 I once again welcome you all to MSP lecture series on interpretive spectroscopy. So let us continue discussion on mass bar spectroscopy today. Of course, you all know what is mass bar effect, mass bar effect is the emission and resonant absorption of nuclear gamma rays under conditions such that the nuclei have negligible recoiling velocities when gamma rays are emitted or absorbed. This is only achieved by working with solid samples in which the nuclear held rigidly in a crystal lattice. The energy and the frequency of the gamma radiation involved corresponds to the transition between the ground state and the short cuted exit state of the nucleate concerned nuclear spins concerned. So, we saw nuclear transitions, spin transitions, spin flipping in case of NMR, electron spin transition, electron transition in case of UV visible and electron spin excitation in case of EPR. Now, we are looking into nuclear transitions in case of mass bar. Properties of selected nuclei which can be observed are studied using mass bar spectroscopy are shown here 57 iron 119, 99 ruthenium 197 gold and natural abundance is 2.2, 8.6, 12.7 and 100 and ground spin state is half, half, 3 by 2, 3 by 2 and exaggerated spin states are 3 by 2, 3 by 2, 5 by 2 and half and radio isotope source we used in case of 57 iron is 57 cobalt, in case of tin is metastable 119 tin and 99 rhodium and then in case of 197 gold it is 197 metastable platinum. And here if you want to illustrate mass bar effect, 57 iron mass per spectroscopy comes very handy to illustrate very nicely the mass bar effect and the basic operators consists of a radioactive source, a solid absorber with the Fe 57 containing sample and a gamma ray detector. Very simple assembly mass per spectroscopy for 57 MP samples the radioactive source is 57 cobalt which is incorporated into stainless steel, 57 cobalt source decay by capture of an extra nuclear electron to give the excited state 57 iron which emits gamma radiation as it decays to its ground state. So, this is the simple technique we use in mass bar, if 57 Fe is present in the same form in both source and absorber resonant absorption occurs and no radiation is transmitted. So, that is the reason we generate excited state 57 Fe from 57 cobalt through decay of an extra nuclear electron. However, if the 57 Fe in the source and the absorber is present in two different forms absorption does not occur and gamma radiation reaches the detector. Moving the source at different velocities towards or away from the absorber has the effect of varying the energy of the gamma radiation that we use it as call it as Doppler effect. The velocity of movement required to bring about maximum absorption relative to stainless steel defined as an arbitrary 0 for iron here is called the isomer shift of 57 iron in the sample with units of millimeters per second and of course, the time scale for mass per spectroscopy is 10 raise to minus 18 seconds. And what structural information one can get from isomer shift data. So, this isomer shift data gives a measure of the electron density on the 57 iron center and isomer shift values can be used to determine the oxygen state of the iron atom. So, that means it gives electron density as well as the oxygen state of iron atom. Similarly, in case of 197 gold mass bar spectroscopy isomer shift can be used to distinguish between gold 1 and gold 3, 3 specific examples I have chosen here from iron chemistry to explain both isomer shift and also to get sexual information. For example, in the cation if you take pentamine nitrosyl iron 2 plus there is an ambiguity in describing the bonding. Since in some instances it has been described as ennoplus nitrosyl cation bound to iron 1 center because if you take iron 2 center and if you add in nitrosyl group what happens it immediately donates one electron that is present in anti-bonding orbital because if you count electron 11 electrons are there in the in the valence shell. So, one will be basically in the anti-bonding orbital once it gives your ennobond will be triple it stabilizes as a result it readily forms a cation. Hence what happens when it comes to the metal it is the assumption that the oxygen state decreases by one unit that is what is told here iron 2 on addition of ennob assuming it is ennoblus it turns an oxygen state from 2 to 1 results of 57 iron mass bar spectroscopy have revealed that the correct description of an ennoblus is not ennoblus in fact