Analysis of models for the Ziegler-Natta stereospecific polymerization on the basis of non-bonded interactions at the catalytic site—I. The Cossee model

The model proposed by Cossee for Ziegler-Natta polymerization of propene is re-examined by computing the non-bonded energies for all sets of possible internal coordinates of the atoms at the catalytic site. For catalytic sites in the bulk of (110) surfaces of TiCl₃-α, it is possible to explain the regiospecificity of the reaction but, at variance with Cossee, we see no evidence that the surface conditions the methyl group of the olefin to protrude out of the crystal. The stereospecificity may arise however from the fixed, chiral orientation of the first carbon—carbon chain bond, due to interactions with the surface. Further computations seem to indicate that the polymerization could occur with much less steric hindrances on edges or reliefs of the surfaces, the relative positions of olefin and chain being exchanged in respect to the Cossee model.     

A new microbial assay for the toxicity detection of contaminated soils

A comparative study to detect toxicity prior to bioremediation treatment was set in order to investigate dehydrogenase activity inhibition of a common soil bacterium caused by soil contaminated with Cu, Pb, and As. A spectrophotometric test with Pseudomonas fluorescens strain ATCC 13525 utilising the 2,3,5-triphenyl tetrazolium chloride (TTC) reduction by microbial dehydrogenase has been adapted for this purpose. Soil samples are incubated for 48 hours at 30 +/- 1 degrees C in 18-ml tubes in the presence of TTC as an artificial electron-acceptor. The reduced TTC forms a reddish colour substance named triphenyl formazan (TPF), which can be extracted from the microbial cells and measured colorimetrically. The rapid response of biological activity in microorganisms and the reported sensitivity to the toxicants in the contaminated samples are reflected by the TTC reduction method, which is a sensitive tool for toxicity screening of contaminated sites, routine monitoring of bioremediation processes, as well as for feasibility studies of bioremediation treatments, in order to assess whether a specific pollutant or any other substance at a site location could inhibit the microbiological processes.     

Thin-layer chromatography of sulphonamides on silica gel G and polyamide layers by means of a polar mobile phase.

A thin-layer chromatographic system based on polyamide or silica gel G layers and an aqueous mobile phase was used for the separation of a series of sulphonamides. A linear relationship between RM values on polyamide layers and the acetone concentration in the mobile phase allowed the calculation of extrapolated RM values. The influence of the nature of the stationary phase on the migration of compounds is discussed.     

Model MDI/butanediol polyurethanes: Molecular structure,morphology, physical and mechanical properties

Model butanediol–MDI–polypropylene oxide polymers have been synthesized to explore the structure–property relationships in urethane/polyether polymers. The results of mechanical, thermal, and spectroscopic analyses agree remarkably well. The phase mixing in these polymers decreases with increasing hard-segment length, while the hard-segment stability increases with increasing hard-segment length. This is demonstrated clearly by dynamic mechanical, differential scanning calorimetry, infrared, and NMR studies. The importance of hydrogen bonding to the stability of the hard segment is strongly supported by the variable-temperature infrared experiments. The critical hard-segment size for phase segregation was shown to be two butanediols per hard segment. The temperature limit of the flatness of the storage modulus was tied to the thermal stability of the hard-segment hydrogen bonding which is controlled largely by the length of the hard segment and the extent of the hydrogen bonding.     

An MS Xα and ETS study of the influence of “d” orbitals on the electron affinities of thio-substituted benzenes

The electron transmission spectra of the o,o′-dimethyl substituted benzenes 2,6-(CH₃)₂C₆H₃-XR (X = O,S, NR: R = H, CH₃) show that the substituents adopt a planar (R = H) or rotated (R = CH₃) conformation depending upon their size and they suggest that in the sulphur derivatives the ²B₁ H anion states are stabilized by S3d orbital participation. MS Xα calculations performed on both the planar and orthogonal forms of the model systems p-(XH)₂-C₆H₄ (X = O, S) support this hypothesis and assign the extra resonance present in the ET spectra of the thio derivatives to electron capture into [ σ_S-R* MOS. strongly mixed with sulphur 3d orbitals ( = 33% d character).     

Correlation functions in the Non Perturbative Renormalization Group and field expansion

The usual procedure of including a finite number of vertices in Non Perturbative Renormalization Group equations in order to obtain n-point correlation functions at finite momenta is analyzed. This is done by exploiting a general method recently introduced which includes simultaneously all vertices although approximating their momentum dependence. The study is performed using the self-energy of the tridimensional scalar model at criticality. At least in this example, low order truncations miss quantities as the critical exponent η by as much as 60%. However, if one goes to high order truncations the procedure seems to converge rapidly.     

Not Carbon s–p Hybridization,but Coordination Number Determines C−H and C−C Bond Length

A fundamental and ubiquitous phenomenon in chemistry is the contraction of both C−H and C−C bonds as the carbon atoms involved vary, in s–p hybridization, along sp³ to sp² to sp. Our quantum chemical bonding analyses based on Kohn–Sham molecular orbital theory show that the generally accepted rationale behind this trend is incorrect. Inspection of the molecular orbitals and their corresponding orbital overlaps reveals that the above-mentioned shortening in C−H and C−C bonds is not determined by an increasing amount of s-character at the carbon atom in these bonds. Instead, we establish that this structural trend is caused by a diminishing steric (Pauli) repulsion between substituents around the pertinent carbon atom, as the coordination number decreases along sp³ to sp² to sp.