Radiation-grafted anion-exchange membranes (AEM) containing pendent benzyltrimethylammonium, 1-benzyl-3-methylimidazolium and 1-benzyl-2,3-dimethylimidazolium functional head-groups were synthesised with ion-exchange capacities in the range 1.7–1.9 meq g−1. The ionic conductivities of the AEMs were also comparable (24.5 ± 1.8 mS cm−1 at 50 °C). The alkali stability (in aqueous potassium hydroxide (1 mol dm−3) at 60 °C) of the 1-benzyl-2,3-dimethylimidazolium head-groups was superior to the 1-benzyl-3-methylimidazolium but inferior to the benzyltrimethylammonium benchmark head-groups. Radiation-grafted AEMs containing pendent 1-benzyl-2,3-dimethylimidazolium head-groups are not suitable for application in electrochemical devices containing highly alkaline environments.
Charcoal is the result of natural and anthropogenic burning events, when biomass is exposed to elevated temperatures under conditions of restricted oxygen. This process produces a range of materials, collectively known as pyrogenic carbon, the most inert fraction of which is known as black carbon (BC). BC degrades extremely slowly and is resistant to diagenetic alteration involving the addition of exogenous carbon, making it a useful target substance for radiocarbon dating particularly of more ancient samples, where contamination issues are critical. We present results of tests using a new method for the quantification and isolation of BC, known as hydropyrolysis (hypy). Results show controlled reductive removal of non-BC organic components in charcoal samples, including lignocellulosic and humic material. The process is reproducible and rapid, making hypy a promising new approach not only for isolation of purified BC for 14 C measurement but also in quantification of different labile and resistant sample C fractions.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
The influence of the intrapore cation on the fluorination of zeolite Y from dilute fluoride solutions has been studied, revealing fluoride reacts with the zeolite framework in the presence of a Brønsted acid to form [SiO3F] and [AlO3F] moieties. 29Si{1H} Cross-polarised MAS NMR indicates the reaction proceeds by the substitution of surface hydroxide moieties for fluoride. The fluorination reaction is strongly influenced by the nature of the intrapore cation. Intrapore Brønsted acids facilitate fluorination of the framework by in situ ion-exchange, releasing the acidic ions to the zeolite surface. The fluorination reaction may be further promoted by the presence of intrapore alkaline earth cations (viz. Mg2+, Ca2+, Sr2+ and Ba2+). The conclusions of this work are significant to the preparation of fluorinated zeolite catalysts, the application of zeolites in defluoridation and the labelling of zeolite-based tracers with 18F for application in positron imaging techniques.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
The bis-guanidino compound H2C{hpp}2 (I; hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) has been converted to the monocation [I-H]+ and isolated as the chloride and tetraphenylborate salts. Solution-state spectroscopic data do not differentiate the protonated guanidinium from the neutral guanidino group but suggest intramolecular "—N—H···N═" hydrogen bonding to form an eight-membered C3N4H heterocycle. Solid-state CPMAS 15N NMR spectroscopy confirms protonation at one of the imine nitrogens, although line broadening is consistent with solid-state proton transfer between guanidine functionalities. X-ray diffraction data have been recorded over the temperature range 50−273 K. Examination of the carbon−nitrogen bond lengths suggests a degree of "partial protonation" of the neutral guanidino group at higher temperatures, with greater localization of the proton at one nitrogen position as the temperature is lowered. Difference electron density maps generated from high-resolution X-ray diffraction studies at 110 K give the first direct experimental evidence for proton transfer in a poly(guanidino) system. Computational analysis of I and its conjugate acid [I-H]+ indicate strong cationic resonance stabilization of the guanidinium group, with the nonprotonated group also stabilized, albeit to a lesser extent. The maximum barrier to proton transfer calculated using the Boese−Martin for kinetics method was 2.8 kcal mol−1, with hydrogen-bond compression evident in the transition state; addition of zero-point vibrational energy values leads to the conclusion that the proton transfer is barrierless, implying that the proton shuttles freely between the two nitrogen atoms. Calculations determining the gas-phase proton affinity and the pKa in acetonitrile both indicate that compound I should behave as a superbase. This has been confirmed by spectrophotometric titrations in MeCN using polyphosphazene references, which give an average pKa of 28.98 ± 0.05. Triadic analysis indicates that the dominant term causing the high basicity is the relaxation energy.
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