Synthetic thermally reversible gel systems. V

Differential thermal analysis has been used to study the fusion of aqueous thermally reversible gels of gelatin and polyacrylylglycinamide (PAG). In the case of gelatin gels, endotherms close to the melting point are readily observed and these are sometimes preceeded by a small exothermic heat of gel reorganization. Calculations are presented to show that breaking of the gelatin gel network requires only a small fraction of the observed endothermic heat of fusion and that most of the heat is required for melting larger crystallites within gelatin aggregates and for perhaps a helix → coil transition. Failure to observe endotherms by DTA over the known temperature range of fusion of PAG gels is consistent with prior measurements and conclusions. The noncrystallinity of PAG gels and soluble aggregates together with a heat of crosslinking of only ?5 to ?10 kcal/mole of crosslinks places the heat of fusion of PAG gels outside the lower limits of DTA sensitivity.     

Notes on Kuhn's ω″ molecular orbital method

(1) It is shown that, contrary to statements sometimes made, it is perfectly possible to obtain consistent sets of charges in Kuhn's ω″ molecular-orbital method, whatever the values of the parameters ω, ω′, ω″.

(2) Particular applications are made to the benzyl cation, and to the non-alternants fulvene, heptafulvene and azulene. It appears that including the ω-terms decreases the magnitude of the largest of the net atomic charges as calculated by the simple Hückel method, and results in an overall smoothing-out process. Further inclusion of the ω′-terms continues this smoothing-out process, but inclusion of the ω″-terms may sometimes slightly reverse this process.

(3) The charge distributions obtained for fulvene and azulene in (1) and (2) lead to molecular dipole moments which are still much too large.

In view of (3) it is not clear that the additional work involved in the inclusion of ω′ and ω″ is justified by the greater accuracy thus obtained.     

Potentiometric titration of hydrogen peroxide with an electropolished stainless steel electrode

The preparation and use of a stainless steel indicator electrode in the accurate potentiometric titration of aqueous hydrogen peroxide are described. The results agree excellently with those obtained by a standard colorimetric weight-titration procedure. The nature of the titration curves, potential changes and the factors affecting them are discussed. Rapid potentiometric titrations directly to the equivalence-point potential are possible, owing to the nature of the titration curve.     

Diffusion-controlled vinyl polymerization. I. The gel effect

It is well known that the reaction rate and molecular weight of vinyl polymers can change markedly during the course of polymerization and that these changes are due to the influence of diffusion on the termination reaction. The chain length dependence of the termination rate constant has been considered in this work and has resulted in a general method of treating the polymerization kinetics and molecular weight distribution. This method is independent of the form of the chain length dependency and is capable of dealing with both disproportionation and recombination modes of termination. A specific model for the termination rate constant with chain length dependence is proposed and is based on free volume theory and entanglement coupling. Master curves for the characteristics of the reaction rate and molecular weight distribution are presented with the application of this model.     

Double-layer effects on simple electrode reactions III. The reduction of Eu3+ and Cr3+ in the presence of thiocyanate anions. Evidence for ligand bridging

The rates of reduction of Eu³⁺ and Cr³⁺ have been measured in mixed perchlorate + thiocyanate electrolytes at constant ionic strength, using low concentrations of thiocyanate to minimize its association with the cationic reactants. The effect of adsorbed thiocyanate anions on the reduction kinetics of Cr³⁺ resembled those produced by iodide and bromide on both Cr³⁺ and Eu³⁺. However, thiocyanate exhibited an unusual catalytic effect on the reduction of Eu³⁺ which was identified as arising from a reaction pathway involving thiocyanate-bridging between the mercury surface and the Eu³⁺. The diagnostic criteria used to support the proposed mechanism included analysis of the rate—potential behavior and of the effects of competitively adsorbed iodide ions upon the reduction rates.     

Comparison of the structural and chemical composition of giant T-even phage heads.

