N‐Acetyl‐l‐tyrosine methyl ester monohydrate at 293 and 123 K

The crystal structure of a protected l ‐tyrosine, namely N‐acetyl‐l ‐tyrosine methyl ester monohydrate, C₁₂H₁₅NO₄·H₂O, was determined at both 293 (2) and 123 (2) K. The structure exhibits a network of O—H...O and N—H...O hydrogen bonds, in which the water molecule plays a crucial role as an acceptor of one and a donor of two hydrogen bonds. Molecules of water and of the protected l ‐tyrosine form hydrogen‐bonded layers perpendicular to [001]. C—H...π interactions are observed in the hydrophobic regions of the structure. The structure is similar to that of N‐acetyl‐l ‐tyrosine ethyl ester monohydrate [Soriano‐García (1993). Acta Cryst. C 49 , 96–97].     

Radiation formation of polymeric nanogels

An alternative method of synthesis of polymeric nanogels is proposed, based not on polymerization, but on intramolecular crosslinking of polymer chains, initiated by pulse irradiation in dilute aqueous solutions. Kinetic data show that for many water-soluble polymers irradiation under these conditions result in intramolecular crosslinking. Preliminary product studies on poly(vinyl alcohol) indicate that in fact internally crosslinked macromolecules can be obtained by this technique.     

Molecular reorientation in the nematic and rotatory phases of 4,4'-di-n-pentyloxyazoxybenzene

Abstract

Results of dielectric relaxation, quasielastic neutron scattering, calorimetric D.S.C. and preliminary X-ray measurements on the fifth member, POAOB, of the 4,4'-di-n-alkoxyazoxybenzene homologous series are presented. It has been found that POAOB exhibits two mesophases: a nematic (N) and an intermediate crystalline phase (CI) just below it. From comparison of the dielectric relaxation and quasielastic neutron scattering studies we can conclude that in the nematic phase the molecule as a whole performs rotational diffusion around the long axis (ω ~ 150ps) and at the same time the two moieties perform faster independent reorientations around the N-δ bonds (ω denotes a benzene ring) with δ ~ 5 ps. The intermediate crystal phase is identified as a solid uniaxial rotational phase in which fast molecular reorientations exist. It seems that the fast reorientations observed in the nematic phase to some extent survive to the crystal I phase. A model of molecular arrangements in the crystal I phase is proposed, and it explains the reduction of the dielectric increment observed on passing from the nematic phase to this phase.     

Synthesis and Reactivity of Metallophosphanes

Abstract

Aspects of the reactivity of metallophosphanes are presented along with the synthesis and structure determinations for new aluminophosphanes.     

Fractal-like scaling in liquid crystals. Odd–even effect of scaling exponents in the 4,4′-di-n-alkyloxyazoxybenzene homologous series

Abstract

A power law behaviour of dielectric loss versus frequency has been found in members of the 4,4′-di-n-alkyloxyazoxybenzene series. For each member the scaling of the data with respect to v/v _max is possible in the liquid-crystalline temperature range; this suggests a fractal-like geometry in nematic and smectic phases. The values of the scaling exponents, indicating a cooperativity of molecular reorientation around the short axes, exhibit an odd–even effect.     

Detailed study of a two component smectic 4TPB–8OCB system with a nematic gap. Phase diagram and viscosity study

Abstract

The phase diagram of a two component system composed of two smectic compounds: 4-octyloxy-4′-cyanobiphenyl (8OCB) and 4-isothiocyanatophenyl 4-butylbenzoate (4TPB) was investigated. Three Miesowicz viscosity coefficients η₁, η₂, η₃ and the refractive indices at different temperatures as well as the enthalpies of the phase transitions were measured. It was stated, that the properties of the induced nematic phase, for example, the nematic phase existing between two smectic regions, are the same as the ones observed in the case of low viscosity nematic mesogens.     

