Pulsed field gradient magic angle spinning NMR self-diffusion measurements in liquids

Several investigations have recently reported the combined use of pulsed field gradient (PFG) with magic angle spinning (MAS) for the analysis of molecular mobility in heterogeneous materials. In contrast, little attention has been devoted so far to delimiting the role of the extra force field induced by sample rotation on the significance and reliability of self-diffusivity measurements. The main purpose of this work is to examine this phenomenon by focusing on pure liquids for which its impact is expected to be largest. Specifically, we show that self-diffusion coefficients can be accurately determined by PFG MAS NMR diffusion measurements in liquids, provided that specific experimental conditions are met. First, the methodology to estimate the gradient uniformity and to properly calibrate its absolute strength is briefly reviewed and applied on a MAS probe equipped with a gradient coil aligned along the rotor spinning axis, the so-called 'magic angle gradient' coil. Second, the influence of MAS on the outcome of PFG MAS diffusion measurements in liquids is investigated for two distinct typical rotors of different active volumes, 12 and 50 microL. While the latter rotor led to totally unreliable results, especially for low viscosity compounds, the former allowed for the determination of accurate self-diffusion coefficients both for fast and slowly diffusing species. Potential implications of this work are the possibility to measure accurate self-diffusion coefficients of sample-limited mixtures or to avoid radiation damping interferences in NMR diffusion measurements. Overall, the outlined methodology should be of interest to anyone who strives to improve the reliability of MAS diffusion studies, both in homogeneous and heterogeneous media.     

Innovative concept for a major breakthrough in atmospheric radioactive xenon detection for nuclear explosion monitoring

The verification regime of the comprehensive test ban treaty (CTBT) is based on a network of three different waveform technologies together with global monitoring of aerosols and noble gas in order to detect, locate and identify a nuclear weapon explosion down to 1 kt TNT equivalent. In case of a low intensity underground or underwater nuclear explosion, it appears that only radioactive gases, especially the noble gas which are difficult to contain, will allow identification of weak yield nuclear tests. Four radioactive xenon isotopes, ^131mXe, ^133mXe, ¹³³Xe and ¹³⁵Xe, are sufficiently produced in fission reactions and exhibit suitable half-lives and radiation emissions to be detected in atmosphere at low level far away from the release site. Four different monitoring CTBT systems, ARIX, ARSA, SAUNA, and SPALAX? have been developed in order to sample and to measure them with high sensitivity. The latest developed by the French Atomic Energy Commission (CEA) is likely to be drastically improved in detection sensitivity (especially for the metastable isotopes) through a higher sampling rate, when equipped with a new conversion electron (CE)/X-ray coincidence spectrometer. This new spectrometer is based on two combined detectors, both exhibiting very low radioactive background: a well-type NaI(Tl) detector for photon detection surrounding a gas cell equipped with two large passivated implanted planar silicon chips for electron detection. It is characterized by a low electron energy threshold and a much better energy resolution for the CE than those usually measured with the existing CTBT equipments. Furthermore, the compact geometry of the spectrometer provides high efficiency for X-ray and for CE associated to the decay modes of the four relevant radioxenons. The paper focus on the design of this new spectrometer and presents spectroscopic performances of a prototype based on recent results achieved from both radioactive xenon standards and air sample measurements. Major improvements in detection sensitivity have been reached and quantified, especially for metastable radioactive isotopes ^131mXe and ^133mXe with a gain in minimum detectable activity (about 2 × 10^?3 Bq) relative to current CTBT SPALAX? system (air sampling frequency normalized to 8 h) of about 70 and 30 respectively.     

A Convergent and Straightforward Synthetic Method to Prepare Deaza-Analogs of Squalene Epoxidase Inhibitors

a general route to deaza-analogs of squalene epoxidase inhibitors of general formula A is described using a very convergent synthetic scheme illustrated by the preparation of compound 2b. A Heck reaction, and an enolate condensation with an appropriate electrophile are the key steps of the expedient elaboration of 2b or analogs.     

Probabilistic parameter estimation in a 2-step chemical kinetics model for n-dodecane jet autoignition

This paper demonstrates the development of a simple chemical kinetics model designed for autoignition of n-dodecane in air using Bayesian inference with a model-error representation. The model error, i.e. intrinsic discrepancy from a high-fidelity benchmark model, is represented by allowing additional variability in selected parameters. Subsequently, we quantify predictive uncertainties in the results of autoignition simulations of homogeneous reactors at realistic diesel engine conditions. We demonstrate that these predictive error bars capture model error as well. The uncertainty propagation is performed using non-intrusive spectral projection that can also be used in principle with larger scale computations, such as large eddy simulation. While the present calibration is performed to match a skeletal mechanism, it can be done with equal success using experimental data only (e.g. shock-tube measurements). Since our method captures the error associated with structural model simplifications, we believe that the optimised model could then lead to better qualified predictions of autoignition delay time in high-fidelity large eddy simulations than the existing detailed mechanisms. This methodology provides a way to reduce the cost of reaction kinetics in simulations systematically, while quantifying the accuracy of predictions of important target quantities.     

Benzohopanes,a novel family of hexacyclic geomarkers in sediments and petroleums.

