 Hello and welcome to this next segment of CDN MOSBIR Spectroscopy for Chemist. My name is Arunabh Dutta and today we are going to discuss about the application of MOSBIR spectroscopy where we are going to look how we can use MOSBIR spectroscopy to understand the properties of ferrocene. So, in the previous segment we are talking about ferrocene and we find out this ferrocene can be present in two forms. One is this eclipsed version where the two rings are actually on top of each other or it can be present in the staggered geometry where the two rings are actually in opposite direction present in that particular molecule and among these two the staggered position is actually more stable because in the eclipsed form it actually faces the steric hindrance coming from this substituents present in the cyclopentadienyl ring which is avoided over here because they are separated at the maximum it is possible in this particular geometry. So, with that we are going to move forward and we find that this ferrocene molecule from now on we are going to draw only the staggered form because now we know that is the most stable state of ferrocene. So, now first this ferrocene was taken and people have drawn this ferrocene in the staggered form and try to measure the MOSBIR spectra for this particular ferrocene molecule and when they run that they found an isomer shift and also quadrupole splitting in this particular molecule. Now, when we talk about isomer shift we found that this is actually coming around 0.53 millimeter per second and quadrupole splitting is quite a large one 2.39 millimeter per second. Now, question is it had to understand why there is a quadrupole splitting why this isomer shift? The isomer shift is representing the iron is in plus 2 oxidation state over here and the quadrupole splitting says that what is the geometry of this particular iron over here in this ferrocene. So, in the staggered conformation the point group is D5D and in this particular condition how the electrons will be there. So, we are mostly talking about the D electrons and the D electron configuration in D5D is as following. So, it is a little bit different that what we expect for a octahedral geometry and if you look into the character table of D5D you will find the D orbital is located in this 3 segments. First one is a doublet E2G which is actually showing you the x square minus y square and xy orbital then comes the A1G dash which is coming for the z square orbital and then comes the E1G orbital which is coming for the xz and yz orbital. And over here iron plus 2 has 6 electrons. So, those 6 electrons goes like this because the difference over here it is actually quite large greater than the peering energy. So, that is why it stands out kind of a low spin system where all the 6 electrons are paired up and over here you can see the electrons are in x square minus y square xy and z square orbital. So, it is not really properly oriented in all different axes xz and yz is totally empty. So, that is why it creates a valence electric field gradient which is going to be nonzero. And then when you look into the lattice interaction it looks pretty similar on the top side that it is actually binding with two similar ligands, but they are oppositely oriented and you can see the orbitals which is actually going to interact over here. It is coming mostly from the top and bottom. So, on the iron center you can say it is having something free on this particular axis. So, that is why it is not really a totally symmetric one. So, it is going to have also the lattice electric field gradient and that is why it has a huge electric field gradient coming from both the components the valence and the lattice component and that is why it is reflected in its huge quadrupole splitting right over here and iron is in 0.53 millimeter per second. To understand a little bit further that how the electrons are actually exchanging between the metal the iron and the ligand in this case the cyclopentyl dinal ring. An experiment was performed with three different derivatives of ferrocene and those derivatives are as following. In one case it is the simple iron C P ring. So, this is the compounds. So, we write it irons C P 2 and then the next compound is the following which is pretty similar to the previous one. But with a slight difference that one of the ring right now have methyl ring methyl groups connected to it rather than the hydrogen. So, now one of them this is C 5 not H 5 but C H 3 whole 5 this remain as C 5 H 5 and this particular system over here it is we write them as C P star this will come it is the all methylated version of the cyclopentyl dinal ring. So, this one we could call C P C P star. And then there is another version of the molecule and you can probably predict what we are going to have where both the rings are actually all methylated. So, you can call them iron C P 2 star. So, these are the three molecules have been prepared and then if you look into the isomer shift and the quadrupolar splitting of that and try to find out what is actually happening. And this is the quadrupolar splitting and try to find out what is happening. So, iron C P 2 the values are already we have mentioned over here. So, it is going to remain the same what happens when you move to iron C P C P star. Interestingly, the isomer shift goes down a little bit goes to 0.5 which typically says that the oxidation state on the iron is probably on the moving towards a further positive side because as we remember if we have an iron center and the oxygen state increases it has less D electron less D electron means it has less shielding on the S electron and the S electron specially specifically the 3 S electron will be having more chance to go into the nucleus. So, it will increase the chance of having S electron density on the nucleus higher which will be multiplied by this change in the radii of the atom in the excited versus ground state as already negative. So, multiply that. So, it is going to move to the negative side. So, it is going to be lower down and that is what we are seeing over here that with C P change to C P star it is moving down to the lower side. The change it is a slight change also in the quadrupolar splitting it goes to 2.44. Now, when you move towards this C P 2 star that is