Microwave dielectric dispersion in ferroelectric telluric acid ammonium phosphate

The paper presents the results of the investigation of dielectric dispersion in monoclinic telluric acid ammonium phosphate crystal over the frequency range 1 MHz to 77 GHz. It is shown that ferroelectric dispersion of the Debye type along the direction perpendicular to the plane (101) occurs in the frequency range 10⁸ to 10¹¹ Hz and is caused by a single relaxational soft mode. The relaxational soft mode is caused by the flipping motion of protons between two potential minima in a hydrogen bond. The frequency of the flipping proton mode in the paraelectric phase varies according to v_s = 0.94 (T – T_o ) GHz and decreases to 1.75 GHz at T_c = 317.3 K. The relaxational mode gives the main contribution to the high static dielectric permittivity of this crystal which fits the Curie-Weiss law. The results confirm the order-disorder nature of a proper ferroelectric phase transition of second order.     

Biased Borate Esterification during Nucleoside Phosphorylase-Catalyzed Reactions: Apparent Equilibrium Shifts and Kinetic Implications**

Biocatalytic nucleoside (trans-)glycosylations catalyzed by nucleoside phosphorylases have evolved into a practical and convenient approach to the preparation of modified nucleosides, which are important pharmaceuticals for the treatment of various cancers and viral infections. However, the obtained yields in these reactions are generally determined exclusively by the innate thermodynamic properties of the nucleosides involved, hampering the biocatalytic access to many sought-after target nucleosides. We herein report an additional means for reaction engineering of these systems. We show how apparent equilibrium shifts in phosphorolysis and glycosylation reactions can be effected through entropically driven, biased esterification of nucleosides and ribosyl phosphates with inorganic borate. Our multifaceted analysis further describes the kinetic implications of this in situ reactant esterification for a model phosphorylase.     

Synthesis of enantiomerically pure cyclohex-2-en-1-ols: development of novel multicomponent reactions

Multicomponent reactions of aldehydes, dienophiles, and alcohols or carboxylic acid anhydrides have been developed for the first time. In situ generation of 1-acyloxy- and 1-alkoxy-1,3-butadiene derivatives in toluene in the presence of electron-deficient dienophiles provides selective and efficient access to functionalized cyclohex-2-ene-1-ols in good yields. Subsequent enzyme-catalyzed kinetic resolution gave the corresponding enantiomers with high enantioselectivity.     

Dielectric relaxation in ferroelectric (CH3)2NH2AL(so4)2 · 6H2O crystal

Abstract

This paper presents the results of the investigation of dielectric dispersion and ultrasonic velocity in the ferroelectric (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal. The crystal shows a critical slowing down process of polarization with an extremely long relaxation time of the dipole system (τ = 1.6 · 10^?7s at the phase transition point). The dielectric response over the frequency range up to 56 GHz in the paraelectric phase can be well described in terms of a monodispersive Debye-type formula. The activation energy of dipoles in the paraelectric phase is 0.11 eV = 8.5 kT_c . The results show that the proper ferroelectric phase transition is nearly critical and of the order-disorder type.     

Anomalien der physikalischen Eigenschaften von Sb2Se3-Kristallen

This paper presents the results of investigations of the d.c. conductivity, complex dielectric constants ϵ* = ϵ' – iϵ″ at 9.3 GHz, birefringence as well as the width of band gap versus temperature in Sb₂Se₃ single crystals. The anomalies in these properties were observed in the temperature range from 400 to 450 which are expected to be related with diffuse phase transition in this semiconductive chain structure compound.     

Dielektrische Eigenschaften und Phasenübergänge von M2SbF5 -Kristallen

This paper presents the results of investigations of complex dielectric constants ϵ* = ϵ′ — ιϵ″ at audio and mikrowave frequencies and d.c. conductivity versus temperature in M₂SbF₅-type ionic single crystals. The anomalies in these properties were observed along the b-axis which are expected to be related to structural phase transitions. An electric field causes the switching effect and negative-resistance I–E curves. In contrast to other antimony compounds the phonon contribution to ϵ(0) is small. However, it is considered that the audio-frequency anomalies of ϵ* are increased by the coupling of order parameter phonon with disorder modes.