ennoblus ligand bound to an Fe 3 center that means basically it takes an electron and then pair of electron comes to the metal like a typical sigma donor iron center will be not iron 2 it should be iron 3. So, this ambiguity is there, but here to overcome this ambiguity 57 Fe mass bar spectroscopy clearly explains these things. If you look into other example hexasino ferrate 4 minus and hexasino ferrate 3 minus iron is in plus 2 and plus 3 states. However, the closeness of isomer shifts for these species suggest that the actual oxygen states are similar it looks very puzzling. If you take hexasino ferrate 4 minus and hexasino ferrate 3 minus the moment you look into it we think the first one is in plus 2 state and the second one is in plus 3 state, but if you look into the isomer shifts observed in 57 iron mass bar spectroscopy is suggest that the actual oxygen states are similar in both the complexes and this may be interpreted in terms of extra electron in a Fe cn 6 4 minus being localized onto the sinoligans rather than sitting on the iron center. So, this is very vital information again this information comes from mass bar spectroscopy. So, difference in isomer shifts can be used to distinguish between different iron environments in the same molecule as well. That means not only it clarifies the ambiguity about oxygen state, but also it can tell you about different environments geometric environments present in a molecule although simply it is it appears like they are indistinguishable. So, the existence of two signals in mass bar spectrum of Fe 3 CO 12 provided the first evidence for the presence of two types of iron atoms in the solid state structure as confirmed by x-ray diffraction method. If you just look into the structure of Fe 3 CO 12 we have two bridging carbonates here less will be different. So, we have less will be different here. So, this we should remember. So, since two iron atoms here have two bridging carbonates whether the one on the top of this triangular planar geometry do not have any bridging. So, that means strictly speaking there are two types of iron centers are there this can be distinguished no doubt from x-ray crystallography, but mass bar spectroscopy also shows two signals suggesting that in 2 is to 1 ratio suggesting that there are two different types of iron environments in the molecule of Fe 3 CO 12. And then of course, this will show you the region of electromagnetic radiation that is used in mass bar spectroscopy we use gamma rays. Then this is a typical layout of a mass bar spectrometer here the cobalt source is there as I mentioned 57 cobalt is considered and it is put into steel. The speed of the carriage is adjusted until the Doppler shifted frequency of the emitted gamma ray matches the corresponding nuclear transition in the sample. The inset shows the nuclear transition responsible for the emission of the gamma ray here and then of course, it comes to the sample and then it goes to the detector and then we get the spectrum. So, then the effect of an electric field gradient so, that is abbreviated as EFG in later if I say EFG it should be understood as electric field gradient. The effect of an electric field gradient and a magnetic field on the energy levels involved in the mass bar technique for iron sample is shown here. The derived spectra show shown depicts the origin of the isomer shift quadrupole coupling and magnetic hyperfine coupling, but no quadrupole splitting. The spectrum of 2 that is K4 efficiency and 6 3 H2O octahedral no spin and D6 it is because it is a plus 2 state D6 is a representative of a highly symmetric environment and shows a single peak with an isomer shift it is a highly symmetric octahedral molecule it shows a single peak with an isomer shift here. The spectrum of 3 F is 0 for 7 H2O is also D6 in a non-symmetrical environment as a result it shows quadrupole splitting here you can see quadrupole splitting here. So, the isomer shift of iron 2 compounds related to metallic iron F is 0 are generally in the range of 1 to 1.5 millimeters per second whereas, isomer shift for iron 3 compounds lie in the range of 0.2 to 0.5 millimeters per second. So, you explain these values in terms of the electric configuration of iron 0 iron 2 and iron 3. So, this is a question I read out again the isomer shifts of iron 2 compounds related to metallic iron F is 0 are generally in the range of 1 to 1.5 millimeters per second whereas, isomer shifts for F