Showing papers in "Physical Chemistry Chemical Physics in 2000"

Systematic XPS studies of metal oxides, hydroxides and peroxides

Abstract: The results of a systematic XPS study, under high controlled conditions, of different basic oxides of transition metals, alkali and alkaline-earth metals are presented; the XPS data of some hydroxides and peroxides are also reported. Variations of the O 1s binding energies are analysed and one point of interest is the large binding energy scale obtained for O 1s peaks all associated with a ‘‘2− ’’ formal charge. Through extended Huckel theory-tight binding (EHT-TB) calculations, attempts are made to rationalize the observed variations. The results illustrate the significant differences between real charges on oxygen atoms in transition metal and alkaline-earth oxides.

Temperature-dependent self-diffusion coefficients of water and six selected molecular liquids for calibration in accurate 1H NMR PFG measurements

Abstract: Pulsed magnetic field gradient (PFG) NMR is today a routine method for the determination of self-diffusion coefficients, D. However, a remaining goal is the improvement of the precision of the method. The best procedure for the determination of accurate diffusion coefficients by PFG NMR is a calibration with a sample of precisely known D-value. In continuation of our previous work on calibration at 25°C (M. Holz and H. Weingartner, J. Magn. Reson., 1991, 92, 115) we present reference data as a function of temperature. Since H2O plays an outstanding role as liquid and as primary standard, we carefully measured self-diffusion coefficients of water by 1H PFG NMR in the temperature range from + 5 to + 55°C and added literature data obtained from tracer methods in the range between 0 and + 100°C. This comparatively large collection of data could then be fitted to a Speedy–Angell power law, showing the excellent congruence of the results of two completely different methods and proofing the certainty of the absolute values for water. In this manner reliable primary standard values with error limits of <1% were obtained, allowing us to adapt the temperature of the standard water to the sample temperature of interest. We further give 1H PFG NMR self-diffusion reference data in the range from + 5 to + 55°C for six easily accessible solvents, which we propose as secondary calibration standards, namely cyclohexane, dioxane, dodecane, DMSO, tetradecane and pentanol, covering a large range of absolute D-values and allowing us to match in addition the absolute D-values of calibration sample and sample under investigation. Furthermore, the gained accurate self-diffusion data are suited for an elaborate check of theoretical approaches in the physics of molecular liquids.

COSMO Implementation in TURBOMOLE: Extension of an efficient quantum chemical code towards liquid systems

Abstract: The most recent algorithmic enhancements of the COSMO solvation model are presented and the implementation in the TURBOMOLE program package is described. Three demonstrative applications covering homogeneous catalysis, tautomeric equilibria, and binary phase diagrams show the efficiency and general applicability of the approach. Especially when combined with the COSMO-RS extension, the method very reliably predicts thermodynamic properties of liquid mixtures.

Pickering emulsions stabilised by Laponite clay particles

Abstract: The type and stability of Pickering emulsions stabilised by disc-like Laponite RD clay particles are described. By establishing the phase diagram of aqueous dispersions as a function of clay and salt (NaCl) concentration, we deduce that toluene-in-water (o/w) emulsions, stable to creaming and coalescence for at least 6 months, are only formed under conditions where the colloidal particles are flocculated. The initial average drop diameter is independent of clay concentration but depends markedly on oil volume fraction, ranging from 10 to 28 μm. Changes in the drop size distributions with time are shown to be due to Ostwald ripening, which, due to the irreversible nature of particle adsorption at oil/water interfaces, is rapid at first and ceases completely at long times. It is suggested that ripening is arrested when the Laplace pressure inside drops of different sizes becomes equal. For optimum conditions, emulsions prepared using a variety of oils including non-polar alkanes and polar alcohols are always o/w even at high oil phase volume fraction, reflecting the hydrophilic nature of this synthetic clay in oil–water systems.

