Theoretical and Experimental Interpretation of the Signals From ΔITA

Computer modelling of time dependences of radon release rate during hydration of 3CaO·SiO₂ was carried out. It was demonstrated that the emanation thermal analysis (ETA) can be used for the characterisation of morphology changes during hydration of Portland cement clinker minerals. The presence of various additives and increased temperature affecting kinetics of hydration were simulated by the mathematical model of the radon release rate during hydration of 3CaO·SiO₂. A good agreement between the mathematical model and ETA experimental results was found.This revised version was published online in November 2005 with corrections to the Cover Date.     

Enantiomeric separation of 5-alkyl-5methylhydantion derivatives by capillary gas chromatography on permethylated β-cyclodextrin and molecular modeling

Permethylated β-cyclodextrin (hereafter designated perMe-β-CD), dililuted or not in polysiloxane, is an efficient chiral discriminant for native, 3-methylated, and 1,3-dimethylated series of 5-methyl-5-(C_nH_{2n + 1})hydantoins. From thermodynamic data obtained with pure perMe-β-CD and with the aid of molecular modeling, it is concluded that: (i) For native hydantoin derivatives having a carbon chain at the 5-position varying from ethyl to nonyl (2?n?9), the retention time is related to their H-bonding capability. The resolution is poor for derivatives with n?4. For n?5, the resolution is considerably improved and this increase in resolution is attributed to the long alkyl chain being capable of being buried inside the cyclodextrin cavity. (ii) For 1,3-dimethylhydantion derivatives having a carbon chain at the 5-position varying from ethyl to hexyl(2?n?6), the retention time is mainly determined by a steric fit between the solute and the inner volume of the macrocyclic cavity. For n = 2, a particular behavior is observed, consistent with a total inclusion of the solutes inside the cavity. By contrast with the native hydantoins, the selectively is high for light derivatives. Thus, the selectivity does not correlate with a strong docking energy (such as the presence of H-bonds postulated with native derivatives). (iii) For 3-methylhydantoin derivatives having a carbon chain at the 5-position varying from ethyl to octyl (2?n?8), as expected, retension time and selectivity are intermediate between that of native hydantoins and 1,3-dimethylhydantoins. For all three series, where n?5, the increase in chain length (n) does not bring about substantial changes in the chromatographic results. Therefore, for these long alkyl chain derivatives, similar dominant interactions with perMe-β-CD are postulated.     

Study of the quaternary system(+) and (−)5-phenyl-5-trifluoromethyl-imidazolidine-2,4-dione/(−)1-phenylethylamine/ethanol at 20°C under atmospheric pressure

The resolution of (±)-5-phenyl-5-trifluoromethyl-imidazolidine-2,4-dione ((±)-CF₃61H hereafter) using 1-phenyl-ethylamine (αMBA hereafter) in ethanol was investigated. At 20°C, the evolution of the resolution efficiency vs. the excess of αMBA was studied by establishing a part of the quaternary system: (+) and (−)-CF₃61H/(−)-αMBA/ethanol. Competition between two equilibria has been highlighted:

Cyclobishelicenes: Shape-Persistent Figure-Eight Aromatic Molecules with Promising Chiroptical Properties

Cyclobis[n]helicenes (n=3 or 5) are chiral D₂-symmetric π-conjugated macrocycles with stable lemniscular, or figure-eight, shapes. The conformational analysis of five different cyclobis[n]helicenes revealed that these molecules can only exist as their lemniscular conformers with high barriers to enantiomerization (>200 kJ mol⁻¹). The enantiomers of a cyclobis[5]helicene were resolved by HPLC and their unusual chiroptical properties were attributed to the inherent chirality of their macrocyclic figure-eight.     

Stereoselective Syntheses,Structures, and Properties of Extremely Distorted Chiral Nanographenes Embedding Hextuple Helicenes

We report a molecular design and concept using π‐system elongation and steric effects from helicenes surrounding a triphenylene core toward stable chiral polycyclic aromatic hydrocarbons (PAHs) with a maximal π‐distortion to tackle their aromaticity, supramolecular and molecular properties. The selective syntheses, and the structural, conformational and chiroptical properties of two diastereomeric large multi‐helicenes of formula C₉₀H₄₈ having a triphenylene core and embedding three [5]helicene units on their inner edges and three [7]helicene units at their periphery are reported based on diastereoselective and, when applicable, enantiospecific Yamamoto‐type cyclotrimerizations of racemic or enantiopure 9,10‐dibromo[7]helicene. Both molecules have an extremely distorted triphenylene core, and one of them exhibits the largest torsion angle recorded so far for a benzene ring (twist=36.9°).     

Attrition-enhanced preferential crystallization combined with racemization leading to redissolution of the antipode nuclei

Preferential crystallization is an effective and cheap resolution method which is limited by the nucleation of the antipode, even in the case of in situ racemization. Grinding (during an in situ racemization experiment) was done to promote the crystals stemming from the seeds by enhancing the dissolution step of smaller ones (resulting from nucleation). Thus, the solid remains of a single chirality even if the supersaturation exceeds the Ostwald limit of the antipode.     

Dehydration Mechanism and Crystallisation Behaviour of Lactose

The dehydration mechanism of α-lactose monohydrate was investigated by several techniques and interpreted on the basis of structural data. Whatever the dehydration conditions (heating or use of hygroscopic organic solvents), the departure of water molecules occurs cooperatively in channels parallel to the c axis of the initial structure. Subsequently, the reorganization leads to the closest packing (hygroscopic metastable form, Lα_H) under heating or to the stable anhydrous form (Lα_S), probably via a nucleation and growth process in ethanol. The use of acetone as dehydrating solvent on single crystals of α-lactose monohydrate led to the unexpected formation of single crystals of the anomeric β-lactose at room temperature, from which the crystal structure of β-lactose could be accurately re-determined. Recrystallization experiments of anhydrous lactose allowed to prepare N-methylpyrrolidinone and DMSO solvates of α-lactose. This revised version was published online in August 2006 with corrections to the Cover Date.     

Enantiospecific Syntheses of Congested Atropisomers through Chiral Bis(aryne) Synthetic Equivalents

The synthetic equivalents of the enantiopure binaphthyl bis(aryne) atropisomers derived from BINOL (1,1′-bi-2,2′-naphtol) featuring a stereogenic axis vicinal to the two reactive triple bonds can be generated for the first time in solution in an enantiospecific manner. Using a two-step sequence based on the bidirectional [4+2] cycloaddition of furan derivatives followed by an aromatizative deoxygenation reaction, several 9,9’-bianthracenyl-based atropisomers could be prepared enantiospecifically in high enantiomeric purity. Alternatively, bidirectional reactions with anthracene, 2-bromostyrene, and perylene as the arynophiles afforded very congested bis(benzotriptycene), bis(tetraphene) and bis(anthra[1,2,3,4-ghi]perylene) nanocarbon atropisomers in equally high enantiomeric purity. In complement, cross reactions with two different arynophiles revealed possible. The unusual atropisomer prototypes described in this study open the way to enantiopure nanographene atropisomers designed for functions.