e 3 compound lie in the range of 0.2 to 0.5 millimeter per second you explain these values in terms of the electronic configuration of iron 0 iron 2 and iron 3. And then now we have to look into the electronic configuration of iron 0 iron 2 and iron 3 which are 4 H 2 3 D 6 all the electrons are intact and then iron 2 yes 4 H electrons are gone 3 D 6 in case of F e 3 one more electron is gone it is 3 D 5 electronic configuration 4 is 0. So, the S electron density at the nucleus is reduced in F e 2 compared to F e 0 producing a large positive isomer shift that means S electrons. The reduction in S electron will show large positive shift that one should remember. So, here we have 2 electrons are there in S and here no electrons are there. So, electron density is reduced that produces a large positive isomer shift when a 3 D electron is removed to produce 3 D 6 to 3 D 5 from F e 3 2 there is a small increase in S electron density at the nucleus as a 3 D electrons are partly screened the nucleus from the inner S circle and the isomer shift becomes less positive that is the reason between 2 to 3 less significant because there is no S electrons involved in it is only changing the taking out one electron whereas, in case of iron 0 to iron 2 or iron 3 we are talking about S electron density that means, it shows how the S electron density influences the isomer shift. There is one more question here variable temperature 57 iron mass bar spectra of this one phenanthraline is given here and the NCSI thiocyanate 2 are there and 2 phenanthraline bident ligands are there iron is in octahedral geometry are shown below here it is shown here variable temperature starting from 77 K to 300 K using the data such as isomer shift a quadrupole splitting data provided in the 377 K spectra and the magnetic susceptibility data explain the variable temperature behavior that is the question. So, now 57 mass bar spectra indicate the significant changes in the iron environment at 185 K at 185 K it is 186 you can say here from here 184 you can consider 182 184 it is significantly changing these are growing here the magnetic data indicate the high spin nature of the complex at room temperature and low spin at low temperature. So, it is a D 6 system showing spin crossover isomer shift and quadrupole splitting parameters for the high spin and low spin isomers. This is another question now 57 iron mass bar spectra for iron 2 and iron 3 octahedral spin crossover complexes recorded different temperature are shown here determine the isomer shift and quadrupole splitting for each spectrum and use these to assign the spectra to the iron axis state and the spin state again this compound is taken. So, here 57 mass bar spectroscopy is a again a powerful way of identifying the oxidation and spin states of iron complexes that we saw in case of iron 0 iron 2 iron 3 complexes. So, now if you just look into the data given for iron 2 and iron 3 octahedral spin crossover complexes the isomer shift is expected to move to more negative values as the axis state changes from iron 2 to iron 3. However, apart from 0.85 millimeters per second you can see here and the value for the 383 Kelvin 383 Kelvin the other values are resembling similar and indicate that the left hand spectra are probably from iron 2 complexes. The delta values alone are not always sufficiently diagnostic octahedral high spin iron 2 has a 5 T 2 G ground term low spin iron 2 has a 1 A 1 G ground term and high spin F e 3 has a 6 A 1 G ground term and low spin F e 3 has a 2 T 2 G ground term. So, this you can just simply look into it how we get it from U V S spectroscopy. So, therefore, electronically induced E F G's are predicted for high spin iron 2 and low spin iron 3 as significant delta E Q values are observed at high temperature in the left hand spectrum and at lower temperature for the right hand spectra. This confirms that the left hand spectra are for iron 2 complexes and the right hand spectra are for the iron 3 complex. This how you should analyze and conclude about the spectra obtained here. Let us look into one more spectrum and this again how to distinguish different geometries of iron compounds iron carbonyl compounds. The 57 mass per spectra and structures of some iron carbonyls are shown here assign the spectra to the character structure. Spectra is given here and also some structures are given we have to assign the spectra to the corresponding structures. The isomer shift