Linear scaling geometry optimisation and transition state search in hybrid delocalised internal coordinates

Abstract: An algorithm for linear scaling geometry optimisation and transition state search using hybrid delocalised internal coordinates (HDLC) has been developed and implemented in the context of a semiempirical quantum-chemistry program (MNDO) and a modular quantum-mechanical/molecular-mechanical (QM/MM) package (ChemShell). Linear scaling is achieved by a divide-and-conquer approach: the system is partitioned into user-defined fragments, and all coordinate manipulations are performed exclusively within these fragments. The optimiser employs a limited-memory quasi-Newton algorithm (L-BFGS) for energy minimisation, and a microiterative scheme for transition state search using a Hessian eigenmode-following algorithm (P-RFO) for the reaction core and the L-BFGS algorithm for the environment. There are automatic procedures for generating redundant sets of internal coordinates and non-redundant sets of HDLC from Cartesian coordinates. The input to the optimiser consists of the initial Cartesian geometry, the fragmentation of the system, the choice of the working coordinate system, and any constraints to be imposed in Cartesian and/or internal coordinates. The optimiser requires an external function that provides the energy and gradient at a given Cartesian geometry. Systems with thousands of atoms have been optimised, and transition states of a model enzymatic reaction have been determined.

Effects of oil type and aqueous phase composition on oil–water mixtures containing particles of intermediate hydrophobicity

Abstract: The behaviour of emulsions stabilised by partially hydrophobic spherical silica particles is described for systems in which the oil type is varied in mixtures with water, and for toluene-containing systems in which water is replaced by other polar solvents. Using contact angle measurements on flat glass substrates, we deduce that particles are more hydrophobic at polar oil–water interfaces (esters and alcohols), preferring water-in-oil (w/o) emulsions, and more hydrophilic at non-polar oil–water interfaces (alkanes), with the preferred emulsion type being oil-in-water (o/w). Formamide systems are similar to those of water, whereas emulsions could not be prepared with glycerol or ethylene glycol. In contrast to a surfactant-stabilised emulsion, we find that the preferred emulsion type depends on the initial location of particles in that the continuous phase is the phase in which particles are first dispersed. Using undecanol as oil, we observe an interesting phenomenon during the complete breakdown of viscous w/o emulsions involving contraction to retain the shape of the vessel. Finally, we determine the partitioning of particles between toluene and water and show that particles can be rendered more hydrophilic by simply increasing the pH of the aqueous phase. An increase in the wettability by water in this way is used to shed new light on the behaviour of different particle-containing systems involved in demulsification, flotation and antifoaming.

Multicenter bond indices as a measure of aromaticity

Abstract: A MO multicenter bond index involving the σ+π electron population is proposed as a measure of aromaticity It is related both to the energetical and to the magnetic criteria The index is applied to linear and angular polycyclic hydrocarbons with benzenoid rings, to hydrocarbons including non-benzenoid rings, to monocyclic azines, benzoazines and other heterocyclic compounds with 5-membered rings The index gives satisfactory values for monocyclic molecules and is shown to be quite suitable in discriminating the relative aromaticity of the different rings in polycyclic compounds

Computational determination of equilibrium geometry and dissociation energy of the water dimer

Abstract: The equilibrium geometry and dissociation energy of the water dimer have been determined as accurately as technically possible. Various quantum chemical methods and high-quality basis sets have been applied—that is, at the level of a nearly complete basis—and both the intermolecular separation and the deformation of the donor and acceptor molecules have been optimized at the level of CCSD(T) theory (coupled-cluster theory with singles and doubles excitations plus a perturbation correction for connected triples). It is found at the CCSD(T) level that the monomer deformation in the dimer amounts to 86% of the deformation computed at the MP2 level (second-order Moller-Plesset perturbation theory) and that the core/valence electron correlation effects at the CCSD(T) level amount to 80% of the same effects at the MP2 level. The equilibrium O···O distance is determined as Re=291.2±0.5 pm and the equilibrium dissociation energy as De=21.0±0.2 kJ mol−1, with respect to dissociation into two isolated water molecules at equilibrium. Accounting for zero-point vibrational energy, the theoretical prediction for the dissociation energy becomes D0=13.8±0.4 kJ mol−1, a result which is open to direct experimental verification.

