Mossbauer spectroscopy exploitation

Table I shows the various Mossbauer nuclides—i.e., the nuclides where the Mossbauer eflFect has actually been seen. Not all of these are as easy to exploit as the Fe and 9Sn cases referred to above. However, with improved techniques a number of these should prove accessible to the chemist. Representative elements of almost all parts of the periodic table are tractable by these techniques. It seems clear, however, that the methods of Mossbauer spectroscopy are no longer technique-oriented but that this field is becoming a problem-oriented discipline. In other words, the Mossbauer effect is now used successfully in many cases not only to demonstrate the effect or to corroborate physical evidence obtained by other means—NMR, or infrared, or kinetic studies— but also to solve new chemical problems.

As will be explained in Chapter 7, spectroscopic methods are a powerful way to probe the active sites of the hydrogenases. Often spectroscopic methods are greatly enhanced by judicious enrichment of the active sites with a stable isotope. For example, Mossbauer spectroscopy detects only the isotope Fe, which is present at only 2.2 per cent abundance in natural iron. Hydrogen atoms, which cannot be seen by X-ray diffraction for example, can be studied by EPR and ENDOR spectroscopy, which exploit the hyperfine interactions between the unpaired electron spin and nuclear spins. More detailed information has been derived from hyperfine interactions with nuclei such as Ni and Se, in the active sites. In FTTR spec-... 

There are four naturally occurring isotopes of iron ( Fe 5.82%, Fe 91.66%, Fe 2.19%, Fe 0.33%), and nine others are known. The most abundant isotope ( Fe) is the most stable nuclear configuration of all the elements in terms of nuclear binding energy per nucleon. This stability, in terms of nuclear equilibrium established in the last moments of supernova events, explains the widespread occurrence of iron in the cosmos. The isotope Fe has practical applications, most notably in Mossbauer spectroscopy, which has been widely exploited to characterize iron coordination complexes. 

The implementation of combinatorial chemistry and automated methods for rapid synthesis, testing, and characterization of catalysts, has opened a wide range of new opportunities in catalysis. However, so far, Mossbauer spectroscopy has not been introduced into this methodology. Two hurdles must be overcome for Mossbauer spectroscopy to become important in high-throughput catalyst characterization the system for recording spectra must be scaled down, and the data acquisition and exploitation systems must be adapted. 

One aspect of Mossbauer spectroscopy that has not been widely exploited in phase transformation studies is time resolution. Studies with conventional techniques are possible over a wide range of time scales, starting from the intrinsic time scale of the Te Mossbauer effect (t <= 10 s) to investigate processes such as electron transfer, to time scales of t 10 s to measure diffusion, to longer timescales of t > 10 s to study phase transitions, oxidation and other chemical reactions. 

The effect is exploited in Mossbauer spectroscopy in which a gamma-ray source is mounted on a moving platform and a similar sample is mounted nearby. A detector measures gamma rays scattered by the sample. The source is moved slowly towards the sample at a varying speed, so as to continuously change the frequency of the emitted gamma radiation by the Doppler effect. A sharp decrease in the signal from the detector at a particular speed (i.e. frequency) indl-... 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

The stability of the electronic configuration is indicated by the fact that each element has the highest ionization energy in its period, though the value decreases down the group as a result of increasing size of the atoms. For the heavier elements is it actually smaller than for first-row elements such as O and F with consequences for the chemical reactivities of the noble gases which will be considered in the next section. Nuclear properties, particularly for xenon, have been exploited for nmr spectroscopy and Mossbauer... 

Many valuable new results may be expected in this respect from the wider ranging application of electron paramagnetic resonance spectroscopy, NMR spectroscopy and solution X-ray examinations, and from the introduction of neutron diffraction and other diffraction methods in the chemistry of non-aqueous solutions. New possibilities in this field, which have by no means been fully exploited, are Mossbauer and ESCA investigations of rapidly frozen solutions. 

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