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Queen Elizabeth II Fellows
Dr Richard Webster
The electrochemical control of
oxidation and reduction processes in organic and inorganic systems is
an area of extensive research in both academia and industry.
Electrochemical techniques are extremely useful in generating
interesting species in unusual oxidation states, or for producing
reactive intermediates (as, for example in the reductive dimerization
of vinyl cyanide in the Monsanto manufacture of Nylon 66), but
provide little intrinsic structural information. To overcome this
limitation, spectroscopic methods have frequently been used in
conjunction with electrochemical methods in order to monitor the
progress of a reaction and to obtain more detailed structural and
mechanistic information. The in situ alliance of
electrochemistry/spectroscopy is particularly valuable in situations
where the species undergoing the redox process would not survive the
transfer from an electrochemical to a spectroscopic cell, or in
situations where it is essential that the spectroscopic analysis
occur concurrently to the electrochemical generation, such as in
kinetic studies. The focus of this research is developing and
utilising spectroscopic techniques (including EPR, UV-VIS, FTIR and
NMR) to study processes involving electron transfer, primarily in
organic systems. Resources and expertise are shared with the
Coordination Chemistry and Spectro-electrochemistry Group.
In situ Electrochemical-NMR Spectroscopy
NMR spectroscopy has capacity for the structural identification of
solution-phase species. However, despite widespread interest in both
NMR spectroscopy and voltammetry, the compatibility problems that
exist between the two procedures have made it extremely difficult to
achieve an in situ combination, with few published attempts.
The principal difficulty is the requirement that a metal electrode be
located within the magnet of a NMR spectrometer, with the current
flowing simultaneously to spectral acquisition. The purpose of the
present study is to design a versatile and robust in situ
electrochemical-NMR spectroscopy cell, capable of operation in a
number of different NMR spectrometers under variable temperature and
anaerobic conditions. The cell is presently undergoing a process of
refinement using the technical expertise of the RSC mechanical,
electrical and glass workshops. The finished cell will be applied to
studying a number of challenging redox systems where important
information regarding the structure, identity or kinetic behaviour of
participating reactants and/or intermediates has not been achievable
with the use of other spectroscopic methods.
ATR-FTIR Spectroscopy of Solution Phase Radicals
The
School recently purchased a FTIR spectrometer (the ReactIR) that
utilises an attenuated total reflectance (ATR) probe in order to
monitor the progress of chemical reactions and obtain kinetic data
from solution phase reactions. It was thought that such a technique
could be useful for obtaining the IR spectra of species that are too
unstable to survive being transferred to a conventional FTIR cell.
After some initial teething problems that required the instrument
being returned to America for repair, successful experiments were
carried out this year where the probe was used to obtain the IR
spectra of reactive carboxylate radical anions and 2,4,6-substituted
phenoxyl radicals. The radicals were prepared by electrolysing the
neutral compounds in a specially built electrochemical cell
constructed in the RSC glassblowing workshop. Future studies will
focus on improving the performance of the electrochemical cell with
the aim of obtaining IR spectra of species that are considerably less
stable, such as a-tocopheroxyl (the
vitamin E radical) and other biologically important phenoxyls.
EPR Spectroscopy of Organic and Transition Metal Species
Work
continues on obtaining the EPR spectra of organic and inorganic
radicals that require special conditions to be stabilised, such as
low temperatures or exotic media. Cyclic voltammetric and controlled
potential electrolysis experiments performed at -40°C in
CH2Cl2 have indicated that tethered
arene-ruthenium(II) complexes can be oxidised by one-electron to form
moderately stable compounds. EPR experiments confirmed that the
oxidation occurred at the metal centre to form the RuIII
complexes. At temperatures below ~ -20°C, the oxidised
species remain stable for long periods of time, but at higher
temperatures quickly decompose to other RuII compounds. A
combination of electrochemistry and EPR spectroscopy helped to
establish the optimal conditions with which to synthesise and isolate
the tethered arene-ruthenium(III) compounds. (with J.R. Adams,
M.A. Bennett)
Processed
wood surfaces exposed outdoors are rapidly degraded because lignin
strongly absorbs UV light, which leads to radical-induced
depolymerisation of lignin and cellulose, the major structural
constituents of wood. We have used EPR spectroscopy to monitor the
concentration of free radicals in wood and to assess the feasibility
of using chemical modifying agents to reduce photo destabilisation.
(with P.D. Evans [Dept. Wood Science, U. British Columbia,
Canada])
Copper Corrosion
Passive
films formed on copper electrodes in neutral or basic solutions have
a duplex structure with an outer layer of CuO or Cu(OH)2
and an inner layer of Cu2O. It is because of the
formation of these thermodynamically and kinetically stable passive
films in approximately neutral pH media that copper tubes are a
widely used and generally reliable conduit for domestic water
supplies. However, one source of pitting corrosion of copper is the
presence of aggressive anions such as Cl- and SO42-,
which accelerate the breakdown of the passive layers. We have
developed a procedure for studying the corrosive nature of the
aforementioned anions in the concentration range typically found in
potable water supplies (20-200 ppm) using artificial copper pits that
are prepared by voltammetrically forcing dissolution of 60 or 80 ?m
copper wires embedded in an epoxy matrix. The morphology and
chemical composition of the pit caps are examined by Raman
spectroscopy, optical microscopy, scanning electrode microscopy (SEM)
and energy dispersive X-ray (EDX) analysis. Raman spectroscopy is
particularly useful for in situ studies because H2O
scatters only weakly between 3100-150 cm-1, thus the
samples do not need to be dried (such as they do for SEM, EDX and XPS
analysis) thereby reducing the probability of alteration of the
corrosion products prior to spectroscopic analysis. (with A. Lowe,
M. Stoll [Engineering, FEIT, ANU], V. Otieno-Alego [U. Canberra])
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