Precursor Anions Research Articles

Coarsening of metal oxide nanoparticles.

In solution phase synthesis of nanoparticles, processes such as coarsening and aggregation can compete with nucleation and growth in modifying the particle size distribution in the system. We show that coarsening of ZnO and TiO2 nanoparticles in solution follows the Lifshitz-Slyozov-Wagner rate law for diffusion controlled coarsening originally derived for colloidal systems with micrometer-sized particles, where the average particle size cubed is proportional to time. The rate constant for growth of ZnO in propanol is in the range 10(-4)-10(-2) nm3 x s(-1) and is dependent on the precursor anion and temperature. The coarsening of TiO2 nanoparticles from aqueous Ti(IV) alkoxide solutions is slower due to the low solubility of TiO2 with the rate constant in the range 10(-5)-10(-3) nm3 x s(-1) for temperatures between 150 degrees C and 220 degrees C. Epitaxial attachment of TiO2 particles becomes significant at higher temperatures and longer times. We show that the dominant parameters controlling the coarsening kinetics are solvent, precursor salt, and temperature.

Spectroscopy of reactive potential energy surfaces

This perspective reviews experiments in which spectroscopy rather than scattering is used to probe reactive potential energy surfaces. The application of negative ion photodetachment to the transition state spectroscopy of benchmark reactions is described, followed by a brief description of recent transition state spectroscopy experiment starting from clustered precursor anions that probe the effects of solvation on transition state spectroscopy and dynamics. Experiments on the spectroscopy of open-shell “pre-reactive” complexes are also discussed.

Dissociation pathways in low energy (0–2 eV) electron attachment to Cl 2O

Dissociative electron attachment (DA) to ClOCl is studied in a high resolution crossed beam experiment. Two complementary ion pairs, Cl −/ClO − and O −/Cl 2 −, are observed. The Cl −/ClO − pair arises from a simple Cl–OCl bond cleavage with the electron sitting on either of the two fragments. The O −/Cl 2 − pair is formed by a concerted reaction with the expulsion of O − (or O) and formation of Cl 2 (or Cl 2 −). Ab initio calculations indicate that in low energy electron attachment an electronically excited state of the precursor anion ( ClOCl − ∗ ( 2 B 2) ) is involved.

A novel anion rearrangement. The conversion of [CC(O)(CN)]− to [NCCCO]− in the gas phase: a joint experimental and theoretical study

The neutral radical N13CCCO has been made by charge stripping of anion [N13CCCO]− in a collision cell of a ZAB 2HF mass spectrometer. The precursor anion is formed in the ion source of the mass spectrometer by the loss of the elements of MeOH from the anion N13C–−CHCO2Me. Neutral N13CCCO can, in turn, be charge stripped to the corresponding cation [N13CCCO]+ which then decomposes by competitive losses of CO and N13C˙. No losses of 13CO or NC˙ are observed in this spectrum: thus, the neutral N13CCCO does not scramble the carbons in the carbon chain during the lifetime (ca. 10−6 s) of the neutral. An attempt to form the neutral radical CC(O)(CN) (which is isomeric with NCCCO) from the precursor anion [CC(O)(CN)]− was unsuccessful, since on formation, [CC(O)(CN)]− undergoes facile rearrangement to the more stable [NCCCO]−. A theoretical investigation at the B3LYP/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory indicates that (i) there are singlet and triplet forms of [CC(O)CN]− which are only 7 kJ mol−1 different in energy, and (ii) both of these ions undergo facile rearrangement to form the respective singlet and triplet forms of [NCCCO]− in strongly exothermic reactions.

Alumina powders from aluminium nitrate-urea and aluminium sulphate-urea reactions — The role of the precursor anion and process conditions on characteristics

Precursor powders for alumina were prepared by aluminium nitrate-urea (NU-) and aluminium sulphate-urea (SU-) reactions. The role of process conditions on the reactions and characteristics of the resultant powders were studied. NU-reactions resulted in an amorphous gel that upon prolonged refluxing at a higher pH crystallized into boehmite powders of 0.3–0.8 μm size. These were agglomerates having elongated fibrillar morphology. SU-reactions resulted in the direct formation of amorphous powders of 2–7 μm size which were agglomerates of spherical particles. Upon prolonged refluxing at higher pH these powders were also found to undergo crystallization into boehmite, with a little modification of their particle size distribution and morphology. The alpha alumina powders formed by calcination of these precursors at 1673 K exhibited the same size distribution and morphological features as that of the precursors. At 1823 K, NU-based powder compacts sintered to above 95% T.D., while SU-based powder compacts could sinter only to about 85%.

Formation of Solvated Ions by Electron Capture in van der Waals Aggregates

AbstractWe report on the formation of solvated anions in beams of molecular aggregates by resonant capture of low‐energy electrons. The initially generated negative ion can be effectively deactivated in the cluster environment (“built‐in 3‐body collision”). By this means anions which are normally inaccessible from the ground neutral state can be stabilized by intermolecular energy transfer. In these experiments halide ions solvated by H2O or CO2 are produced from mixed clusters of these molecules and CF3Cl or C2Cl4. The appearance of such products is found to depend strongly on the electronic state of the precursor anion.