Influence of the boundaries in the impedance of porous film electrodes

Abstract: This paper presents an enhanced model for the impedance of porous film electrodes. The impedance of a transmission line with two transport channels, a crosswise element and arbitrary terminal loads is solved analytically. The local impedances at the boundaries represent a frequency-dependent response of the blocking of ionic and electronic charge carriers at the two faces of the electrode region. A general expression is found that contains, as particular cases, a number of models of impedance for porous electrodes used in the literature. Some examples of the generalised transmission line illustrate the use of the model in the interpretation of experimental data. First, a polarisable electrode showing low-frequency dispersion of the constant phase element (CPE) type is analysed, and diagnosis criteria are derived to recognise whether the dispersion is caused by the boundary or the inner surface. Secondly, the manifestation in the impedance of the failure of a porous electrode due to direct charge transfer between the substrate and a redox couple in solution is investigated.

Luminescence of nanocrystalline ZnSe:Mn2+

Abstract: Nanocrystalline ZnS:Pb2+ is synthesised ia a precipitation method. The influence of the size of the nanocrystals and the sulfide concentration used in the synthesis on the luminescence properties is investigated. Nanocrystalline ZnS:Pb2+ shows a white emission under UV excitation with a rather high quantum efficiency (∽5%). At least two luminescence centres are involved. One centre is identified as a Pb2+ ion located on a regular Zn2+ site and gives a red emission under 480 nm excitation. The luminescence properties of this emission are characteristic of 3P0 → 1S0 (A-band) or charge transfer (D-band) transitions on Pb2+ ions. The other centres are not as well defined and give a broad green emission band under 380 nm excitation and also show luminescence properties typically observed for Pb2+. The green emission probably originates from a charge-transfer like D-band emission of Pb2+ in ZnS close to a defect (e.g. an S2− vacancy or an O2− ion on an S2− site). A relation between the temperature quenching of the emissions and the band gap is observed and indicates that photoionisation occurs. The higher quenching temperature for the Pb2+ luminescence in smaller particles can be explained by widening of the band gap as a result of quantum size effects in the ZnS host.

An empirical correlation between stretching vibration redshift and hydrogen bond length

Abstract: A correlation is found between the magnitude of the redshift (Δν=20 to 2500 cm−1 relative to the free molecule) of the stretching mode of H-bonded A–H groups in an A–H···B complex, and the length of the H-bond (rH···B=0.28 to 0.12 nm). The correlation is based on both new spectral data for narrow decoupled H-bands in cold isotopically diluted carbohydrate crystals with known H-bond distances and on literature spectral and structural data from a total of 36 systems. Once established, additional data for H-bonded crystals (hydrates, acid salts of carboxylic acids) and for gas phase dimer systems also fit this correlation quite well. Hydrogen bond enthalpies in the range of −ΔH=10–80 kJ mol−1 correlate with the inverse third power of the H-bond length. Literature experimental data on −ΔH and rH···B of ten gas phase dimers confirm this relationship.

On the determination of excitation energies using density functional theory

Abstract: The use of time-dependent density functional theory (TDDFT) is considered for the determination of electronic excitation energies. Using beryllium and methylene as examples, we highlight (i) problems with Rydberg excitations arising from neglect of the integer discontinuity in the potential; (ii) the absence of pure double excitations in calculations using conventional exchange-correlation functionals; (iii) quantitative differences between excitation energies determined using TDDFT and the ‘delta SCF’ method; (iv) non-additivity of excitation energies calculated using TDDFT from different electronic states; (v) an apparent failure to predict single excitations to states that are lower than the reference states and (vi) the difference in quality between excitations to singlet and triplet states.