both of them are C P star methylated the value go further down 0.49 and this one go to 2.47. Now, we want to understand why it is happening. The most important thing over here is the change in the isomer shift. You can see it is actually going down as you are moving from C P to C P star. So, as we go to more C P to C P star my delta value actually goes down which actually says that my iron is actually oxidation state is slightly increasing the formal oxidation state we are talking about and over here why it is increasing. So, that is actually giving us a very nice idea of what is the interaction happening between iron and the cyclopentyl dinel rings over here. So, when you talk about cyclopentyl dinel rings over here these are actually negatively charged anions which are trying to give electron to the iron and this is actually exchanging electron density with the iron. Now, when you put this methyl groups over here the methyl groups actually have one very interesting property it actually pushes some electron density from the methyl groups towards the ring which we can say the plus I effect the inductive effect and this is happening over here also on both the sides and as it is showing this inductive effect the electron density on the C P ring is actually increasing. And as it increasing the electron density there is interaction between the iron and the C P ring and it is pulling off more electron density from the iron to balance this negative charge on the C P ring. And as a result iron is losing a little bit more electron density when the C P star rings are present compared to the original C P rings and that is reflected on the MOSBUS spectroscopy the slight change it is showing that here my rings are actually moving out more electron density towards it and that is actually pulling some electron density from the iron and slightly lowering the actual D electron density on the iron which is reflecting on the MOSBUS spectroscopy where I am getting it moves towards a little bit on the negative side. So, higher induction state is coming because of the C P star rings having this plus I effect. So, that is actually increasing negative charge on the rings and that actually moves electron density from iron which is changing its oxidation state towards the more higher side and that is why we see this particular trend of change in the isotopic isotope isomer shift in this particular MOSBUS spectroscopy then why we are seeing this change in quadrupole splitting. So, now quadrupole splitting we see a change over here for two reasons first reason when we see this particular change that is because I am bringing more asymmetry because this is a C P star ring versus C P ring. So, obviously the lattice electric field gradient is getting more affected and that is showing up over here a shift towards the much higher side. And over here it is changing further 2.44 to 2.47 because when this two C P star rings are here it is pushing electron density from here and both of them are trying to pull electron density out of this iron which is further changing the electron density around the iron in this particular orientation which is already a little bit asymmetric and now you are pulling more electron density out because there is no electron density in the plane of the iron perpendicular to the way I have drawn and now it is pulling more electron density out because of the two C P star ring it is getting more and more asymmetric which is shown over here and that is what is actually happening and that is reflecting on this particular MOSBUS spectroscopy results for this ferrocene derivatives. Now, move next what happens if I take ferrocene if I take ferrocene and oxidize it so from iron plus 2 I am going to iron plus 3 so this is ferrocene and this is ferrocenium again we look for the isomer shift the quadrupolar splitting and we try to find out what will be the difference between these two ferrocene the values we already know 0.53 and 2.39. Now, take a look what happens to the ferrocenium so we are expecting it is already going to iron plus 3 so we should go down in the delta isomer value because it should move to the negative side because iron is now going to hydroxylation state less D electron less shielding S electrons are not shielded by D electrons that means it has more chance to go towards the nucleus increase the S electron density on to the nucleus which has a direct effect on the delta value if you remember delta value depends on the psi 0 square sample minus psi 0 square the source and that is going to be more values it will be higher value and that is multiplied with delta R by R which is actually negative value because atomic radii of excited state of 3 by 2 of iron is actually less than ground state so it actually shrinks down and that is why it is bringing a negative value which is actually multiplied with this higher value it should move to as a negative side let us find out what is actually happening in the experiment. In experiment we interestingly found the value is remaining almost similar not for change this is remaining almost similar and very interestingly the eq value actually goes down a lot so it is almost negligible so when you are doing this experiment what do you find so say this is what you are finding for the ferrocene and then we do the same molecule but after the oxidation finding this where it is almost shrinking down to a singlet rather a doublet because this value is actually so low that it is almost negligible and not only that if I take the average of this the delta value is remaining almost same now here delta eq is almost negligible whereas in previously there was a significantly high value of delta eq now the question is how do I analyze this particular data and try to make a meaning out of it so this is actually very interesting data which shows that MOSFET can give you a very interesting insight how the electron distributions actually happening in different molecules following the redox change so over here iron plus 2 got changed to iron plus 3 and we expected the delta value would be changing towards more negative and that is actually not happening at all why because once the iron plus 3 is formed from iron plus 2 the anions over here the cp anion it is not remaining spectator because these are as we just