delta isomer shift is given by delta for each spectra is close to 0 and does not vary much between the 3 carbonyl compounds because all of them have 3 D 6 4 S 2 electronic configuration iron is in 0 state. However, there are marked difference in the quadrupole splitting quadrupole splitting is delta E Q and then isomer shift is small delta. So, it is observed in 3 spectra and this is related to different electric field gradient at the iron nucleus. In this case the greatest contribution to the E F G come from structural and geometrical effects rather than electronic ones. So, that means, in case of F E CO 5 with iron is in trigonal bipranded geometry 3 carbonyls in the plane and 2 in the axial position and the electric field has axial symmetry which is different in z direction compared to that in x as well as y directions which are essentially the same. So, therefore, there will be an appreciable electric field gradient and hence delta E Q observed in the spectrum of F E CO 5. In F E 2 CO 9 there are 2 equivalent iron atoms. So, there will be 1 multiplied here and as each is in approximately actahedral configuration in case of E F G and F E will be smaller than in F E CO 5. And then in case of F E 3 CO 12 there are 2 different iron environments that can be clearly seen from the structure. So, 2 multiplets are expected one from the iron at the top of the triangle and one from the bottom of the triangle and as there is in a 1 is to 2. So, bottom 2 are equivalent and the top one is different. So, 2 is to 1 ratio or 1 is to 2 ratio. This will be reflected in the peak intensities even this one can make out. The iron on top is in a more symmetric and almost regular actahedral environment with no bridging carbonyl ligands whereas, the 2 iron atoms at the bottom have a mix of both terminal as well as bridging carbonyls. On this basis see a widely spaced quadruple doublet with twice the intensity of a narrowly spaced quadruple doublet is expected. Narrowly spaced quadruple doublet is for the top one. The spectrum A with a large delta E Q is assigned to F E CO 5 and the spectrum B with a small delta E Q is assigned to F E 2 CO 9 and the spectrum C with 2 multiplets is assigned to F E 3 CO 12. So, this is how we can even identify the spectrum based on this information. So, it is shown here. So, this is for F E CO 5 and then here this is for F E 2 CO 9 you can see both are them appears to be in a typical regular actahedral geometry and whereas, here we have this one is in a typical actahedral geometry and then these two have two bridging carbonyls. These two are different type one type and this is one. Obviously, you should we will expect this in a 2 is to 1 ratio that is what is observed here. 57 F E mass bar experiments were the first to identify the structure of F E 3 CO 12. Of course, F E 3 CO 12 structure was confirmed by single critical extra diffraction studies and of course, when you look into IR, IR also shows you the presence of bridging carbonyls, but it would not tell you whether only two have bridging carbonyls and other one does not have that information does not come from IR spectroscopy. There is a limitation of that one whereas, here even 13 C NMR may not be able to give this information about the difference in the geometrical arrangement of carbonyl groups around two iron atoms whereas, here without any ambiguity it can show you two different type of iron environments. So, now one more question here, a acetyl pridine thiosemic carbosone can act as a tridentate ligand either in its neutral thion form this is neutral form or as an anionic ligand through tautomerism can N H H can come here and see S H becomes and H goes off it becomes an anionic ATP minus or H ATP here. It can form as a neutral or deprotonation of the thiol tautomer to give ATP minus. So, this forms complexes of this type H A P T twice Cl 2 and then tridentate ligand. So, it is iron 2 is said di cationic ligand. So, two anions are there and in this case it is an anionic ligand one is there one is there. So, Cl 2 is here. So, it is iron 3 state. So, use the isomer shift and quadruple splitting data provided in the spectrum in the 57 mass per spectra 2 and identify that state and spin states and hence assign the spectra to the complexes here. For 57 mass per spectra the isomer shift is expected to be lower value for Fe 3 then Fe 2 on this basis the spectra A is assigned to iron 3 with delta value of 0.065 millimetres per second and