Thermodynamic quantities for the acid dissociation of alkylammonium ions in water over a wide temperature range

Enthalpies of protonation in aqueous solution have been determined from calorimetric measurements between 0 and 100°C for methylpentylamine, butyldimethylamine, and quinuclidine; between 0 and 125°C for di-iso-propylamine; and between 0 and 75°C for 1-azacycloheptane. Values ofpK_a at 25°C have been determined from pH measurements using a glass electrode for di-iso-propyl-, methylpentyl-, and butyldimethylamine. The calorimetric results are used together with values ofpK_a at 25°C to derive expressions which allow the calculation of accuratepK_a values over a large temperature range. The heat capacity and entropy changes for the protonation of aqueous alkylamines found in the present and in previous studies are discussed in terms of differences in hydrophobic solvation of the alkyl groups in the neutral amines and the ions.     

Inter-laboratory exercise with an aim to compare methods for 90Sr and 239,240Pu determination in environmental soil samples

In order to deliver reliable results for a multitude of different scenarios, e.g. emergency preparedness, environmental monitoring, nuclear decommissioning and waste management, there is a constant process of method development in the field of radioanalytical chemistry. This work presents the results of a method comparison exercise aimed at quantifying ⁹⁰Sr and ^239,240Pu in environmental soil samples, with the intention of evaluating the performance and applicability of different methods. From the methods examined in this work, recommendations are given in order to find a radioanalytical measurement procedure, for ⁹⁰Sr and ^239,240Pu analysis, which is fit-for-purpose for a particular scenario.

     

Polymer-supported pyridine-bis(oxazoline). Application to ytterbium-catalyzed silylcyanation of benzaldehyde

[reaction: see text] Terminal acetylenes containing hydroxy and carboxylic acid groups were subjected to Sonogashira coupling with 4-bromo-2,6-bis[(R)-4-phenyloxazolin-2-yl]pyridine and the resulting pybox derivatives were immobilized on Tentagel resins. Ytterbium(III) chloride complexes of the polymeric ligands catalyzed the addition of trimethylsilyl cyanide to benzaldehyde with 80-81% ee. The ligands were reused more than 30 times without any loss in selectivity or activity, and the metal complexes could be recovered and reused at least four times, although with slightly decreasing activity.     

Partitioned partial least squares regression with application to a batch fermentation process

The performance of Partial Least Squares regression (PLS) in predicting the output with multivariate cross‐ and autocorrelated data is studied. With many correlated predictors of varying importance PLS does not always predict well and we propose a modified algorithm, Partitioned Partial Least Squares (PPLS). In PPLS the predictors are partitioned into smaller subgroups and the important subgroups with high prediction power are identified. Finally, regular PLS analysis using only those subgroups is performed. The proposed Partitioned PLS (PPLS) algorithm is used in the analysis of data from a real pharmaceutical batch fermentation process for which the process variables follow certain profiles during a specific fermentation period. We observed that PPLS leads to a more accurate prediction of the yield of the fermentation process and an easier interpretation, since fewer predictors are used in the final PLS prediction. In the application important issues such as alignment of the profiles from one batch to another and standardization of the predictors are also addressed. For instance, in PPLS noise magnification due to standardization does not seem to create problems as it might in regular PLS. Finally, PPLS is compared to several recently proposed functional PLS and PCR methods and a genetic algorithm for variable selection. More specifically for a couple of publicly available data sets with near infrared spectra it is shown that overall PPLS has lower cross‐validated error than PLS, PCR and the functional modifications hereof, and is similar in performance to a more complex genetic algorithm. Copyright © 2011 John Wiley & Sons, Ltd.     

Dual Lewis acid-Lewis base activation in enantioselective cyanation of aldehydes using acetyl cyanide and cyanoformate as cyanide sources

Dual activation by a chiral Lewis acid and an achiral or chiral Lewis base enabled cyanation of both aromatic and aliphatic aldehydes with acetyl cyanide and ethyl cyanoformate to provide direct access to O-acetylated and O-alkoxycarbonylated cyanohydrins, respectively, under mild conditions. With a combination of a Ti-salen catalyst and Et3N, benzaldehyde was converted to the O-acetylated cyanohydrin with 94% ee within 10 h at -40 degrees C in 89% isolated yield.     

Comparing results of X-ray diffraction, µ-Raman spectroscopy and neutron diffraction when identifying chemical phases in seized nuclear material,during a comparative nuclear forensics exercise

Journal of Radioanalytical and Nuclear Chemistry - This work presents the results for identification of chemical phases obtained by several laboratories as a part of an international nuclear...     

B-norsteroids from Hymenoscyphus pseudoalbidus

Two viridin-related B-norsteroids, B-norviridiol lactone (1) and B-norviridin enol (2), both possessing distinct unprecedented carbon skeletons, were isolated from a liquid culture of the ash dieback-causing fungus Hymenoscyphus pseudoalbidus. Compound 2 was found to degrade to a third B-norsteroidal compound, 1β-hydroxy-2α-hydro-asterogynin A (3), which was later detected in the original culture. The proposed structure of 1 is, regarding connectivity, identical to the original erroneous structure for TAEMC161, which was later reassigned as viridiol. Compound 2 showed an unprecedented 1H-13C HMBC correlation through an intramolecular hydrogen bond. The five-membered B-ring of compounds 1-3 was proposed to be formed by a benzilic acid rearrangement. The known compound asterogynin A was found to be formed from 3 by a β-elimination of water. All compounds were characterized by NMR spectroscopy, LC-HRMS and polarimetry.     

A synthetic approach to 2,3,4-substituted pyridines useful as scaffolds for tripeptidomimetics

The synthesis of 2,3,4-substituted pyridine derivatives useful as scaffolds in the development of peptidomimetics is described. The use of a variety of electrophiles in a halogen-dance reaction to produce 3-alkyl-2-fluoro-4-iodo-pyridine derivatives as ‘functionalized scaffolds’ and the possibility to differentiate between the reactivities of the two halogen handles have been explored. Coupling of amino acid derivatives in the 4-position of the pyridine was found to proceed efficiently by conversion of iodo-pyridine to a Grignard derivative, which was allowed to react with a protected amino aldehyde. Substitution of fluorine in the 2-position of the pyridine was found to be facile with alkoxide nucleophiles, whereas amines were much less reactive.     

Thermodynamic quantities for the solution and protonation of four C6-amines in water over a wide temperature range

The enthalpies of solution ofn-hexyl- and cyclohexylamine have been determined at temperatures up to 125°C, of di-n-propylamine up to 100°C, and of triethylamine up to 75°C from reaction-solution calorimetric experiments. Quadratic polynomials were found to give good representations of the experimental values, implying a linear variation with temperature of the heatcapacity changes. The enthalpies of protonation of the aqueous amines have been determined in the same temperature range. Linear expressions give satisfactory representations of the protonation enthalpies of the hexylamines and dipropylamine, whereas a quadratic expression is required to represent the triethylamine values. The calorimetric results are used together with reported pK _a values at 25°C to calculate the dissociation constants of the corresponding ammonium ions between 0 and 200°C. At 0°C triethylamine was found to be the weakest base, the relative base strengths being triethyln-hexyl相似文献