Ultrasonic Broadband Spectrometry of Liquids A Research Tool in Pure and Applied Chemistry and Chemical Physics

This review is a survey of the many scientific applications of ultrasonic broadbandspectrometry (absorption and velocity measurements with coherent sound waves)in liquids and liquid systems, covering, at present, a frequency range from nearly10 kHz to 10 GHz. Ultrasonic spectrometry has proved to be an almost universalresearch tool in many laboratories, one that is useful for investigation of variouschemical, biochemical, and physicochemical systems. Sound waves traversingliquids induce periodic perturbations in pressure and temperature, which can shiftequilibria, resulting in characteristic sound absorption and velocity dispersionspectra. An analysis of such spectra yields valuable information about thermodynamic and kinetic parameters of the particular system that is often difficult toobtain by other methods. Since such periodic perturbations imposed on the systemare incremental in nearly all cases, the system can be studied under equilibriumconditions. All nonlinear effects (heating, nonconstant fluid compressibility, andothers) are negligible, permitting, for instance, the application of linearized rateequations. In this review, various examples of measured broadband spectra arepresented. Related elementary processes are discussed. Among these are ionicand molecular reactions, including mechanisms of association and complexation,proton transfer, solvation, isomerization, interconversion, side-group rotation,hydrogen-bonding, as well as stacking processes and micelle formation. Specialattention will be given to the extensive research on chemical relaxation.Fundamental early and recent publications are cited and discussed. Many referencesare included with particular emphasis on less well known research and publicationsfrom countries of the former USSR. This review aims at a demonstration of thewidespread applications of modern ultrasonic techniques in many fields ofliquid-state research.     

One-dimensional spin-polarized quantum-wire states in Au on Ni(110)

Au chain structures have been prepared on Ni(110). Au6 s,p-derived features in photoemission spectra are identified as quantum-wire states due to their strong dispersion along the chains and absence of dispersion perpendicular to the chains in agreement with our ab initio calculation of the electronic structure. Spin analysis reveals that the states have minority-spin character showing that the confinement of electrons in the chain structure depends on the electron spin.     

Nets and generalized quadrangles

There is a new method of constructing generalized quadrangles (GQs) given by S. Löwe, which is based on covering of nets; all GQs with a regular point can be represented in this way. Here we give a method of constructing GQs with a regular point using the so-called content functions on nets. In the last part of the paper we lay the foundations for a research project aiming to use the more general notion of content to classify GQs and maybe to construct new ones.Both authors acknowledge the financial support by CRUI and DAAD in the frame of Programma Vigoni, which made this work possible.     

Beyond nanosizing: an approach to shape analysis of fluorescent nanostructures by SMI-microscopy

Using spatially modulated illumination (SMI) light microscopy it is possible to measure the sizes of fluorescent structures that have an extension far below the conventional optical resolution limit (“subresolution size”). Presently, the sizes are determined as the object extension along the optical axis of the SMI microscope. For this, however, “a priori” assumptions on the fluorochrome distribution (“shape”) within the examined fluorescent structure have to be made. Usually it is assumed that the fluorochrome follows a Gauss-distribution or a spherical distribution. In this report we overcome the necessity to make an assumption on the shape of the fluorochrome distribution. We introduce two new experimentally obtained parameters which allow the determination of a shape measure to describe the spatial distribution of the fluorescent dye. This becomes possible by independent measurements with different excitation wavelengths. As an example, we present shape parameter measurements on individual fluorescent microspheres with a nominal geometrical diameter (“size”) of 190 nm. In the case investigated, the experimental shape correlated well with a homogeneous fluorochrome distribution (“spherical shape”) but not with a variety of other “shapes”.     

Direct contact versus solvent-shared ion pairs in NiCl2 electrolytes monitored by multiplet effects at Ni(II) L edge X-ray absorption

We investigate the local electronic structure in aqueous NiCl2 electrolytes by Ni L edge X-ray absorption spectroscopy. The experimental findings are interpreted in conjunction with multiplet calculations of the electronic structure and the resulting spectral shape. In contrast to the situation in the solid, the electronic structure in the electrolyte reflects the absence of direct contact Ni-Cl ion pairs. We observe a systematic change of the intensity ratio of singlet- and triplet-related spectral features as a function of electrolyte concentration. These changes can be described theoretically by a change in the weight of transition matrix contributions with different symmetries. We interpret these findings as being due to progressive distortions of the local symmetry induced by solvent-shared ion pairs.     

Ultrafast hybridization screening in Fe3+ aqueous solution

We report here on the electron binding energies and ultrafast electronic relaxation of the Fe(3+)(aq) complex in FeCl(3) aqueous solution as measured by soft X-ray photoelectron (PE) spectroscopy from a vacuum liquid microjet. Covalent mixing between the 3d valence orbitals of the iron cation and the molecular orbitals of water in the ground-state solution is directly revealed by spectroscopy of the highest partially occupied molecular orbitals. Valence PE spectra, obtained for photon energies near the iron 2p absorption edge, exhibit large resonant enhancements. These resonant PE features identify 3d-O2p transient hybridization between iron and water-derived orbitals and are an indication of charge transfer within the electronically excited Fe(3+)(aq)* complex. Charge transfer from water to iron is also revealed by the 2p core-level PE spectrum, and the asymmetric peak shape additionally identifies the characteristic multiplet interactions in the 2p core-hole state. The electronic structure of water molecules in the first hydration shell is selectively probed by Auger decay from water molecules, at excitation energies well below the O1s absorption edge of neat water. These experiments lay the groundwork for establishing resonant PE spectroscopy for the study of electronic-structure dynamics in the large family of transition metal (aqueous) solutions.     

Oxidation of Flavonols in an Electrochemical Flow Cell Coupled Online with ESI‐MS

Flavonols, a class of flavonoids, are jointly responsible for antioxidant activities as free radical acceptors in flowers, fruits and vegetables. Due to their electroactive behaviour, the oxidation of chrysin, flavonol, kaempferol, morin, quercetin and myricetin in methanol was performed in an electrochemical flow cell with glassy carbon as the working electrode. This flow cell was coupled online with electrospray ionisation mass spectrometry to identify the possible oxidation products. Based on these experiments, it was concluded that when certain structural requirement are present, the initial electrode reaction of flavonols involves an electron delivery electrochemical step followed by a chemical step via methanol addition. The redox activity of the selected species explains the number of observed multiple methoxylation. Moreover, we identified a second two‐electron, two‐proton oxidation of the formed oxidation products of quercetin and myricetin for the first time.