Mossbauer Spectroscopy



   Mossbauer Spectroscopy:

Isotope
Natural Abundance (%)
57Fe
2.2
99Ru
12.0
119Sn
8.6
125Te
57.3
127I
7.0
129Xe
26.2
197Au
100.0
            Just as there are ground and excited states of atoms and molecules (electronic, vibrational and the like), so too there exist both ground and excited states of nuclei-they have just been mentioned as sometimes providing a method of measuring NQR spectra.  In decaying from an excited state a nucleus may emit light, just as an atom or molecule may. In the case of nuclei, this light is of very short wavelength, it is y radiation.  If this γ radiation falls on another, identical, nucleus it may be absorbed, leaving the second nucleus in an excited state.  Nuclei of any one element which are in different chemical environments will have slightly different energy levels, but  the  environment-induced  changes  are  so  small  that  it  is  possible  to compensate for them  with a  Doppler shift of the γ radiation,  achieved  by moving the  emitting  nucleus  either  towards  or away  from  the  absorber.
           

                  In Mossbauer spectroscopy the absorption of·; rays by the sample is recorded as a function of the velocity of the source. Solid samples are used; the source may be moved by attaching it to the diaphragm of a loudspeaker driven by a suitable signal generator.  The effect has been observed for  relatively few nuclei  at the  concentrations  at which  they  occur  in  most coordination compounds, of which  57Fe and 119Sn have been the  most widely  studied. 
           

          The  difference in absorption  velocity and  that  of a  suitable  reference standard is  called  the  isomer (or  chemical) shift; it  is  denoted δ and  is  usually  expressed  in  units of mm s-1 or cm s-1. The chemical  environment  affects  the  nuclear  energy levels  through  those electrons which  are  in orbitals  which  allow  them  to make contact  with the nucleus.  This means that only s electrons can directly affect isomer shifts since for all other orbitals the nucleus is contained in a nodal plane (electrons in p, d or f orbitals can only influence isomer shifts through their incomplete shielding of the  nucleus,  leading  to a  change in effective  nuclear charge which is  felt  by the  s  electrons. These general-ionizations find application in the observation that the isomer shift of Fe(C0)5 is  greater than  that  of Fe(CO)4-2.


COMPOUND
ISOMERS SHIFT (mm s-1)
COMPOUND
ISOMERS SHIFT (mm s-1)
High Spin FeIII
ca.  0.3-0.5
High Spin FeII
ca.  0.9-1.5
FeF3 (Oh)
0.49
FeF2 (Oh)
1.48
FeCl3 (Oh)
0.46
FeCl2 (Oh)
1.16
[FeF4]- (Tδ)
0.30
FeBr2 (Oh)
1.12
Low Spin FeIII
ca.  0.3-0.5
[FeCl2(H2O)4] (Oh)
1.36
[Fe(CN)6]3- (Oh)
-0.12
[FeCl4]4- (Tδ)
0.90


Low Spin FeII
ca.  0.9-1.5


[Fe(CN)6]4- (Oh)
-0.04

 
          

                 Indeed, a particular use of Mossbauer data has been to indicate the valence state of an atom empirical parameters are available which compensate for the effects of change of substituent and coordination number.  So, isomer shift data have been used to conclude that the π bonding ability of ligands decreases in the order

NO+ >CO> CN- >SO4-2> PPh3> N02- > NH3
            
            An excellent example of the use of Mossbauer spectroscopy in structure determination is provided by Fe3(C0)12. Although the dark green crystals of this compound are easy to prepare dissolve Fe(C0)5 in aqueous alkali to give the Fe(CO)4-2 anion and oxidize this with solid Mn02 to give Fe3(C0)12-its structure was uncertain for over 30 years Although eight structures, all incorrect, had  been proposed for  Fe3(C0)12, it was the ninth, suggested on the basis of its Mossbauer spectrum, which eventually proved to be correct. The most evident thing is that it corresponds, approximately, to three peaks of equal intensity. It would be wrong to conclude that each corresponds to a different type of iron atom. If the iron atoms were all different then they could not all be in high symmetry environments and so quadrupole splitting would be expected on most of the peaks.





 


            Given that the chemical nature of the three iron atoms is so similar and so similar isomer shifts are to be expected the only reasonable interpretation of the spectrum is that the outer two lines are the quadrupole split components arising from a peak of intensity two, centered at about the same position as the central peak (which itself has but a small quadrupole splitting and is of intensity one). So, it seems that there are two equivalent, low-symmetry, iron atoms and one of high symmetry. The high-symmetry iron atom is on the right (it has but two types of bond, one to terminal CO ligands, and the other to Fe atoms). The low-symmetry pair are on the left.  The X-ray crystallographic work showed the bridging CO ligands to be asymmetric, off-center. This means that the low-symmetry iron atoms are each involved in four different bonds, two to bridging CO ligands one long, one short one to the terminal CO ligands and one to the unique iron atom. A more recent study of the Mossbauer spectrum over a temperature range has provided explanations for the asymmetries in the peak-intensity patterns entirely in accord with the accepted structure.




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