Species formed after NO adsorption and NO+O2 co-adsorption on TiO2: an FTIR spectroscopic study

Abstract: Adsorption of NO and its co-adsorption with oxygen on TiO2 (Degussa P-25) were studied by FTIR spectroscopy It was found that NO adsorption results in its disproportionation to NO− (1170 cm−1), N2O22− (1335 cm−1) and nitrates (1650–1550 and 1240–1220 cm−1) The nitrate bands develop with time and coordinated NO (ca 1900 cm−1) is formed Addition of oxygen to NO results in a strong increase in concentration of the nitrates and formation of NO+ (2206 cm−1) In addition, species assigned to nitrocomplexes (1520 and 1284 cm−1) are found The stability and reactivity of the different surface compounds as well as their interconversion are studied and discussed

A new table of the thermodynamic quantities of ionic hydration: values and some applications (enthalpy–entropy compensation and Born radii)

Abstract: Absolute single-ion thermodynamic quantities of hydration at 298.15 K are derivable from the conventional enthalpies and entropies if the values of S°(Haq+) and ΔhydH°(H+) are known. Here we suggest S°(Haq+) = −5.5 J K−1 mol-1 based on the thermodynamics of the dissociation of water. This assignment, in turn, corresponds to ΔhydH°(H+) = −1078 kJ mol-1 according to a self-consistent analysis of Krestov. Using these values, as a main result, the anions are more strongly hydrated than usually thought, in line with recent calculations. Only the group 1, 2, and 15 nobel gas ions are dealt with. For each series, the conventional enthalpies and entropies are linearly related to one another. From these linear free energy relationships (LFERs) a relationship between S°(Haq+) and ΔhydH°(H+) is derived. Further, a connection is detected between the Born radii rB, calculated from the free energies of hydration, and the distances d, corresponding to the upper limits of the experimental first peak position of the ionoxygen radial distribution curves, upon implication, in the case of a cation, the covalent radius rcov of oxygen, and in the case of an anion, the water radius rwater, Finally, from the differences between the enthalpies and free energies of hydration the temperature derivatives of the Born radii are determined.

Water and its anomalies in perspective: Tetrahedral liquids with and without liquid-liquid phase transitions

Abstract: We show how water with its small proton bridge between tetrahedrally coordinated oxygen centers, forms a provocative series with (i) the archetypal glassformer and strong liquid, SiO2, (ii) the analog ionic compound BeF2 in which the Be ions are bridged by fluoride ions, and (iii) elemental Si with no bridge at all. As the size of the bridging unit decreases from the polarizable oxide of silica through the unpolarizable fluoride of BeF2, the tiny proton of water, and finally vanishes at silicon, the constraints on the network tighten. As the constraints become more severe, the departures from simple activated diffusion behavior of SiO2 during heating of the amorphous phase at ambient pressure, become more pronounced. In the case of Si, the extreme of a first-order transition from strong to extremely fragile liquid state is manifested in the supercooled regime. With water the behavior is intermediate. In both cases the anomalous regime is located in the supercooled state which greatly complicates its investigation and leads to much controversy. In the case of BeF2, for which the exceptional viscosity behavior can now be understood, new MD results show that a weakened form of the anomalies exists. The weakened anomaly should be observable in detail in experimentally accessible ambient pressure conditions, because now it occurs in the thermodynamically stable domain, at temperatures which lie above those studied to date. The observable anomalies should include not only a density maximum but also a density minimum, and a smeared lambda type heat capacity anomaly. In view of the phase diagrams deduced for various water models, our observations suggest that the second critical point scenario proposed for water, if correct, might be observable under thermodynamically stable conditions at high pressures in the case of BeF2. In the cases of water and Si, in which the interaction potentials cause the critical point to lie at relatively low or negative pressures, respectively, the glassy state proves to have unusual properties intermediate between those of normal glasses and crystals. It is suggested that such glasses, which will include many Si analogs such as Ge, InSb, etc, might constitute a distinct class of amorphous material with the attributes (low residual entropy, etc.) of the hypothetical perfect glass state.

