Solvent Effects on the Enthalpy and Entropy of Activation for the Hydrolysis of <Emphasis Type="Italic">β</Emphasis>-Lactones

The hydrolysis of β-propiolactone and β-butyrolactone in binary water + dioxane mixtures was investigated by kinetic studies. The following conclusions were reached: First, β-propiolactone is more reactive than β-butyrolactone across the range of water + dioxane compositions. This observation was rationalized in terms of the electric charge flow caused by the β-butyrolactone’s methyl substituent. Second, hydrolysis of these lactones is essentially enthalpy controlled. Third, an increase in the dioxane percentage, which relaxes the intermolecular hydrogen bonds in the ordered structure of water, reduces the enthalpy of activation ΔH ^# and simultaneously increases the entropy of activation ΔS ^#(absolute value) for solvent compositions up to 60% dioxane. Fourth, plotting ΔH ^#/ΔS ^# against the solvent composition yields an N-shaped curve. This results is a consequence of the quadratic and cubic terms appearing in the expressions of ΔH ^# and ΔS ^# as functions of the solvent media composition. Fifth, an ABC classification was set up to characterize the behavior of ΔH ^#/ΔS ^# for the solvolysis of these lactones.     

Biological applications of gold nanoparticles

This critical review gives a short overview of the widespread use of gold nanoparticles in biology. We have identified four classes of applications in which gold nanoparticles have been used so far: labelling, delivering, heating, and sensing. For each of these applications the underlying mechanisms and concepts, the specific features of the gold nanoparticles needed for this application, as well as several examples are described (142 references).     

Spectroscopic Identification of Active Sites of Oxygen-Doped Carbon for Selective Oxygen Reduction to Hydrogen Peroxide

The electrochemical synthesis of hydrogen peroxide (H₂O₂) via a two-electron (2 e⁻) oxygen reduction reaction (ORR) process provides a promising alternative to replace the energy-intensive anthraquinone process. Herein, we develop a facile template-protected strategy to synthesize a highly active quinone-rich porous carbon catalyst for H₂O₂ electrochemical production. The optimized PCC₉₀₀ material exhibits remarkable activity and selectivity, of which the onset potential reaches 0.83 V vs. reversible hydrogen electrode in 0.1 M KOH and the H₂O₂ selectivity is over 95 % in a wide potential range. Comprehensive synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with electrocatalytic characterizations reveals the positive correlation between quinone content and 2 e⁻ ORR performance. The effectiveness of chair-form quinone groups as the most efficient active sites is highlighted by the molecule-mimic strategy and theoretical analysis.     

Supramolecular architecture in Langmuir and Langmuir-Blodgett films incorporating a chiral azobenzene

This article describes the synthesis and fabrication of Langmuir and Langmuir-Blodgett (LB) films incorporating a chiral azobenzene derivative, namely, ( S)-4- sec-butyloxy-4'-[5'-(methyloxycarbonyl)pentyl-1'-oxy]azobenzene, abbreviated as AZO-C4(S). Appropriate conditions for the fabrication of monolayers of AZO-C4(S) at the air-water interface have been established, and the resulting Langmuir films have been characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and UV-vis reflection spectroscopy. The results indicate the formation of an ordered trilayer at the air-water interface with UV-vis reflection spectroscopy showing a new supramolecular architecture for multilayered films as well as the formation of J aggregates. Films were transferred onto solid substrates, with AFM revealing well-ordered multilayered films without 3D defects. Infrared and UV-vis absorption spectroscopy indicate that the supramolecular architecture may be favored by the formation of H bonds between acid groups in neighboring layers and pi-pi intermolecular interactions. Circular dichroism spectra reveal chiro-optical activity in multilayered LB films.     

Front Cover: High-level ab initio evidence of bipyramidal Cu5 clusters as fluxional Jahn-Teller molecules (ChemPhysChem 19/2023)

Novel highly selective synthesis techniques have enable the production of atomically precise monodisperse metal clusters (AMCs) of subnanometer size. These AMCs exhibit ‘molecule-like’ structures that have distinct physical and chemical properties, significantly different from those of nanoparticles and bulk material. In this work, we study copper pentamer Cu₅ clusters as model AMCs by applying both density functional theory (DFT) and high-level (wave-function-based) ab initio methods, including those which are capable of accounting for the multi-state multi-reference character of the wavefunction at the conical intersection (CI) between different electronic states and augmenting the electronic basis set till achieving well-converged energy values and structures. After assessing the accuracy of a high-level multi-multireference ab initio protocol for the well-known Cu₃ case, we apply it to demonstrate that bypiramidal Cu₅ clusters are distorted Jahn-Teller (JT) molecules. The method is further used to evaluate the accuracy of single-reference approaches, finding that the coupled cluster singles and doubles and perturbative triples CCSD(T) method delivers the results closer to our ab initio predictions and that dispersion-corrected DFT can outperform the CCSD method. Finally, we discuss how JT effects and, more generally, conical intersections, are intimately connected to the fluxionality of AMCs, giving them a ‘floppy’ character that ultimately facilitates their interaction with environmental molecules and thus enhances their functioning as catalysts.     

