Solvent and Zn2+ Binding Effects on the Photo-physical Properties of (Dibenzoylmethanato)boron Difluoride

Absorption and emission spectroscopic studies of (dibenzoylmethanato)boron difluoride (1bf) in various polar and non-polar, protic and aprotic solvents are reported. The solvatochromic shifts of the spectral bands were examined in terms of solvent properties, including donor and acceptor numbers, followed by multilinear regression in which several solvent parameters were simultaneously analyzed. This π-conjugated positively charged system exhibits excellent solvatochromism. Variations in the electronic absorption spectral characteristics of 1bf were studied in solution in the presence of zinc perchlorate. Absorption spectral studies indicate stable complex formation between the zinc ion and 1bf in the ground state in aprotic dipolar benzonitrile rather than in protic polar solvent methanol. Zinc ion binding of 1bf was theoretically rationalized through frontier molecular orbital interaction.     

A Model for Flow and Deformation in Unsaturated Swelling Porous Media

A thermomechanical theory for multiphase transport in unsaturated swelling porous media is developed on the basis of Hybrid Mixture Theory (saturated systems can also be modeled as a special case of this general theory). The aim is to comprehensively and non-empirically describe the effect of viscoelastic deformation on fluid transport (and vice versa) for swelling porous materials. Three phases are considered in the system: the swelling solid matrix s, liquid l, and air a. The Coleman–Noll procedure is used to obtain the restrictions on the form of the constitutive equations. The form of Darcy’s law for the fluid phase, which takes into account both Fickian and non-Fickian transport, is slightly different from the forms obtained by other researchers though all the terms have been included. When the fluid phases interact with the swelling solid porous matrix, deformation occurs. Viscoelastic large deformation of the solid matrix is investigated. A simple form of differential-integral equation is obtained for the fluid transport under isothermal conditions, which can be coupled with the deformation of the solid matrix to solve for transport in an unsaturated system. The modeling theory thus developed, which involves two-way coupling of the viscoelastic solid deformation and fluid transport, can be applied to study the processing of biopolymers, for example, soaking of foodstuffs and stress-crack predictions. Moreover, extension and modification of this modeling theory can be applied to study a vast variety of problems, such as drying of gels, consolidation of clays, drug delivery, and absorption of liquids in diapers.     

Direct Conversion of Alcohols into Alkenes by Dehydrogenative Coupling with Hydrazine/Hydrazone Catalyzed by Manganese

We have developed unprecedented methods for the direct transformation of primary alcohols to alkenes in the presence of hydrazine, and for the synthesis of mixed alkenes by the reaction of alcohols with hydrazones. The reactions are catalyzed by a manganese pincer complex and proceed in absence of added base or hydrogen acceptors, liberating dihydrogen, dinitrogen, and water as the only byproducts. The proposed mechanism, based on preparation of proposed intermediates and control experiments, suggests that the transformation occurs through metal–ligand cooperative N?H activation of a hydrazone intermediate.     

Selective Encapsulation of Disaccharide Xylobiose by an Aromatic Foldamer Helical Capsule

Xylobiose sequestration in a helical aromatic oligoamide capsule was evidenced by circular dichroism, NMR spectroscopy, and crystallography. The preparation of the 5 kDa oligoamide sequence was made possible by the transient use of acid‐labile dimethoxybenzyl tertiary amide substituents that disrupt helical folding and prevent double helix formation. Binding of other disaccharides was not detected. Crystallographic data revealed a complex composed of a d ‐xylobiose α anomer and two water molecules accommodated in the right‐handed helix. The disaccharide was found to adopt an unusual all‐axial compact conformation. A dense network of 18 hydrogen bonds forms between the guest, the cavity wall, and the two water molecules.     

Kinetic and Mechanistic Studies on the Oxidation of Nitrogen(III) (HNO2/$\rm{{{NO}}_2^ - }$) by the Tris(biguanide)manganese(IV) Ion in Aqueous Acidic Media

The kinetics of a net two‐electron transfer between an authentic Mn^IV complex, [Mn(bigH)₃]⁴⁺ (Fig. 1; bigH = biguanide = C₂N₅H₇), and nitrite in aqueous solution in the pH interval 2.00–3.60 is described. Stoichiometric data for the reaction clearly indicates Δ[Mn^IV]/Δ[N^III]_T = 1.07 ± 0.10, and is detected as the oxidized product of nitrite ([N^III]_T = [HNO₂] + [ ]). Though both HNO₂ and are found to be reactive, the latter is kinetically superior in reducing the fully protonated Mn^IV complex. Proton‐coupled electron transfer (PCET; 1e, 1H⁺) reduces the activation barrier for the thermodynamically unfavorable reaction of weakly oxidizing Mn^IV species. At the end of the redox process, the ligand bigH is released, and the high protonation constants of the ligand carry the overall reaction to completion.     