say redox non-innocent ligand so they also change their property as it is undergoing a redox change in the metal center so when you go to a iron plus 3 center the molecule try to stabilize itself because now previously it was very well stabilized iron plus 2 now it is becoming iron plus 3 3 there is a change so there is a change in the states of the energies that we have shown earlier so that previously it was a e2g a1g star a1g dash and e1g so one will be same and the rest the other electron will be absent in the case of iron plus 3 and present in the case of iron plus 2 so once this electron is gone the ligand also changes how it is interacting towards the iron it changes the electron density it is sharing with the iron once it becomes iron 3 plus it is supposed to have less electron density but when it has become much more charged system there is a lot of electron density coming from the cp ring towards the iron because now it is a more charge it can help the cp ring to disperse its charge and that is the extra charge is coming from the cp ring to the iron plus 3 to ensure that the overall electron density remains same on iron even after it is getting oxidized because when you say we are getting oxidized we are thinking it is the only metal center getting oxidized but it is a molecule so the electron get redistributed over the oxidation and after the oxidation when we try to take one electron out of the iron cp rings are replenishing that electron density and that is actually showing up over here and iron is getting electron density back from them and it is showing from the mosba spectroscopy yes iron even after the oxidation having similar electron density of d electron and that is why it is showing same isomership value. So, it is one of the most interesting example where you are seeing that even after oxidation the ferrocene iron center ferrocene iron centers are keeping the similar electron density. Now, why this delta eq value is actually shrinking so much that is actually coming from a different reason. So, when we talk about this iron 3 plus system we say it is a spin half system because you can see it is a spin half system but over here there is a very strong interaction between the cp and iron which is bringing a lot of orbital motion into the picture and this orbital moment multiply with this spin moment and it creates a strong spin orbit coupling and through this spin orbit coupling it can create further states beyond just only the spin state because now the orbital motion is also coming into the picture the spin orbit coupling will give generation to multiple state and it is actually found that this iron after this oxidation can be present in two such states which are known as Kramer's doublet and you can learn about that more when you are talking about the systems from IPR which can have a spin state beyond half. So, this Kramer doublet means you can present in two different states and either of the states can have different amount of quadrupole splitting because it is generating different amounts of electric field gradient and very interestingly this electric field gradient what we are seeing over here in these two different states are actually coming against each other it is actually negating each other and that effect is shown over here different EFG and they are actually opposing to each other and that actually gives almost negligible EFG which is coming into the picture almost negligible quadrupole splitting and that is why even iron plus C state in ferrocenium it is going to give us almost negligible delta equilibrium and that is coming over here and we can get well distinguished signal when it is a ferrocene versus when it is a ferrocenium iron and that is what we are seeing over here ferrocene versus ferrocene. So, let recap what we have discussed over here so ferrocene system it is going to show us a very well separated doublet the doublet is coming because it has a very differed electric field gradient coming from the lattice point of view and also from the valence point of view. Now, when you oxidize it the iron plus to go to iron plus 3 but it does not remain as iron plus 3 formal state because now the anions the cyclopentadienyl anions giving more electron density back to the iron and officially although we are writing its iron plus 3 but practically it is still in iron plus 2 state because 0.5 charge you can say it is coming from this cyclopentadienyl anions and which is showcased by this delta or isomer shift values for the ferrocene and ferrocene anions on the other hand the delta aq value shrinks down from a significant 2.4 millimeter per second to almost close to 0 why it is happening that is because this iron plus 3 state actually now a spin active state a appear active system and this actually having an unsaturated spin it is also interacting I have a lot of orbital moment because now the cp ring and iron increase their interaction. So, it is having a lot of orbital moment of a spin moment they will combine it will create spin orbital momentum that means spin orbit coupling once it creates a spin orbit coupling it generates multiple state and those multiple states known as Cramer doublet showcase that within very similar energy you can have multiple state and which this system can go forward with respect to the energy it is getting from the temperature and it can populate either of the state and each of this state has electric field gradient but they are actually kind of opposing to each other. So, they cancel each other out and an average I am going to see a 0 electric field gradient and almost 0 quadruple a speed and that is what is actually happening in the case of ferrocene. So, that is why ferrocene is a well separated doublet ferrocinium on the other hand it is actually almost a singless structure although the very important thing is that over here we are seeing the delta value almost same for ferrocene and ferrocene. The only give away of that which is the ferrocene which is the ferrocene is the quadruple splitting ferrocene has quadruple splitting ferrocene do not. So, with that we like to conclude this particular segment over here and we will continue our journey MOSBUS spectroscopy and understanding ferrocene and ferrocinium kind of systems in the next segment. Thank you very much.