surfactant-templated mesoporous silica sublayer (pore size 2–6 nm) is deposited on the Anodisk support by dip-coating, in order to provide a smooth transition from the pore size of the support (20 or 100 nm) to that of the membrane (3–4 Å). Subsequently, the microporous gas separation membrane layer is deposited by spin-coating, resulting in a defect-free dual-layer micro-/mesoporous silica membrane exhibiting high permeance and high selectivity for size selective gas separations. For example, in the case of CO₂:N₂ separation, the CO₂ permeance reached 3.0 MPU (1 MPU = 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹) coupled with a CO₂:N₂ separation factor in excess of 80 at 25 °C. This processing scheme can be utilized for laboratory-scale development of other types of microporous or dense inorganic membranes, taking advantage of the availability, low cost and low permeation resistance of anodic alumina (or other metal oxide) meso- and macroporous supports.     

Effect of silica/alumina ratio and structure-directing agent on the physical and chemical properties of SAPO-34

Journal of Sol-Gel Science and Technology - This work describes how the silica/alumina ratio (SAR) affects the framework, texture, chemical composition, and acidity of SAPO-34 molecular sieves...     

Solvent donor and acceptor properties of pyridine

Polarographic and cyclovoltammetric measurements on the perchlorates of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Tl⁺, Ba²⁺ and Ni²⁺ as well as on the trifluoromethane sulfonates of Zn²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Cu⁺, Mn²⁺ and Co²⁺ were carried out. The data allowed the evaluation of the different donor behavior of pyridine towards hard, border line and soft cations. The conclusions drawn from electrochemical investigations were supported by Gibbs energies of transfer for cations, which were derived from both electrochemical measurements based on the bis(biphenyl)chromium assumption and from solubility studies based on the tetraphenylarsonium tetraphenylborate assumption. The acceptor properties of pyridine were obtained from the solvatochromic dyes bis(cyano)bis(1,10-phenanthroline)iron(II) and bis(cyano)bis(3,4,7,8-tetramethyl-1,10-phenanthroline)iron(II) and the results were compared with the acceptor number and the E _T -value.     

Fast neutron activation analysis of bulk coal samples for alumina,silica and ash

The fast neutron activation technique was applied to bulk samples (≈11 kg) of Australian black coal. The determination of alumina is based on the reaction ²⁷Al(n,p)-²⁷Mg by counting the 0.844-MeV peak (t_{$\text{1}\text{2}$} = 9.4 min). Silica is determined by means of the reaction ²⁸Si(n,p)²⁸Al; the 1.78-MeV peak (t_{$\text{1}\text{2}$} = 2.3 min) is counted and a correction for the interference from alumina is applied. The ash content is based on the correlation between ash and the sum of alumina and silica. The accuracies (1 SD) for the determination of alumina, silica and ash were 0.52% Al₂O₃, 0.79% SiO₂ and 1.02% ash, respectively. The ash, alumina and silica contents of the samples were in the ranges 8.8–37.5%, 1.3–10.3% and 6.4–22%, respectively.     

Adsorption properties of alumina

Summary Besides the three previously reported (3) surface species (i. e. outer, inner complex and a py⁺-species) which are formed on adsorption and desorption of pyridine on dehydroxylated alumina at temperatures above 200 °C, a fourth low temperature species is identified. The dependence of the surface concentrations of the various species and of the acid sites which they occupy is discussed usingPeris, surface model ofγ-Al₂O₃. It can be shown to be highly probable that the inner complex and the py⁺-species are formed solely on triplet oxide defects. These acid sites have to be distinguished with respect to there nearest neighbours. At least two types of triplet defects must be postulated; type A preferentially contains adjoined oxide ions and may possibly correspond toPeris α-sites, type B preferentially contains adjoined hydroxyl ions and therefore exhibits the higherLewis acidity. Whereas the py⁺species is formed on this latter type of triplet defect, outer and inner complexes presumably are formed on all types of triplet oxide vacancies.