A novel family of hexacyclic hopanoids, the benzohopanes 1 (C₃₂–C₃₅), has been identified in geological sources by X-ray crystallography. These compounds must be formed from bacterial C₃₅ hopanoid precursors during early stages of sedimentation.     

Interaction entre groupes aromatique et polaire voisins—III: Synthese et etudes physico-chimiques d'acides p-x-phenyl-3 bicyclo[2.2.1] heptene-5 carboxyliques-2 et de leurs esters methyliques

The synthesis, ¹H and ¹³C NMR spectra of acids 1 to 4 and methyl esters 5 to 8 are described. Interaction of aromatic and polar groups in the cis acids must be steric in origin and not due to charge transfer complex or hydrogen bonding. The structure of 1e,3a,5e and 7e has been determined by X-Ray crystallography. ¹H and ¹³C NMR studies indicate that, in the cis compounds, the proximity of aromatic and polar groups tend to impose to them some largely predominant conformations which should be analogous in the crystal or in solution.     

Transformations thermiques des photooxydes meso des acenes—III: Dimeres de l'o-quinodimethane issu du photooxyde d'anthracene

Thermolysis of the anthracene photooxide 1a, in solution, can afford successively three dimers (I→II→III) the structures of which have been established by NMR and X-ray analysis for the first one. Their origin is the unstable o-quinodimethane diether 3a, coming from 1a by isomenzation, which at 80° dimerizes to 8a (dimer I) by an unusual [_π8_s+_π6_s] concerted cycloaddition. Above 110°, 8a isomerizes into 9a (dimer II) by a concerted [1,5] suprafacial sigmatropic migration. Finally, at higher temperature (~200°), both dimers are partly converted into the symetrical dimer 10a (dimer III), probably by a radical pathway. Thermolysis runned in presence of N-methylmaleimide show the non-reversibility of these reactions, giving two adducts, 15a and 16a, derived from 9a, the structures of which are deduced from NMR data. An explanation of the nature and high selectivity of the proceeding processes is given by considering orbital factors.     

Molecular Recognition of Anionic Substrates. Crystal Structures of the Supramolecular Inclusion Complexes of Terephthalate and Isophthalate Dianions with a Bis-intercaland Receptor Molecule

Molecular recognition of flat substrates requires the design of receptor molecules containing complementary flat units. If two such units are incorporated into a macrocyclic framework, a face to face inclusion of a planar substrate may take place, leading to an intercalative supramolecular structure. The water-soluble macrocyclic bis-intercaland receptor 1.4H+, containing two naphthalene subunits, linked by two positively charged oxy-bis- ethylamine binding sites, is able to bind strongly flat organic anions. The crystal structures of the terephthalate 2 and isophthalate 3 inclusion complexes are reported here. Complex 2, triclinic, P-1(N°), a = 7.717(3), b = 10.625(6), c = 16.238(9) Å, = 99.00(7), = 99.70(6), = 109.46(4)°, Z = 1. Complex 3, triclinic, P1 (N°1), a = 7.513(10), b = 10.640(9), c = 16.164(10) Å, = 98.81(5), = 99.77(10), = 109.36(12)°, Z = 1. Comparison of the environment (water molecules, anions and macrocycle) in the two X-ray structures highlights the formation of a similar organized assembly with the two different substrates.     

Réarrangements en milieu acide trifluoroacétique de deux alcaloïdes indoliques: la desformocorymine et la dihydrocorymine

The rearrangement in trifluoroacetic acid of two indole alkaloids of the echitamine series, desformocorymine (14) and dihydrocorymine (9) , has been investigated. Desformocorymine (14) was tranformed into a mixture of carbinolamines 17a , b , with the akuammiline skeleton, which were reduced (Et₃SiH, CF₃CO₂H) into an isomer 12 of cathafoline (6). This sequence constitutes the first example of an interconversion of the corymine skeleton into the akuammiline skeleton (Scheme 2). In the case of dihydrocorymine (9) , the rearrangement followed a different pathway owing to the formation of a hemiacetal between the primary alcohol CH₂(17)-OH and a carbonyl formed at C(3). Treatment of this hemiacetal 26 with aqueous base led to its opening with concomitant formation of a lactam. ¹³C-NMR seems to indicate that this lactam exists under a hydrated form 27. This highly unstable intermediate was cleanly transformed (MeONa-MeOH) into a 2-acyl indole 30 (Scheme 4), the structure of which was determined by X-ray crystallography. The formation of this acylindole involves the rupture of the C(7)? C(16) bond; it is the reverse of the reaction generally postulated as occurring in the biogenesis of the pentacyclic alkaloids. The structure of a by-product 34 was established as 17-hydroxymethylvincoridine by X-ray crystallography. The acid-catalyzed rearrangements involve the rupture of the Ph-N? C? N chromophore, with formation of a carbonyl at C(3). The reversibility of these steps is used in an easy correlation of dihydrocorymine and of 3-epidihydrocorymine via their trifluoroacetates.     

The microwave-assisted synthesis of a 2-carboxyphosphole

Using a one-pot four-step reaction a readily available phospholide was converted efficiently to a 2-carboxyphosphole. This compound can be used as a building block for the synthesis of phosphole-containing peptides, where the phosphole can serve as a coordination site for late transition metals and may affect the secondary structure of the peptide.