A study has been made of the structure of the capsids of T4D giant phage produced from mutants in gene 23 and temperature-sensitive mutants in gene 24, and T4D and T2L giant phage formed by the addition of L-canavanine followed by an Larginine chase in the growth medium. All the giant phage capsids have been shown to be built according to the same geometrical architecture. This consists of a near-hexagonal surface net, lattice constant 129.5 A, folded into a left-handed T = 13 prolate icosahedron elongated along one of its fivefold symmetry axes. Their only apparent difference from wild-type T-even phage capsids is their abnormally elongated tubular part. A comparison of the capsomere morphologies and protein compositions of the giant phage capsids showed that all T4D giants are identical but differ from T2L: The T4D capsomere has a complex (6 + 6 + 1)-type morphology, whereas the T2L has a simple 6-type. T2L phage, however, lack two capsid proteins, "soc" and "hoc", present in T4D. The difference in capsomere morphology can therefore be related to the difference in the protein compositions of these two phage. Possible differences between the initiation and means of length regulation of giant phage heads and the aberrant polyheads are discussed.     

Spectrophotometric determination of germanium with Catechol Violet and cetyltrimethylammonium bromide

A ternary complex between germanium, Catechol Violet (CV) and cetyltrimethylanunoniuni bromide is proposed for the determination of germanium. The stoichiometric ratio Ge:CV is 1:2. Beer's law is obeyed from 0.1 to 1.0 ppm of Ge. The method is highly selective. Interference from Sn(IV), Fe(III), Bi(III), Cr(VI), Mo(VI), V(V) and Sb(III) in mg amounts is eliminated by extracting the germanium into carbon tetrachloride from 9M HC1 and then stripping into water before the photometric determination.     

Cyclobutane polymers from acrylonitrile dimer

A wide variety of polyesters and polyamides was prepared from 1,2- and 1,3-disubstituted cyclobutane derivatives. In general, polyamides containing the trans-disubstituted cyclobutane ring were crystalline. Polyesters containing predominantly the trans-isomer tended to be crystalline but those containing mixtures of cis-trans isomers were amorphous. Mechanical properties have been determined wherever possible. Appreciable yields of cyclic dimers were isolated during the preparation of certain polyesters. These cyclic dimers readily polymerized when treated with dibutylzinc. Examination of acrylonitrile dimer (1,2-dicyanocyclobutane) by gas chromatography failed to reveal the presence of any 1,3-dicyanocyclobutanes.     

Double asymmetric induction in benchrotrene derivatives

Attachment of a Cr(CO)₃ moiety to one of the aromatic rings in rigid diaryl ketones of very low prochirality followed by addition of a chiral and optically pure reagent leads to satisfactory asymmetric induction ~40%. Asymmetric induction is smaller in the case of acyclic systems.     

Phase transitions in chlorobenzene-cis-decalin mixtures

The dielectric constant ?' and loss tangent tan δ of chlorobenzene-cis-decalin mixtures have been measured in the temperature range 77 K to 330 K and frequency range 0.1 to 100 kHz. On cooling, ?' increases with decreasing temperature upto about 135 K, after which it drops rapidly with decreasing T followed by a slow decrease. This indicates that the liquid mixture goes to an amorphous phase which transforms to a glass phase of restricted dipole rotation below T_g; however, the peak in ?' is due to relaxation in the amorphous phase (α relaxation) and does not give an exact T_g. On heating, the behaviour of the cooling curve is retraced upto 160 K, after which ?' drops suddenly to a value lower than that at 77 K in the glass phase. This indicates the transition to a crystalline phase in which dipole rotational freedom is completely lost. The crystalline phase changes to a eutectic liquid phase of high ?' at a temperature (200 K) lower than the melting point of chlorobenzene and cis-decalin. Dielectric dispersion is observed only in the glass and amorphous phases. The dielectric relaxation time is independent of the concentration of chlorobenzene.