Electron capture induced processes in molecules and molecular aggregates

Negative ion formation following resonant electron capture by molecules and molecular aggregates is studied in beam experiments for some prototype systems (CF 3Cl, CF 4, C 2F 4, CF 3Cl/H 2O). Analysis of the excess kinetic energy of the reaction products is performed in order to observe the change in decomposition dynamics on proceeding from single molecules to clusters. It is shown that electron capture by aggregates may ultimately generate monomeric anions in their relaxed geometries which are not accessible in the isolated compound. In some cases a relaxation energy of several electronvolts has to be distributed among the target cluster. Fragment ions formed with remarkably high translational energy are effectively slowed down when arising from aggregates at the same attachment energy. Clusters of tetrafluoroethylene show an additional resonance at low attachment energies; these exclusively decompose into (C 2F 4) − n , n ⩾ 2. Aggregates composed of H 2O and CF 3Cl give rise to the solvated ions (H 2O) n ·Cl − ( n ⩾ 1), (H 2O) n ·F − ( n ⩾ 1) and (H 2O) n ·OH − ( n ⩾ 2). In the case of (H 2O) n ·Cl − it is shown that the formation probability depends strongly on the electronic state of the precursor anion within the cluster.

Crystal Structure of [(C2H5)4N]4Cu7Cl14O2: A Cu14Cl28O48−Cluster

The synthesis and crystal structure of a novel copper(ll) halide cluster, (Et4N)8Cu14Cl28O4, is reported. The cluster anion Cu14Cl28O4 8− can be Viewed as being formed by the dimerization of two Cu7Cl14O2 4− fragments. The hypothetical precursor anion of this latter cluster, Cu7Cl15O2 5− in tum can be viewed as being formed from. the known parent Cu7Cl14O2 4− cluster anions by fusion of pairs across a common CuCI3 face. The fusion and dimenzation processes cause substantial distortion of the trigonal bipyramidal coordmation geometry of the copper(II) ions in the parent cluster. For three of the copper(II) ions this Yields a 4+1 geometry distinct from either trigonal blpyramldal or square pyramidal. The dimerization of the two Cu7 clusters forces a severe distortion upon one additional copper(II) ion, this time towards a tetrahedral geometry. The implications of these distortions for the catalytic properties reactivities and magnetic behaviour of the parent complex are discussed.

Influence of the catalyst precursor anion in catalysis of water vapour gasification of carbon by potassium: 1. Activation of the catalyst precursors

The activation of various potassium salts as raw materials for potassium catalysed water vapour gasification was studied with a natural graphite and a high volatile bituminous coal using thermogravimetry, differential thermoanalysis and gas analysis. The result of the study is a self-consistent general scheme of activation, whereby potassium hydroxide is formed from all salts. Potassium hydroxide therefore represents the key component of all the activation processes, from which finally the active species, a non-stoichiometric potassium/oxygen compound K x O y ( y < x) is formed.

The influence of the catalyst precursor anion in catalysis of water vapour gasification of carbon by potassium: 2. Catalytic activity as influenced by activation and deactivation reactions

Based on a fundamental clarification of the activity of various potassium salts as precursors for potassium catalysed water vapour gasification in part 1 of this paper, the catalytic activity was studied by gasification experiments in a fixed bed flow reactor using equimolar argon/water vapour and hydrogen/water vapour mixtures. The catalytic activities found with the various potassium salts (hydroxide, carbonate, nitrate, sulphate, chloride) characterized by the onset of catalysed gasification and the maximum gasification rates during linear heating (4 K min −1), were found to be identical with the kinetics of the activation, confirming the following sequence: KOH ~ K 2 CO 3 ~ KNO 3 > K 2 SO 4 > KCl. As potassium hydroxide represents the key component in the activation of all salts, the active species has to be formed from this compound. The active species is defined as a non-stoichiometric potassium-oxygen compound K x O y ( y < x) with varying oxygen contents. It acts as a dissociation centre for water and transfers the oxygen to the carbon surface, from which carbon monoxide is finally desorbed. A high selectivity towards carbon dioxide found in argon/water vapour and a high selectivity towards methane in hydrogen/water vapour are explained by the shift reaction and methanation of primary formed carbon monoxide being catalysed by the actual active species of different oxygen content. Deactivation of the active species by sulphur does not occur. There is also strong irreversible deactivation of the active species by silicates. Inhibition by hydrogen is interpreted by blocking of active sites at the carbon surface.

ESR and INDO study of radical conformations in irradiated single crystals of L-histidine free base

Radicals produced in x-irradiated single crystals of L-histidine free base in the temperature range 77–300 K have been investigated using ESR spectroscopy at 9.5 and 35 GHz. The main species trapped at 77 K are radical anions located at the carboxyl group. These are converted to deamination radicals upon warming. Conversion of the anion into the deamination radical is not found if the former is bleached by uv illumination at 77 K. At room temperature, only radicals resulting from deamination are observed. Two different conformations of the deamination have been found. The one obtained at room temperature is produced from a precursor with identical composition but different conformation. The latter, which is stable at reduced temperatures, is in the open conformation like both the precursor anion radical and the parent molecule. The room temperature radical however adopts a closed conformation. The identifications are supported by MO-SCF calculations at the INDO level. A detailed analysis of the anion radical and the deamination radical by INDO is presented.