Computational electrochemistry: aqueous one-electron oxidation potentials for substituted anilines

Abstract: Semiempirical molecular orbital theory and density functional theory are used to compute one-electron oxidation potentials for aniline and a set of 21 mono- and di-substituted anilines in aqueous solution. Linear relationships between theoretical predictions and experiment are constructed and provide mean unsigned errors as low as 0.02 V over a training set of 13 anilines; the error rises to 0.09 V over a test set of eight additional anilines. A good correlation is also found between oxidation potential and a simple computed property, namely the energy of the highest occupied molecular orbital for neutral anilines in aqueous solution. For the particular case of the substituted anilines, a strong correlation between oxidation potential and pKa is found, and a still stronger correlation between oxidation potential and physical organic descriptors for aromatic substituents is also found, albeit over a reduced data set.

All-electron ab-initio molecular dynamics

Abstract: We present an all-electron implementation of the Gaussian and augmented plane wave density functional method (GAPW method), which allows ab-initio density functional calculations for periodic and non-periodic systems. The GAPW method uses a Gaussian basis set to expand the Kohn–Sham orbitals, whereas an augmented plane wave basis set is introduced as an auxiliary basis set to expand the electronic charge density. The results of the all-electron calculations for a representative set of small molecules are reported to demonstrate the accuracy and reliability of the GAPW method. Furthermore, its performance is shown for some larger systems, including calculations on unbranched alkane chains up to n-C100H202 with 1804 basis functions and a fully hydrated crystalline RNA duplex (sodium guanylyl-3′-5′-cytidine nonahydrate) with 368 atoms and 3168 basis functions. Finally, as a first test an all-electron ab-initio molecular dynamics (MD) run was performed for 32 water molecules in a simple cubic box under ambient conditions. A standard single processor workstation (IBM 397) was used for all the presented calculations.

Optical spectroscopy of nanocrystalline cubic Y2O3:Er3+ obtained by combustion synthesis

Abstract: We report the emission and upconversion spectra and the lifetimes of Er3+ doped Y2O3 nanocrystalline and bulk samples We found that when the nanocrystal and bulk samples of Y180Er020O3 were excited at 815 nm, the overall emission intensity was stronger for the bulk sample However, the relative intensity of the (2H11/2, 4S3/2)→4I15/2/4F9/2→4I15/2 transitions is 2:3 and 3:2 for the nanocrystal and bulk sample, respectively The decay times obtained for the nanocrystalline samples are in general significantly faster than those observed for the bulk sample We attribute this to the adsorption of CO2 on the surface of the nanocrystalline samples The effect of Er3+ concentration on the decay time of the nanocrystalline samples is also discussed

Self-aggregation of Methylene Blue in aqueous medium and aqueous solutions of Bu4NBr and urea

Abstract: The self-aggregation of Methylene Blue [MB] (1×10−6–4×10−4 M) in water and in aqueous solutions of Bu4NBr (0.1–0.3 M) and urea (0.1–4 M) has been investigated by recording electronic spectra in the wavelength range 550 to 700 nm. Analysis of the spectral data yielded the dimer dissociation constant, and individual characteristic monomer and dimer spectra. By applying the exciton model for the first time, the interaction energy between the MB molecules in the dimer species has been evaluated. Further, information about the dimer geometry and the twist angle between the dipoles of the MB molecules in the dimer species has been obtained. Similar data have also been obtained for solutions of MB in aqueous urea and Bu4NBr solutions. The significant observation is the change in the dimer geometry from sandwich type to end-on-end type in the presence of urea and Bu4NBr. It is observed that the contribution to the interaction energy is mainly from van der Waal's type and dispersion forces, in addition to short-range forces involving multipoles. However, the contribution from hydrogen-bonding interactions between MB and water molecules is found to be significantly low. It is proposed that water structural effects and hydrophobic interactions are the major factors in the phenomenon of aggregation.