also 2.430 millimetres per second and the second one is assigned to Fe 2 having delta equals 0.264 millimetres per second and delta E q equals 0.537 millimetres per second. So, then how it is done you can see here the quadruple splitting value delta E q can be used to identify the spin state using the same methodology as in Fe 3 CO 12 we used as these complexes are also 6 coordinate. Therefore, the structural E F G is likely to be small and E F G to be dominated by electronic effects. Then now look into these two cases four cases actually octahedral high spin Fe 2 has a 5 T 2 G ground term and octahedral low spin Fe 2 plus has a 1 A 1 G ground term and then octahedral high spin Fe 3 has a 6 A 1 G ground term and octahedral low spin Fe 3 has a T 2 G ground term. So, therefore, electronically induced E F G's and hence appreciable delta Q L S are predicted for high spin Fe 2 and low spin Fe 3 with small E F E G's and a values for low spin Fe 2 and high spin Fe 3. So, the this value in spectrum A is 2.430 millimits per second and that in spectrum B it is 5.37 millimits per second using this information in conjunction with the isomer shift data. We can assign the spectrum A to the again low spin Fe 3 complex that means this anionic ligand here and spectrum B to the low spin Fe 2 complex where H A P D is a neutral tridentate ligand. So, this is a very good example where a large quadruple doublet does not automatically imply high spin iron 2 state. Now, let us look into one more example ferrodoxins are non iron sulphur proteins ferrodoxins are iron sulphur proteins that mediate electron transfer in a variety of metabolic reactions. Electron transfer is accompanied by iron redox chemistry in the Fe 2 S 2 core here. The 57 Fe mass per spectra of the oxidized and reduce forms of Cendismus ferrodoxin are shown in the figure here this is the one that is shown. Use this data to account for the electronic and magnetic behavior of ferrodoxin in its oxidized and reduced forms. Some of these problems are taken from library text as well as Oxford classes. Figure shows the structure of the Fe 2 S 2 core and the 57 Fe mass per spectra of the oxidized and reduced forms of Cendismus ferrodoxin. So, therefore, here 57 iron mass per spectra show convincingly the difference between the oxidized form of ferrodoxin with 2 iron 3 centers and the reduced form of 1 Fe 3 and 1 Fe 2 center here. You can see clearly here. So, that means in the oxidized version there is a simple quadrupole doublet with isomer shift of plus 0.20 millimits per second and then we have delta eq equals 0.60 millimits per second, but on reduction this is replaced by a pair of quadrupole doublets. In the oxidized form both iron atoms in the Fe 2 S 2 core are tetrahedral high spin Fe 3 plus. Therefore, each Fe is expected to have a 6 A ground term giving rise to a small or negligible valence E fg and hence small delta eq value. So, we can compare the eq value. As the iron atoms are strongly anti ferromagnetically coupled by both exchange and super exchange mechanisms through the sulfur atoms, the ground state is in fact a spin singlet. So, in the reduced form there is a central quadrupole doublet at 0.22 millimits per second with the delta eq equals 0.59 per second both of which are very similar to those in the spectrum of the oxidized one if you just see here. Both of which are very similar to those in the spectrum of the oxidized form. In addition there is a second quadrupole doublet at 0.56 considerably larger with delta eq value of 2.75, whereas 0.60 and it was 0.59 millimits per second. The increase in the isomer shift is consistent with this quadrupole doublet belonging to iron 2. The large quadrupole splitting is consistent with a large E fg caused by the non cubic electron density at in tetrahedral high spin iron 2 which has a 5 E ground term. Therefore, in the reduced form there is one tetrahedral high spin iron 3 with a 6 A term giving rise to a small delta eq and a tetrahedral high spin f e 2 with a 5 2 term giving rise to a large delta eq value, but as these are also anti ferromagnetically coupled overall it is a spin doublet system. This is how the data is interpreted. I think I should stop here. Maybe in my next lecture I will be talking more about problems. As I mentioned I will be solving problems on almost all spectroscopic aspects and I am sure you are going to enjoy these problems. So until then have an excellent time. Thank you so much.