Water Molecules in Hydrotalcite‐like Layered Double Hydroxides: Interplay between the Hydration of the Anions and the Metal Hydroxide Layer

Layered double hydroxides comprise positively charged metal hydroxide layers and intercalated anions. These materials are obtained from aqueous medium both in nature and in the laboratory. Consequently the layered double hydroxides include a considerable amount of water. The presented study was designed to determine the proportion of water associated with the hydration sphere of the anion as opposed to that of the metal hydroxide slab. Among the two differently bound water species observed in all layered double hydroxides, the weakly bound water is associated with the metal hydroxide layer and is lost at 100 °C, whereas the strongly bound water is in the hydration sphere of the anion and is lost at higher temperatures (100 °C ≤ T ≤ 250 °C). This is in contrast to the better known cationic clays, wherein all the intercalated water is generally found to be in the hydration sphere of the cations. Further the water molecules in layered double hydroxides also bond to each other, leading to the incorporation of water in excess of what is predicted by the Miyata formula (Miyata, 1975) based on crystal chemical considerations. The excess water is one of the reasons for the poor crystallinity of layered hydroxides.     

Solution Thermodynamics of <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis><Superscript>/</Superscript>-Ethylenebis-(salicylideneiminato)-diaquochromium(III) Chloride in Aqueous Dimethylsulphoxide

The densities, viscosities and refractive indices of N,N ^/-ethylene-bis(salicylideneiminato)-diaquochromium(III) chloride, [Cr(salen)(H₂O)₂]Cl, in aqueous dimethylsulfoxide (DMSO) with different mass fractions (w ₂ = 0.20, 0.40, 0.60, 0.80 and 1.00) of DMSO were determined at 298.15, 308.15 and 318.15 K under atmospheric pressure. From measured densities, viscosities and refractive indices the apparent molar volumes (V _φ ), standard partial molar volume (V _φ ⁰ ), the slope (S _V ^* ), standard isobaric partial molar expansibility (φ _E ⁰ ) and its temperature dependence (?φ _E ⁰ /?T) _p , the viscosity B-coefficient, its temperature dependence (?B/?T), solvation number (S _n ) and apparent molar refractivity (R _D ^φ ), etc., were calculated and discussed on the basis of ion–ion and ion–solvent interactions. These results revealed that the solutions are characterized by ion–solvent interactions rather than by ion–ion interactions and the complex behaves as a long range structure maker. Thermodynamics of viscous flow was discussed in terms of transition state theory.     

Electronic Structure Insights into the Solvation of Magnesium Ions with Cyclic and Acyclic Carbonates

A computational framework to rank the solvation behavior of Mg²⁺ in carbonates by using molecular dynamics simulations and density functional theory is reported. Based on the binding energies and enthalpies of solvation calculated at the M06‐2X/6‐311++G(d,p) level of theory and the free energies of solvation from ABF‐MD simulations, we find that ethylene carbonate (EC) and the ethylene carbonate:propylene carbonate (EC:PC) binary mixture are the best carbonate solvents for interacting with Mg²⁺. Natural bond orbital and quantum theory of atoms in molecules analyses support the thermochemistry calculations with the highest values of charge transfer, perturbative stabilization energies, electron densities, and Wiberg bond indices being observed in the Mg²⁺(EC) and Mg²⁺(EC:PC) complexes. The plots of the noncovalent interactions indicate that those responsible for the formation of Mg²⁺ carbonate complexes are strong‐to‐weak attractive interactions, depending on the regions that are interacting. Finally, density of state calculations indicate that the interactions between Mg²⁺ and the carbonate solvents affects the HOMO and LUMO states of all carbonate solvents and moves them to more negative energy values.     

Conservation of order, disorder, and "crystallinity" during anion-exchange reactions among layered double hydroxides (LDHs) of Zn with Al

Carbonate and chloride ions mediate an ordered stacking of metal hydroxide slabs to yield ordered layered double hydroxides (LDHs) of Zn with Al, by virtue of their ability to occupy crystallographically well-defined interlayer sites. Other anions such as ClO(4)- (T(d)), BrO(3)- (C(3v)), and NO(3)- (coordination symmetry C(2v)) whose symmetry does not match the symmetry of the interlayer sites (D(3h) or O(h)) introduce a significant number of stacking faults, leading to turbostratic disorder. SO(4)(2-) ions (coordination symmetry C(3v)) alter the long-range stacking of the metal hydroxide slabs to nucleate a different polytype. The degree of disorder is also affected by the method of synthesis. Anion-exchange reactions yield a solid with a greater degree of order if the incoming ion is a CO3(2-) or Cl-. Incoming NO(3)- ions yield an interstratified phase, whereas incoming SO(4)(2-) ions generate turbostratic disorder. Conservation or its converse, elimination, of stacking disorders during anion exchange is the net result of several competing factors such as (i) the orientation of the hydroxyl groups in the interlayer region, (ii) the symmetry of the interlayer sites, (iii) the symmetry of the incoming ion, and (iv) the configuration of the anion. These short-range interactions ultimately affect the long-range stacking order or "crystallinity" of the LDH.     