Neutron scattering by liquid crystals. A new analysis

Abstract

This paper presents a new analysis of all (old and new) quasi-elastic neutron scattering results obtained for seven members of the PAA series (n = 1,…, 7). The analysis is based upon a discussion of the parameter p, which is the deviation of the intensity of the elastic component from the intensity demanded by a reorientational model. If the value of this parameter is negative, it may be interpreted as an indication of an additional motion which has not been taken into account in the model. The first model analysed in this way for all seven substances in the nematic phase was that of the rotational diffusion of the whole molecule in its stretched (trans) conformation, around the long axis. A very large negative values of p (p ? -60 per cent) was obtained, clearly showing that some motions have been neglected. The second model was that of the rotational diffusion of moieties consisting of benzene rings plus alkoxy side-chains. The diffusion occurs around the benzene para-axes, and the alkoxy side-chains are assumed to be in their trans conformations. Less negative values of p (p ? -10 per cent) were obtained, indicating that other motions still exist. Since these must take place in the alkyl chains, we make a third step in the analysis, in which we retain the second model but now take substances which are alkyl-deuterated (which means masking for the neutron incoherent scattering method). Now values of p which are nearly zero are obtained which means that the motions previously ignored indeed exist in the alkyl chains. An even-odd effect observed in the dependence of p on n indicates that even molecules are more mobile than odd ones, probably due to less steric hindrance in more ordered, even substances.

A critical discussion on reorientational correlation times is also presented.     

Dielectric studies of the 4-n-alkyl-4'-thiocyanatobiphenyl (nBT) homologous series (n=2-10) in the isotropic and E phases

Results of the dielectric studies of nine members of the nBT (4-n-alkyl-4'-thiocyanatobiphenyl, n= 2-10) homologous series in the crystal E (E) and isotropic (I) phases are presented. The dependence of the static permittivity ?_s in the isotropic phase, the longitudinal relaxation times τ_is and τ_∥, and the activation enthalpies ΔH _is and ΔH _∥ on the number of carbon atoms in the alkyl chain n are analysed. A considerable increase in the retardation factor g= τ_∥/τ_is with decreasing n is observed. The results are compared with those obtained for similar two-ring homologous series. The parameters characterizing the molecular rotations around the short axis in the E phase (τ_∥ and ΔH _∥) indicate a hardening of this solid-like phase with shortening of the alkyl chain.     

Radiation-Induced Oxidation Reactions of 2-Selenouracil in Aqueous Solutions: Comparison with Sulfur Analog of Uracil

One-electron oxidation of 2-selenouracil (2-SeU) by hydroxyl (^●OH) and azide (^●N₃) radicals leads to various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by the density functional theory (DFT) method. The transient absorption spectra recorded in the reactions of ^●OH with 2-SeU are dominated by an absorption band with an λ_max = 440 nm, the intensity of which depends on the concentration of 2-SeU and pH. Based on the combination of conductometric and DFT studies, the transient absorption band observed both at low and high concentrations of 2-SeU was assigned to the dimeric 2c-3e Se-Se-bonded radical in neutral form (2^●). The dimeric radical (2^●) is formed in the reaction of a selenyl-type radical (6^●) with 2-SeU, and both radicals are in equilibrium with K_eq = 1.3 × 10⁴ M⁻¹ at pH 4 (below the pK_a of 2-SeU). Similar equilibrium with K_eq = 4.4 × 10³ M⁻¹ was determined for pH 10 (above the pK_a of 2-SeU), which admittedly involves the same radical (6^●) but with a dimeric 2c-3e Se-Se bonded radical in anionic form (2^●−). In turn, at the lowest concentration of 2-SeU (0.05 mM) and pH 10, the transient absorption spectrum is dominated by an absorption band with an λ_max = 390 nm, which was assigned to the ^●OH adduct to the double bond at C5 carbon atom (3^●) based on DFT calculations. Similar spectral and kinetic features were also observed during the ^●N₃-induced oxidation of 2-SeU. In principle, our results mostly revealed similarities in one-electron oxidation pathways of 2-SeU and 2-thiouracil (2-TU). The major difference concerns the stability of dimeric radicals with a 2c-3e chalcogen-chalcogen bond in favor of 2-SeU.