Monitoring of hydroxylated polycyclic aromatic hydrocarbons in human tissues: Targeted and untargeted approaches using liquid chromatography-high resolution mass spectrometry

Hydroxylated polycyclic aromatic hydrocarbons are metabolites of persistent organic pollutants which are formed during the bioactivation process of biological matrices and whose toxicity is being studied. The aim of this work was the development of a novel analytical method for the determination of these metabolites in human tissues, known to have bioaccumulated their parent compounds. Samples were treated by salting-out assisted liquid-liquid extraction and the extracts were analyzed by ultra-high performance liquid chromatography coupled to mass spectrometry with a hybrid quadrupole-time-of-flight analyzer. The proposed method achieved limits of detection in the 0.15–9.0 ng/g range for the five target analytes (1-hydroxynaphthalene, 1-hydroxypyrene, 2-hydroxynaphthalene, 7-hydroxybenzo[a]pyrene, and 9-hydroxyphenanthrene). The quantification was achieved by matrix-matched calibration using 2,2´-biphenol as internal standard. For all compounds, relative standard deviation, calculated for six successive analyses, was below 12.1%, demonstrating good precision for the developed method. None of the target compounds was detected in the 34 studied samples. Moreover, an untargeted approach was applied to study the presence of other metabolites in the samples, as well as their conjugated forms and related compounds. For this objective, a homemade mass spectrometry database covering 81 compounds was created and none of them was detected in the samples.     

Porous membrane-protected molecularly imprinted polymer micro-solid-phase extraction for analysis of urinary cocaine and its metabolites using liquid chromatography – Tandem mass spectrometry

Porous membrane-protected micro-solid phase extraction (μ-SPE) using a molecularly imprinted polymer (MIP) as an adsorbent has been proposed as an integrated extraction-cleanup procedure for isolating cocaine (COC) and its metabolites [benzoylecgonine (BZE), ecgonine methyl ester (EME), and cocaethylene (CE)] from human urine. MIP beads have been synthesized using COC as a template molecule, ethylene dimethacrylate (EDMA) as a functional monomer, divinylbenzene (DVB) as a cross-linker, and 2,2′-azobisisobutyronitrile (AIBN) as an initiator. High performance liquid chromatography – tandem mass spectrometry (HPLC-MS/MS) has been used for quantifying the analytes after MIP-μ-SPE. Variables such as urine pH, adsorption temperature and time, mechanical (orbital-horizontal) stirring; and composition of the eluting solution and eluting time, were evaluated. The proposed method was shown to be precise and accurate [relative standard deviations (RSDs) of intra- and inter-day tests ranging from 3 to 8% and from 2 to 10%, respectively]; and analytical recoveries in the range of 89–100%). In addition, excellent accuracy was also verified after analyzing a FDT +25% control material for BZE. The detection limits were in the range of 0.16–1.7 ng L⁻¹, low enough for confirmative conclusions regarding cocaine abuse. The method was finally applied for screening/quantifying cocaine and metabolites in urine samples from poly-drug abusers.     

Structure and rheology of wormlike micelles

In a semi-dilute aqueous solution under certain conditions, surfactant molecules will self assemble to form wormlike micelles. The micelles are dynamic in structure since they can break and reform, providing an additional mode of relaxation. The viscoelastic properties of the wormlike micelles can be predicted using simple theological models. For many surfactant solutions the mechanical data can be related to the optical data by the stress-optical rule. From the viscoelastic data it is possible to estimate the breaking time of the micelle. The techniques of birefringence and small angle light scattering are used to study the microstructure of a surfactant solution under simple shear and extensional flow. The sample under investigation is a solution of cetyltrimethylammonium bromide and sodium salicylate in water, with a salt to surfactant ratio of 7.7. Below a critical shear rate, the birefringence increases linearly with shear rate and the stress-optical rule is valid. The SALS patterns reveal distinctive butterfly patterns indicating that scattering is a result of concentration fluctuations that moderately couple to the flow. However, above a critical shear rate the birefringence plateaus and the stress-optical rule is no longer valid. SALS patterns show both a bright streak and a butterfly pattern. The bright streak is caused by elongated structures aligned in the direction of the flow. The oriented structures occur when the characteristic time of flow is faster than the breaking time of the micelles.Dedicated to Prof. Dr. J. Meissner on the occasion of his retirement from the chair of Polymer Physics at the Eidgenössische Technische Hochschule (ETH) Zürich, Switzerland