Solution NMR studies of supramolecular complexes of [60]- and [70]fullerenes with mono O-substituted calix[6]arene

Supramolecular complexation of [60]- and [70]fullerenes with 37-allyl-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4-tert butyl)calix[6]arene (I) has been studied in CCl(4) medium by NMR spectrometric method. All of the complexes are found to be stable with 1:1 stoichiometry. Formation constants (K) of the above supramolecular complexes have been determined from systematic variation of NMR chemical shifts of specific protons of I in the presence of [60]- and [70]fullerenes. Trends in the K value suggest that [70]fullerene binds more strongly with I relative to [60]fullerene. Both PM3 and ab initio calculations reveal that the intermolecular interaction in the [70]fullerene/I complex proceeds through quite deep energy minima.     

Anomalous dielectric relaxation of water molecules at the surface of an aqueous micelle

Dielectric relaxation of aqueous solutions of micelles, proteins, and many complex systems shows an anomalous dispersion at frequencies intermediate between those corresponding to the rotational motion of bulk water and that of the organized assembly or macromolecule. The precise origin of this anomalous dispersion is not well-understood. In this work we employ large scale atomistic molecular dynamics simulations to investigate the dielectric relaxation (DR) of water molecules in an aqueous micellar solution of cesium pentadecafluorooctanoate. The simulations clearly show the presence of a slow component in the moment-moment time correlation function [PhiMW(t)] of water molecules, with a time constant of about 40 ps, in contrast to only 9 ps for bulk water. Interestingly, the orientational time correlation function [Cmu(t)] of individual water molecules at the surface exhibits a component with a time constant of about 19 ps. We show that these two time constants can be related by the well-known micro-macrorelations of statistical mechanics. In addition, the reorientation of surface water molecules exhibits a very slow component that decays with a time constant of about 500 ps. An analysis of hydrogen bond lifetime and of the rotational relaxation in the coordinate frame fixed on the micellar body seems to suggest that the 500 ps component owes its origin to the existence of an extended hydrogen bond network of water molecules at the surface. However, this ultraslow component is not found in the total moment-moment time correlation function of water molecules in the solution. The slow DR of hydration water is found to be well correlated with the slow solvation dynamics of cesium ions at the water-micelle interface.     

Tris(salicylhydroxamato)manganese(III)

Summary Hydroxamic acids show a degree of selectivity towards transition metal ions having symmetrical d-electron configuration, e.g. vanadium(V) (d⁰) and iron(III) (d⁶⁵). Hydroxamato complexes of metal ions having unsymmetrical d-electron distribution are rare. Thus for manganese(III) (d⁴) only some thiohydroxamato complexes⁽¹⁾ have been characterised so far. In this communication we report on the first synthesis of a salicylhydroxamato complex of manganese(III). Such investigations are of interest because these higher valent manganese complexes are potentially models for the water-splitting complex present in photosystem II⁽²).     

Decomposition kinetics ofbis(biguanide)silver(III) cation in acid perchlorate media

Summary In acid perchlorate media, the title complex undergoes intramolecular redox decomposition generating ultimately Ag⁺ ion and oxidation products of the ligand. The reaction follows a simple first-order process, and the observed pseudo-first-order rate constant is given by k_obs=k₀+kK_a/[H⁺] where K_a is the deprotonation constant of the parent complex; pK_a is approximately 5.9 at 30°. The values of 10⁵ k₀(s^–1) and 10⁷ kK_a (Ms^–1) at 30°, I=1.0 M, are 9.3±0.1 and 11.8±1.3; corresponding H (kJ/mol), S (JK^–1 M^–1) values are 105±0.5, 23±1 and 79±8,-96±5, respectively. The results are compared with those for similar reaction of (ethylenebisbiguanide)silver(III) and effect of change in ligand structure on kinetic behaviours of these complexes is discussed.