Ab initio studies of the dimers (HgH2)2 and (HgMe2)2. Metallophilic attraction and the van der Waals radii of mercury

Abstract: The title compounds are used to determine, for the first time, the energy of the ‘metallophilic’ attraction between two Hg(II) compounds. The dispersion and electrostatic multipole components to this attraction are analyzed. The present purely theoretical molecular data suggest a mercury(II) van der Waals radius of 175(7) pm.

From atoms to crystallites: adsorption on oxide-supported metal particles

Abstract: The properties of metal clusters and nanoparticles are attracting more and more attention in fundamental and applied research. The application of such structures in heterogeneous catalysis is a particularly important area. Nevertheless, there is only limited fundamental knowledge about the dependence of adsorption behaviour and catalytic activity on particle size. In this article, we describe an approach to tackle such questions on the basis of model systems which are prepared by metal vapour deposition under ultrahigh vacuum conditions. A thin alumina film grown on a metal substrate is used as support. This has the advantage that scanning tunneling microscopy and photoelectron spectroscopy can be applied without any charging problems. Firstly, structural data covering palladium, rhodium and iridium deposits demonstrate that a wide spectrum of particle sizes and morphologies may be obtained by taking advantage of kinetically controlled nucleation and growth processes. Subsequently, we discuss the adsorption of simple molecules on the particles, such as CO and ethene. These studies, which have been carried out using infrared and photoemission spectroscopy, illustrate possible variations in the adsorption and reaction behaviour as a function of particle size. Aspects which will be considered include: adsorption at facets and defects, size-dependent variation of adsorption sites, formation of surface complexes, decomposition of molecules as well as co-adsorption phenomena.

Contracted all-electron Gaussian basis sets for atoms Rb to Xe

Abstract: Two series of contracted all-electron basis sets are presented for Rb to Xe which were obtained by full optimization of atomic SCF energies. The smaller, economic basis is of split valence type. The larger is of triple zeta type for the 4d atomic orbitals and better than double zeta quality for remaining valence orbitals.

Selective catalytic reduction of NOx with CH3OH, C2H5OH and C3H6 in the presence of O2 over Ag/Al2O3 catalyst: Role of surface nitrate species

Abstract: The involvement of the reaction of surface nitrate [NO3−(ads)] species with different reductants (C3H6, C2H5OH and CH3OH) in the selective catalytic reduction of nitrogen oxides (NOx) over a Ag/Al2O3 catalyst has been studied by in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and temperature-programmed desorption (TPD). When an NO/O2 mixture was exposed to a Ag/Al2O3 catalyst at 150 °C, three kinds of nitrate species (bridging, monodentate and bidentate) were observed by DRIFT. The thermal stability of the monodentate NO3−(ads) species was higher than that of the bridging and bidentate NO3−(ads) species, which was confirmed by DRIFT and TPD measurements. The monodentate NO3−(ads) species reacted effectively with C2H5OH and CH3OH in the presence of O2 to form surface isocyanate [NCO(ads)] species at 250 °C, whereas the bridging and bidentate NO3−(ads) species reacted minimally. All NO3−(ads) species were largely unreactive with C3H6 in the presence of O2 at temperatures below 250 °C. On the other hand, the order of reactivity in the reduction of NOx at temperatures below 350 °C was in good agreement with that of both the reactivity of the monodentate NO3−(ads) species and the ease of NCO(a) formation (C2H5OH>CH3OH>C3H6). Based on these findings, the involvement of the reactivity of NO3−(ads) species and the formation of NCO(ads) species in the selective reduction of NOx are discussed.

