Dielectric relaxation and ac conduction in multiferroic TbMnO3 ceramics: Impedance spectroscopy analysis

Polycrystalline samples of Tb_1−xAl_xMnO₃ (x = 0, 0.1, 0.2) have been synthesized by means of standard high-temperature solid-state reaction technique. Detailed studies on the effect of compositional variation of aluminum (Al) on the electrical behavior (complex impedance Z*, complex modulus M*, and relaxation mechanisms) of the parent TbMnO₃ have been performed by using the nondestructive complex impedance spectroscopy technique at temperatures above room temperature. In the temperature range covered, the impedance plots signalize that the grains are the unique responsible for the conduction mechanism of the concerned material. The impedance spectra are well modeled in terms of electrical equivalent circuit with a grain resistance (R_g) and constant phase element impedance (Z_CPE). The conductivity data of the undoped and Al-doped samples are well fitted by the universal Jonscher's power law. The resulting fitting parameters indicate that for the studied samples, the hopping process occurs between neighboring sites. Activation energy values for dc conductivity are calculated for undoped and Al-doped samples and found to decrease when Al is incorporated. In turn, the emergence of single arc in the complex modulus spectrum for all the compositions of Al suggests that for the studied samples only one type of relaxation behavior is present at the selected temperatures. A non-Debye-type relaxation is clearly verified. The relaxation process in the present samples seems to be composition and temperature dependent, particularly at higher frequencies.     

Convenient Route to Primary (Z)-Allyl Amines and Homologs

A convenient two-step procedure for the synthesis of primary (Z)-allyl amines, (Z)-homoallyl amines [(Z)-but-3-enylamines], and (Z)-pent-4-enylamines using the Wittig reaction was achieved. The use of nonstabilized ylides from triphenylphosphonium salt, potassium salt, and apolar solvent produced (Z/E)-geometric isomer ratios generally greater than 1.6. The amine moiety was masked using a phtalimide group that was removed successfully in the last step of the process in two different conditions, NH₂NH₂/EtOH/rt or CH₃NH₂/EtOH/rt. However, in some cases, reduction of the C = C double bond in the deprotection with hydrazine was concomitantly observed.     

Ionic liquid–water mixtures as solvents for poly(N-vinylimidazole)

Ionic liquids extend the Hofmeister series and create a wide range of new possibilities for processes involving salt effects on both soluble and crosslinked systems. This work reports on some mixtures of water with NaCl or an ionic liquid, either 1-ethyl-3-methylimidazolium tosylate or 1-hexyl-3-methylimidazolium chloride, which are better solvents for linear poly(N-vinylimidazole) (L-PVI) than water, i.e., that exhibit a salting-in effect. The intensity of the salt effects was measured on the basis of the polymer solubility, the decrease in polymer–solvent interaction parameter (measured by light scattering), and the increase of coil size (measured through the intrinsic viscosity). It was thus found that the intensity of the salting-in effect of either NaCl or 1-hexyl-3-methylimidazolium chloride on L-PVI is different (larger for the ionic liquid), which denotes that salt effects are not under anion control, and the mechanisms operating in the linear and crosslinked polymers are different. These results are discussed after accounting for the role of ion–polymer interactions.     

Iodide‐Induced Shuttling of a Halogen‐ and Hydrogen‐Bonding Two‐Station Rotaxane

The first example of utilizing halogen‐bonding anion recognition to facilitate molecular motion in an interlocked structure is described. A halogen‐bonding and hydrogen‐bonding bistable rotaxane is prepared and demonstrated to undergo shuttling of the macrocycle component from the hydrogen‐bonding station to the halogen‐bonding station upon iodide recognition. In contrast, chloride‐anion binding reinforces the macrocycle to reside at the hydrogen‐bonding station.     

Synthesis of Prochiral Imines from Aromatic Ketones Using Magnesium Perchlorate as Catalytic Promotor

We report the synthesis of new prochiral imines by the reaction of nonsymmetric benzophenone derivates, bencylamines, and anilines in good yields. All the products were characterized by ¹H and ¹³C NMR as an isomeric E/Z mixture. Also, the molecular structures for five derivatives determined by single-crystal x-ray experiments are presented.

Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file.     

Pro- $$\mathcal {C}$$ C congruence properties for groups of rooted tree automorphisms

We propose a generalisation of the congruence subgroup problem for groups acting on rooted trees. Instead of only comparing the profinite completion to that given by level stabilizers, we also compare pro- $$\mathcal {C}$$ completions of the group, where $$\mathcal {C}$$ is a pseudo-variety of finite groups. A group acting on a rooted, locally finite tree has the $$\mathcal {C}$$ -congruence subgroup property ( $$\mathcal {C}$$ -CSP) if its pro- $$\mathcal {C}$$ completion coincides with the completion with respect to level stabilizers. We give a sufficient condition for a weakly regular branch group to have the $$\mathcal {C}$$ -CSP. In the case where $$\mathcal {C}$$ is also closed under extensions (for instance the class of all finite p-groups for some prime p), our sufficient condition is also necessary. We apply the criterion to show that the Basilica group and the GGS-groups with constant defining vector (odd prime relatives of the Basilica group) have the p-CSP.     

Separation of chlorophylls and their degradation products in marine phytoplankton by reversed-phase high-performance liquid chromatography

Summary An RP-HPLC procedure which separates chlorophylls and their degradation products is described. By employing an amonium acetate buffered mobile phase and a linear gradient elution, complex mixtures of chloroand phaeopigments are separated in less than 30 minutes. The method has been applied to the control of chlorophyll enzymatic degradation in cultures of two algal species, and has also been successfully used in the analysis of algal xanthophylls.     

(Trimethylsilylmethyl)tributyltin: A convenient in situ precursor of trimethylsilylmethyllithium

The transmetalation reaction between (trimethylsilylmethyl)tributyltin and $\text{n}$-butyllithium proceeds in quantitative yield at 0°C in a mixture of tetrahydrofuran and hexane to afford trimethylsilylmethyllithium and tetrabutyltin. The methylenation of aldehydes and ketones utilizing this $\text{in situ}$ procedure is demonstrated.     

Photoprotective Potential of Baccharis antioquensis (Asteraceae) as Natural Sunscreen

In the quest for new natural agents of photoprotection, we evaluated the photoprotective and antioxidant activity of B. antioquensis leaf extracts as well as its phenolic composition. The methanolic extract treated with activated carbon showed the highest absorption coefficients for UVA‐UVB radiation, as well as an antioxidant capacity comparable to butylated hydroxy toluene. Furthermore, the formulation containing this extract showed suitable sensorial and photostable characteristics for topical use, and significant values of UVAPF, critical wavelength (λ_c), UVA/UVB ratio and sun protection factor (5.3, 378 nm, 0.78 and 9.1 ± 0.1, respectively). In addition, three glycoside derivatives of quercetin, a kaempferol glycoside and a derivative of caffeic acid were the main polyphenolic compounds identified. These results demonstrate the potential of B. antioquensis extracts to be used as active components of novel, natural sunscreens.