Electron pairing and chemical bonds. Molecular structure from the analysis of pair densities and related quantities

The Fermi holes are presented as a new means of analysis and visualisation of molecular structure. Based on these quantities it is possible to get clear and highly visual insight into the structure of molecular fragments (functional groups) even in molecules with complex bonding patterns like multicenter bonding, hypervalence, etc. In addition to allowing the detection and localization of multicenter bonding, the new approach also brings some new interesting possibilities for the quantitative evaluation of molecular similarity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation of heats of transition in scanning calorimetry

A procedure for the evaluation of heats of transition in scanning calorimetry has been developed. A formula for evaluation of the energy of fusion of a compound at its melting point is derived that takes into account the baseline shift that is attributable to the heat capacity change on melting. Several other calorimeter parameters of importance are discussed. These include heat exchange between calorimeter vessel and jacket, the time constant of the instrument, the scanning rate, and the heater placement.     

Li(thf)3cyclo-(P4tBu4CH)]--synthesis, molecular structure and dynamic behaviour

[Li(thf)3cyclo-(P4tBu4CH)] (2-Li), containing the first tetraphosphacyclopentanide anion cyclo-(P4tBu4CH)- (2), was prepared, and its dynamic behaviour in solution analysed by variable-temperature 31P NMR spectroscopy.     

Phase discrimination by automated BKD

Automated backscatter Kikuchi diffraction in the SEM enables rapid discrimination of phases and the automated acquisition of phase-distribution maps at sub-micron resolution, if the phases belong to different Laue groups and/or if their lattice constants are significantly different. The acquired database can be further used for quantitative determination of phase contents, for construction of crystal orientation maps, for characterization of grain and phase boundaries, for crystal texture analysis on a grain-specific level by calculating pole figures and orientation density functions separately for each phase, and for orientation stereology in general. Pattern-quality maps provide a clear reproduction of the microstructure on a meso-scale level.     

Pervaporative transport modelling in a ternary system: ethyltertiarybutylether/ethanol/polyurethaneimide

The study deals with the analysis of diffusion and mass transfer modelling during pervaporation in a true ternary system involving a polar liquid mixture (ETBE/EtOH) and a polar block copolymer (polyurethaneimide or PUI). A survey of methods of pervaporative transfer modelling in ternary systems is first developed. From differential permeation experiments carried out with both pure liquids, it appears that both permeants obey a Fickian law. Moreover, the diffusional behaviour is consistent with Long's model, which has thus been assumed for the related ternary system. An extension of the Brun's model is then derived, which takes into account the diffusion coupling as well the significant deviation from sorption ideality. From a practical point of view, the calculated values of fluxes show generally good agreement with the experimental results, although a small deviation occurs for mixtures of low ethanol content. Diffusion coefficients of both pure solvents corresponding to transient or steady state are compared. A very good agreement is found for the aprotic permeant (ETBE). whereas the diffusion coefficient of ethanol in transient state is only the quarter of the value corresponding to steady state. The results are discussed in comparison with related investigations in the literature, involving specific liquid-polymer interactions.     

Structural requirements in chiral diphosphine-rhodium complexes. VI. Inhibition of the asymmetric hydrogenation of Z-methyl-α-acetamidocinnamate catalyed by diop-rhodium catalyst in the presence of Z-adamantyl or bornyl-α-acetamidocinnamate.

The half-life period (6.6 mm.) for the hydrogenation of Z-methyl-a-acetamidocinnamate (catalyzed by a neutral DIOPRh complex) was found to be the same when the Me ester reduction was performed in the presence of equimolar quantities of the corresponding i-Pr or t-Bu ester unsaturated stbstrates. Neither the Me nor the i-Pr or t-Bu esters underwent appreciable Z,E-isomerization. The formation of N-acetylphenylalanine methyl ester product suffered inhibition when the hydrogenation reaction was performed in the presence of the corresponding bornyl or 1-adamantyl unsaturated esters (half-life period of the Me ester: 27 ~ 40 mm., respectively. The greater the inhibition of the Me ester unsaturated substrate, the more the bulky inhibitor itself underwent Z,E-isomerization. In the presence of inhibitors, the Me unsaturated substrate did not undergo appreciable Z,E-isomerization.     

Optical electron transfer processes. The dependence of intervalence line shape and transition energy on chromophore concentration

Optical electron transfer in the mixed-valence cation of biferrocenylacetylene (BF⁺) has been examined in CD₂Cl₂ solvent. The intervalence absorption line shape is relatively narrow at both low and high chromophore concentrations, but broader at intermediate concentrations. The transition energy for metal-to-metal charge transfer increases from ≈4440 cm⁻¹ at infinite dilution to 5995 cm⁻¹ for 3.8 mM BF⁺. Related effects exist due to added electrolyte. Neither the electrolyte nor chromophore concentration effects are expected from a simple reading of electron transfer theories. Nevertheless, both phenomena can be understood and within the context of theory upon careful consideration of the effects of ion-pairing (and tripling) equilibria upon electron-transfer energetics.     

Facile homogeneous hydrogenations of hindered olefins with [Ir(cod)py(PCy3)]PF6

The title complex readily hydrogenates a number of hindered steroidal olefin groups from the α face, without reducing ketone carbonyl groups, carbon—halogen bonds or cyclopropane rings.     

Metal salt catalyzed carbenoid—XIII: On the mechanisms of cyclopropanation and allylic CH insertions by diazo esters in the presence of olefins and homogeneous copper catalysts

Evidence is presented demonstrating the existence of two paths to the title processes which arise from a common intermediate. A rationale involving catalyzed addition of the diazo compound to the olefin and carbenoid addition to the olefin is proposed. The penultimate intermediate has one new CC bond formed. It is partitioned between products by forming the second CC bond or formation of a hydrocarbenoid allyl complex which collapses to the allylic CH insertion products. Cyclopropanation occurs stereospecifically. The proposed mechanism accounts for the stereospecificity of cyclopropanation, the variance of syn/anti ratios with catalyst concentration when diazoacetic ester is employed and optical inductions with chiral catalysts. The question of whether the alleged carbenoid and/or the penultimate intermediate contain N₂ is not answered although it is felt that a cupro-cyclobutane intermediate is the most probable intermediate before product partitioning.