The adsorption of sulfate and phosphate on Au(111) and Au(100) electrodes: an in situ STM study

Abstract: We have studied by in situ STM the adsorption of sulfate and phosphate species on Au(111) and Au(100) single crystal electrodes in acid (0.1 M H2SO4 and 0.1 M H3PO4) and neutral (0.1 M Na2SO4 and 0.1 M KH2PO4 + 0.1 M K2HPO4) solutions. The well-known (bi)sulfate structures on Au(111) and Au(100) in 0.1 M H2SO4 were compared with a new one found for phosphate species on Au(100) in 0.1 M H3PO4. The presence of non-uniform anion–anion distances in all these cases indicates that this is a characteristic feature common to oxoanion adlayers, due to their ability to form hydrogen-bridge bonds through the lone pairs of their oxygen atoms. No ordered adsorption was observed in neutral solutions, which indicates that the coadsorption of hydronium ions is necessary to stabilise the ordered oxoanion adlattices. The absence of current spikes in the cyclic voltammograms of Au(100) in 0.1 M H2SO4 and 0.1 M H3PO4, indicative of the formation of ordered adlayers, was explained by the fact that in these cases the adlayers are composed of many small domains.

Robust and variational fitting

Abstract: Fitting used to simplify various approximations to the molecular Schrodinger equation is discussed. Robust fits correct the target function to first order in the error made due to fitting. Variational fits are compatible with the variational principle. A full variation of the fitted energy is used to generate the Fock matrix. Robust and variational fitting of the electric and magnetic fields of the electrons is defined. Robust-fitted MP2 (RF-MP2) methods are defined. Independent of how the charge distributions are fit, robust-fitted errors are quadratic in the difference between the fits and fitted quantities. Analytic, robust and variational fits of various fractional powers of electronic densities and other linear-combination-of-atomic-orbitals (LCAO) objects are described.

Photocatalytic oxygen evolution on α-Fe2O3 films using Fe3+ ion as a sacrificial oxidizing agent

Abstract: Photocatalytic oxygen evolution on α-Fe2O3 films was studied using the Fe3+ ion as a sacrificial oxidizing agent. The reaction conditions affecting the oxygen evolution rate, i.e. anion, Fe3+ concentration, pH, Fe3+/Fe2+ equilibrium concentration and dependence of irradiation wavelength, were investigated. The reaction rate increased with increase of both the Fe3+ ion concentration and the solution pH. Oxygen evolution ceased at an Fe3+ :Fe2+ ion concentration ratio of 3:7–4: 6. The reaction rate decreased rapidly for longer-wavelength irradiation (above around 400 nm), which is considerably shorter than the α-Fe2O3 bandgap of ca. 600 nm (2.1 eV).

Inverse micellar phases of phospholipids and glycolipids. Invited Lecture

Abstract: We here review the current knowledge of lyotropic liquid-crystalline phases having structures based upon periodic three-dimensional packings of discrete inverse micellar aggregates. These have been shown during the last decade to be rather common in lipid systems, particularly in lipid mixtures. The most frequently observed such structure is a cubic phase of crystallographic spacegroup Fd3m. We have previously determined the structure of this inverse micellar cubic phase, both by X-ray diffraction (V. Luzzati, R. Vargas, A. Gulik, P. Mariani, J. M. Seddon and E. Rivas, Biochemistry, 1992, 31, 279) and by freeze fracture electron microscopy (FFEM) (H. Delacroix, T. Gulik-Krzywicki and J. M. Seddon, J. Mol. Biol., 1996, 258, 88), and have determined the effect of pressure on its stability (P. M. Duesing, J. M. Seddon, R. H. Templer and D. A. Mannock, Langmuir, 1997, 13, 265). We have measured the limiting hydration of the Fd3m phase in a number of lipid systems. We have found that electrical conductivity measurements can offer a simple way of distinguishing bicontinuous from discontinuous (inverse micellar) phases. We have also discovered a new optically-isotropic phase in a few lipid systems, at lower hydrations than the Fd3m cubic phase. The X-ray diffraction pattern does not appear to index as cubic, and we assume that this phase consists of a non-cubic complex 3-D packing of inverse micelles. It gives rise to three different fracture planes by FFEM, one of which is identical to the [111] fracture plane of the Fd3m cubic phase, with two different sub-domains with alternating aspects along the [111] direction being present. This implies that the two inverse micellar phases may be related by a restacking transition of planes normal to the cubic [111] direction, analogous to the fcc–hcp restacking transition of hard spheres.