Thin film processing of axially modified phthalocyanine derivatives

A new class of hydroxyl terminated silicon phthalocyanine (SiPc) derivatives was prepared by capping dihydroxy SiPc with a variety of compounds containing two or more hydroxyl groups. The unique feature of these polyhydroxy SiPc derivatives is their function as crosslinkers that allows their processability into polyurethane films on reacting with isophorone diisocyanate. The films possess high impact resistance, thermostability at 125°C for over 10 h, and low absorption in the visible region. Solid polymeric films incorporating over 20% (w/w) of Pc rings were prepared without phase segregation. The kinetic parameters of polymerization, such as reaction conversion versus time and temperature, heat of reaction (ΔH), activation energy (ΔE), and reaction order (n) were investigated. Some of the polyhydroxy SiPc derivatives exhibit very high solubility in common organic solvents and possess adequate optical transparency for use as nonlinear optical materials in vision protection against laser light. © 1996 John Wiley & Sons, Inc.     

Volume effect of laser produced plasma on X-ray emissions

An investigation of x-ray emission from Cu plasma produced by 1.054 μm Nd:glass laser pulses of 5 ns duration, at 2 × 10¹² − 2 × 10¹³ W cm⁻² is reported. The x-ray emission has been studied as a function of target position with respect to the laser beam focus position. It has been observed that x-ray emissions from ns duration plasma show a volume effect similar to subnanosecond plasmas. Due to this effect the x-ray yield increases when target is moved away relative to the best focal plane of the laser beam. This result supports the theoretical model of Tallents and has also been testified independently using suitably modified theoretical model for our experimental conditions. While above result is in good agreement with similar experimental results obtained for sub-nanosecond laser produced plasmas, it differs from result claiming filamentation rather than pure geometrical effect leading to x-ray enhancement for ns plasmas.     

Structural Synthon Approach to Predict the Possible Polytypes of Layered Double Hydroxides

The complete universe of possible polytypes of layered double hydroxides (LDH) is predicted on the basis of symmetry arguments. A single [MX₂] (X = OH) layer, also defined as a structural synthon, belongs to the layer group P$\bar{3}$ 2/m1. These layers can be stacked in such a way as to conserve the unique 3‐axis of the layer in the resultant crystal. The different stacking sequences that facilitate symmetry conservation, yield the different possible polytypes of rhombohedral and hexagonal symmetries. More polytypes can be envisaged by including stacking sequences that systematically destroy the principal symmetry elements of the structural synthon. Thereby, stacking sequences that destroy the 3‐axis, while retaining the 2‐axis, yield possible polytypes of monoclinic symmetry. The nitrate‐containing LDH of zinc and aluminum crystallizes in a faulted structure in which, the planar faults are shown to arise on account of stacking sequences whose local symmetry is monoclinic. This approach to polytype prediction expands on an earlier reported method by Bookin and Drits and is very general with important implications for other classes of layered materials.     

Phenoxide bridged tetranuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes: Syntheses,characterization and fluorescence studies

A tetranucleating compartmental Schiff base ligand system has been derived from 2,6-diformyl-4-methylphenol, p-phenylenediamine and 2-aminomethylbenzimidazole. Phenoxide bridged later first row transition metal(II) complexes of this ligand have been prepared by conventional sequential route. Ligand and complexes were characterized by routine physicochemical characterizations. The mutual influence of metal centers in terms of cooperative effect on the electronic, magnetic, electrochemical and structural properties was investigated. The Schiff-bases exhibit fluorescence originating from intraligand (π→π?) transitions. Metal mediated fluorescence enhancement is observed on complexation with Zn(II), whereas metal mediated fluorescence quenching occurs in all other complexes.     

Synthesis of the Benzoate Intercalated Layered Double ­Hydroxide of Nickel and Aluminum: Application of Mering's Rule

Homogeneous precipitation from a mixed metal nitrate solution using benzamide as a hydrolysis agent results in the formation of a benzoate‐intercalated layered double hydroxide of nickel and aluminum with a basal spacing of 15.6 Å. The alternation in the line widths of the basal reflections has been shown to arise by the random interstratification of a 8.8 Å phase with an altered orientation of the benzoate ions. This work is an illustration of the application of Mering's rules to model disorder in layered double hydroxides.     

Second-Order Nonlinear Optical Properties of a Polymer Exhibiting Optical Transparency Down to 340 nm

Abstract

The synthesis and characterization of an epoxy-based nonlinear optical (NLO) polymer exhibiting optical transparency down to 340 nm is reported. The synthesized polymers show spectroscopic properties (NMR, IR, UV) in accordance with the proposed structures. A glass transition temperature (T_g) of 92°C and a thermal degradation temperature (T_d) of 322°C were recorded. The poled polymer film exhibits stable second-order nonlinear optical activity (d₃₃ = 4.2 pm/V) over a period of 800 hours as characterized by the temporal response of the second harmonic signal at room temperature.