---

Zusammenfassung Au?er den früher beschriebenen (3) Oberfl?chenverbindungen (?u?erer und innerer Komplex und py⁺-Spezies), die bei der Adsorption und Desorption von Pyridin an dehydroxyliertem Aluminiumoxid bei Temperatur n oberhalb 200 °C gebildet werden. wird eine vierte Spezies bei Adsorption unterhalb 200 °C gefunden. Die Konzentrationsabh?ngigkeit der verschiedenen Spezies und der S?urezentren, an. denen sie gebildet werden, wird im Rahmen desPerischen Modells derγ-Al₂O₃-Oberfl?che diskutiert. Mit hoher Wahrscheinlichkeit werden der innere Komplex und die py⁺-Spezies ausschlie?lich an Dreifachdefekten gebildet. Diese S?urezentren müssen hinsichtlich der Art ihrer n?chsten Nachbarn unterschieden werden. Wenigstens zwei Typen von Dreifachdefekten müssen vorliegen, n?mlich Typ A-Zentren, die bevorzugt Sauerstoff ionen enthalten und m?glicherweise mitPeris α-Zentren verglichen werden k?nnen, und Typ B-Zentren, die bevorzugt Hydroxylgruppen enthalten und daher die h?here Acidit?t besitzen. Die py⁺-Spezies wird ausschlie?lich an den Typ B-Zentren gebildet, innerer und ?u?erer Komplex dagegen an beiden Arten von Dreifachdefekten.

     

Extinction coefficients of hydroxyl- and deuteroxyl groups on silica and alumina

Summary Extinction coefficient of the hydroxyl groups on silica, and of deuteroxyl groups on silica and alumina were investigated, using FTIR spectroscopy. A desorption method was applied, i.e. the spectrum of the oxide was taken after a heating period, and an other one obtained after heating to a next higher temperature was subtracted. The amount of water desorbed during this temperature interval was determined from gas phase IR spectra.

---

Extinktionskoeffizienten von Hydroxyl- und Deuteroxylgruppen an Siliciumdioxid und Aluminiumoxid

     

ESR study of products of transformation of 3,6-tert-butylpyrocatechol on alumina, zinca, silica, and titania

The formation of semiquinone and phenoxazyl radicals and metallocomplexes with semiquinone ligands was observed by ESR during the interaction of di-tert-butylpyrocatechol with Al₂O₃, ZnO, SiO₂, and TiO₂. In the case of different modifications of SiO₂, admixtures of TiO₂ exhibit a higher reactivity in complex formation with the organic substrate. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1975–1978, October, 1998.     

Synthesis and properties of alumina aerogels

The synthesis of alumina aerogels has been carried out by hydrolysis of aluminium isopropylate as an aerogel precursor dissolved in isopropanol or methanol, followed by gelation of the sols obtained and drying under supercritical conditions and calcination. The influence of two main preparation parameters, precursor concentration and reagents (water to aluminium isopropylate) mole ratio, on the physicochemical properties of aerogels was investigated.     

Synthesis,structures and properties of two donor–acceptor acridone-based compounds

Two donor–acceptor acridone-based compounds, namely, 2-{10-[4-(diphenylamino)phenyl]acridin-9-ylidene}malononitrile ( TPA-AD-DCN ), C₃₄H₂₂N₄, and 2-{10-[4-(9H-carbazol-9-yl)phenyl]acridin-9-ylidene}malononitrile ( CzPh-AD-DCN ), C₃₄H₂₀N₄, have been synthesized in high yield and their structures determined. TPA-AD-DCN and CzPh-AD-DCN crystallized in the centrosymmetric space groups P and P2₁/c, respectively. Both molecules adopt a `butterfly-like' configuration of the common part of the structure and differences occur within the substituents on the acridine N atom. A Hirshfeld surface analysis showed that the H…H and C…H/H…C contacts constitute a high percentage of the intermolecular interactions. The optical and electrochemical properties, as well as theoretical calculations, of TPA-AD-DCN and CzPh-AD-DCN support the structural characterization of these materials. As crystallization-induced emission materials, TPA-AD-DCN and CzPh-AD-DCN are anticipated to be of potential use in the construction of promising optoelectronic materials.