Anton Neubrand

A high‐pressure NMR probehead for measurements at 400 MHz

A new high‐pressure NMR probehead designed for a 400 MHz widebore NMR spectrometer is described. For the first time, the electrical leads of the rf circuit, the tubes for the thermostating liquid and the high‐pressure fluid, as well as the wires of the Pt‐100 resistor for temperature control, are all fitted at the bottom of the high‐pressure vessel. The sample can easily be removed through a top plug, which allows a relatively fast exchange of the sample. A rather simple design leads to a low‐cost construction. The high‐pressure vessel and the rf circuit are placed inside a standard probehead jacket, such that the high‐pressure probehead can be fitted into the superconducting magnet in the same way as a commercial probehead.

Ionic and hydrogen bonding dynamics coupled to optical and thermal electron transfer: temperature, pressure and viscosity effects

Abstract Electron transfer processes, along with the coupling to the solvent continuum, can also be essentially coupled to, and controlled by the ionic and specific hydrogen bonding dynamics, occuring within the vicinity of the reaction zone. The hexacyanoferrate species, Fe(CN) 6 4−/3− , and related cyanometalate couples lend credence to their consideration as model redox systems suitable for studying the role of such medium dynamics. Recent experimental findings for the temperature, pressure and solvent composition control of three physically related paths for electron transfer (thermal homogeneous, thermal electrochemical, and optically induced homogeneous) detected for this system, allow for the discrimination of microscopic mechanisms responsible for different catalytic effects of electrolyte cations and the solvent control effect of added dextrose. It is argued that for an adequate description of the overall catalytic effect of electrolyte cations, three different factors, viz., the preequilibrium effect of ionic atmosphere, the dynamic effect of stoichiometric ion reorganization, and the superexchange effect of a bridging counter ion should be taken into consideration. The latter two effects may also be “inhibiting”, depending on the composition and geometry of the reactive ionic associate in the precursor and transition states, and the symmetry requirements for the latter. Variation of these three factors in the cases of three different electron exchange paths, leads to the observed interplay of kinetic constants and activation parameters (as well as the CT-band parameters). Participation of the specifically hydrogen bonded water in the first solvation shell of cyanometalate ions, detectable through the differential O-D overtone spectroscopy of heavy water, may be responsible for other peculiarities of the considered redox couple, viz., the solvent isotope effects detected for the homogeneous thermal kinetic constant, the CT-band maximum shift for the optically induced process, and the solvent friction mechanism for the thermal electrochemical process.

The double-well cage effect for a ferrocyanide/dimethylviologen ion pair in viscous media: kinetic and thermodynamic probings through the CT transition band

Abstract The ion pair recombination kinetics following the electric field induced predissociation, and the ion pair formation kinetics following direct mixing, were studied by temperature jump and stopped-flow techniques, respectively, in viscous water/glycerol mixtures. The rate constant detected for the recombination process was virtually independent of the glycerol concentration within 65–80% v/v, while direct mixing revealed viscosity-sensitive dispersive kinetics. Thermodynamic parameters for the ion pair formation pseudo-equilibrium under the high viscosity conditions were also determined, which, together with the kinetic data, point to at least a two-step mechanism for the ion pair formation.

Mechanism of water exchange on five-coordinate copper(II) complexes

The effects of temperature and pressure on the water exchange reaction of the five-coordinate complexes [Cu(tmpa)(H2O)]2+ and [Cu(fz)2(H2O)]2− (tmpa = tris(2-pyridylmethyl)amine; fz = ferrozine = 5,6-bis(4-sulfonatophenyl)-3-(2-pyridyl)-1,2,4-triazine) were studied by employing 17O NMR spectroscopy. The former compound shows a solvent exchange rate constant kex of (8.6 ± 0.4) × 106 s−1 at 298 K and ambient pressure, within a factor of three of that for the corresponding process for [Cu(tren)(H2O)]2+ (tren = 2,2′,2″-triaminotriethylamine). The activation parameters ΔH#, ΔS# and ΔV# for the reaction are 43.0 ± 1.5 kJ mol−1, +32 ± 6 J K−1 mol−1 and −3.0 ± 0.1 cm3 mol−1, respectively. For [Cu(fz)2(H2O)]2−, kex is (3.5 ± 2.6) × 105 s−1 at 298 K and ambient pressure, which is about an order of magnitude less than for [Cu(tren)(H2O)]2+. The ΔH#, ΔS# and ΔV# values for the water exchange are 25.9 ± 2.2 kJ mol−1, −52 ± 7 J K−1 mol−1 and −4.7 ± 0.2 cm3 mol−1. The activation volume is modestly negative for both reactions and therefore implies an associative interchange (Ia) mechanism. The results are discussed in reference to data for water exchange reactions of Cu(II) complexes available from the literature.

Anionopentaaminecobalt(III) complexes with polyamine ligands 27. Activation volumes for spontaneous and Hg2+-assisted chloride release reactions☆

Abstract Activation volumes (Δ V≠) for the spontaneous and Hg2+-assisted chloride release reactions from some [CoCl(N)5]2+ complexes have been estimated from the effect of pressure (5–150 MPa, I = 1.0 M HClO4)_on the reaction rates. Δ V≠ data for the spontaneous aquation process are all small negative (−4 to −9 cm3 mol−1) whereas for the Hg2+-assisted process, Δ V≠ are small positive (+0.5 to +3 cm[su3 mol−1). The implications are discussed in terms of an interchange substitution mechanism.

A Millisecond Time-Scale UV-VIS Spectrophotometer Based on a Double-Diode Array Detector Coupled to a Stopped-Flow Instrument

Abstract Details are reported of the construction, calibration and testing of a fast-scanning spectrometer consisting of a double-diode array optical simultaneous multichannel analyzer (DDA-OSMA) coupled to stopped-flow instrument. This set-up enables the recording of high quality absorbance spectra as well as absorbance-time traces as a function of wavelength, on a millisecond time scale. The electron-transfer reaction between Fe(CN)6 3- and L-ascorbic acid is used to demonstrate the performance of the system.