Concomitant polymorphism in the stereoselective synthesis of a β‐benzyl‐β‐hydroxyaspartate analogue

Two concomitant polymorphs, (I) and (II), of a β‐benzyl‐β‐hydroxyaspartate analogue [systematic name: dibenzyl 2‐benzyl‐2‐hydroxy‐3‐(4‐methylphenylsulfonamido)succinate], C₃₂H₃₁NO₇S, crystallize from a mixture of ethyl acetate and cyclohexane at ambient temperature. The structure of (I) has triclinic (P) symmetry and that of (II) monoclinic (P2₁/c) symmetry. Both crystal structures are made up of a stacking of homochiral racemic dimers (2S,3S and 2R,3R) which are internally connected by a similar R₂²(9) hydrogen‐bonding pattern consisting of intermolecular N—H...O and O—H...O hydrogen bonds. The centroid of the racemic dimer lies on an inversion centre. The main structural difference between the two polymorphs is the conformational orientation of two of the four aromatic rings present in the molecule. Polymorph (II) is found to be twinned by reticular merohedry with twin index 3 and twin fractions 0.854 (1) and 0.146 (1).     

An empirical charge transfer potential with correct dissociation limits

The empirical valence bond (EVB) method [J. Chem. Phys. 52, 1262 (1970)] has always embodied charge transfer processes. The mechanism of that behavior is examined here and recast for use as a new empirical potential energy surface for large-scale simulations. A two-state model is explored. The main features of the model are: (1) explicit decomposition of the total system electron density is invoked; (2) the charge is defined through the density decomposition into constituent contributions; (3) the charge transfer behavior is controlled through the resonance energy matrix elements which cannot be ignored; and (4) a reference-state approach, similar in spirit to the EVB method, is used to define the resonance state energy contributions in terms of "knowable" quantities. With equal validity, the new potential energy can be expressed as a nonthermal ensemble average with a nonlinear but analytical charge dependence in the occupation number. Dissociation to neutral species for a gas-phase process is preserved. A variant of constrained search density functional theory is advocated as the preferred way to define an energy for a given charge.     

Aromaticity or non aromaticity in germanazenes? Theoretical studies on germanazenes, the N-cyano analogs and their carbodiimide isomers

Theoretical studies of germanazene rings [(Ge_II-NR)_2,3; R = H, Me, CN, Ph] have been performed at the DFT/B3LYP level. The fully optimized geometrical structures display four or six-membered planar rings of alternating germanium and nitrogen, in good agreement with the available X-ray experimental data. The hypothetical molecule (GeN-H)₂ presents only a small distortion from planarity. Although the planar conformation could indicate some degree of delocalization, the stabilization energy - estimated using the concept of homodesmotic reactions - indicates very little or no aromatic character in these molecules. The easy experimental formation of these germanazenes can be explained by di- (or tri-)merisation of the transient monomeric germylene-imine GeNR in its triplet state. When R = CN, in conformity with the experimental results, the most stable species is the isomeric carbodiimide form (GeNCN)_n, a result which is easily explained by the maximum spin density on the terminal nitrogen in the calculated monomer.     

Three-Dimensional Naturally Reductive Homogeneous Lorentzian Manifolds

We give a complete classification of three-dimensional naturally reductive Lorentzian spaces, using a classification of the self adjoint Ricci operators.      

Synthesis and characterization of copolymers based on styrene and partially fluorinated acrylates

Copolymers of styrene and fluorinated acrylate monomers with F-octylalkyl, F(CF₂)₈(CH₂)_n^′ side groups were prepared by free radical polymerization. Thermal behaviour of the resulting polymers was investigated by DSC and TGA. Even if at the macroscopic scale the polymers surfaces are homogeneous and clear, the analysis indicates that all samples exhibit two glass transitions temperatures. This discontinuity may be regarded as an indication for microphase separation of fluorine-rich and polystyrene-rich microphases. Water and hexadecane contact angles measurements show that these polymers are quite surface active in the solid state. Surface and bulk organizations were investigated by XPS analysis. A strong correlation between bulk organization and surface properties of the polymers could be established. Preferential adsorption of fluorinated segments at the material surface were more pronounced than expected in the bulk.