Temperature–dependent electrophoretic mobility and hydrodynamic radius measurements of poly(N-isopropylacrylamide) microgel particles: structural insights

Abstract: Electrophoretic mobility and hydrodynamic radius measurements of poly(N-isopropylacrylamide) [poly(NIPAM)] microgel particles dispersed in water were made as a function of temperature and ionic strength. The data reveal differences between the temperatures at which the particle volume and electrophoretic mobility undergo significant changes. The temperature at which the volume phase transition (VPTT) occurred was 5–6°C lower than the temperature at which pronounced electrophoretic mobility changes occurred. The data appear to be consistent with three stages of collapse for the microgel particles over the temperature range 25–50°C. Ohshima's equations for the electrophoretic mobility of particles covered by an ion-impenetrable surface charge layer were successfully used to fit the data. Electrokinetic equations for hard-sphere systems could not be used for our system. The successful application of the Ohshima theory is evidence that poly(NIPAM) microgel particles prepared by surfactant-free emulsion polymerisation consist of a core–shell structure. It is proposed that the core is relatively highly cross-linked and has a low concentration of ionic groups; whereas, the shell is lightly cross-linked and can be considered as a polyelectrolyte layer. The softness parameter obtained from fitting the Ohshima model to data obtained at 46°C was much higher than expected for a rigid particle and suggests incomplete collapse of the shell at this temperature.

IR and Raman assignments for zinc phthalocyanine from DFT calculations

Abstract: Density functional theory (DFT) calculations have been used to predict the IR and Raman spectra for zinc phthalocyanine (ZnPc). There is very good agreement in the frequency data (RMS error of 10 cm−1 for IR and 12 cm−1 for Raman scattering, with half of the bands in both cases predicted to within 6 cm−1). The precision of the data makes it possible to provide reliable frequency assignments and hence to interpret vibrational changes in terms of structural modifications. The sensitivity of the three IR bands between 700 and 800 cm−1 to the phthalocyanine polymorphic form is explained by the assignment of two modes to out-of-plane vibrations and one mode to an in-plane vibration. The out-of-plane vibrations are expected to be more sensitive to the solid-state packing. The most intense band in the Raman scattering, predicted at 1517 cm−1 for ZnPc, has large displacements on the C–N–C bridge bonds and shifts significantly with metal ion size, making it a good marker for changes in the metal ion environment.

The reactivity of chlorine atoms in aqueous solution. Part III. The reactions of Cl• with solutes

Abstract: Laser flash photolysis of chloroacetone was used to measure the rate constants and activation energies for the reactions of the Cl• atom with a number of oxygen-containing compounds and inorganic anions in aqueous solution. For the organic compounds there is a strong correlation at 25°C between k(Cl•+RH) and k(•OH+RH), where RH is CH3OH, CH3CH2OH, CH3CH(OH)CH3, (CH3)3COH, HCHO, CH3CHO, CH3CO2H, HCO2H and HCO2−, respectively. For CH3CO2−, k(Cl•+RH)k(•OH+RH), and for CH3COCH3 and CH3COCH2Cl, k(Cl•+RH)k(•OH+RH). Possible reasons for these differences are discussed in terms of preferential attack by Cl• at O–H groups in the neutral molecules, rather than H-abstraction from C–H as with •OH, and electron transfer for the reactions of Cl• with the anions. For the inorganic anions X=OCN−, NO3−, SO42−, ClO3−, SCN−, HCO3−, N3−, NO2−, HSO3, k(Cl•+X) ranges from 1.0×108 (NO3−) to 5.3×109 dm3 mol−1 s−1 (SCN−) but there is no strong correlation between k and the reduction potential of X. Comparison of the reactivity of Cl• with reported rate constants for the reactions of Cl2•− indicates that, in many cases, these rate constants are largely accounted for by the fraction of Cl• present in equilibrium with Cl2•−. The implications of these results for atmospheric chemistry are discussed.