 Welcome to this next segment of CD spectroscopy and MOSBAR spectroscopy for chemists. My name is Arnab Dutta and I am an associate professor in the department of IIT Bombay. Over here today we are going to discuss a few more applications of MOSBAR spectroscopy where we will look into the various aspect of MOSBAR spectroscopy that will allow us to unravel the mixed valence system in different molecular complexes. Last day we have discussed the mixed valence complexes. So we will recap that for a minute and then we will go to our example. So over here our goal is to use MOSBAR spectroscopy and if we can look into mixed valence complexes. So when we talk about mixed valence complexes, when we talk about mixed valence complexes it comes for 3 different classes. Class 1 where there are localized for an example we are taking iron plus 2 and iron plus 3. So they remain on their original position and no further change is observed that is class 1. So there are 2 different charges but they are not mixing it at all. Then there is a class 2 which is showing dual localization that means if you have an iron 2 and an iron 3 it is going to exchange an electron and iron 2 becomes iron 3 by losing one electron and iron 3 center become iron 2 by taking one electron. So this is happening and there are 2 different setups that is possible. So there is mixing of charge but the thing is here is this one are actually distinguishable. By distinguishable I mean I can still find one which one is iron 3 and which one is iron 2 I can find it out by various spectroscopic measurement. Then comes class 3 this is also delocalized one. There is also electrons is getting exchanged and say iron is exchanging between iron 3 and 2 on either of sets but it is happening so fast that now I cannot separate between iron 2 and 3 they are indistinguishable and from the spectroscopic experiments what I am getting a charge of 2.5 plus which is in between 3 and 2 and this is the average one that we are actually getting there in the system. And this particular system is known as the class 3 which is a delocalized iron but indistinguishable between iron 2 and 3 if it is distinguishable then it is class 2 and class 1 is totally separated. So these are the 3 different systems actually looked into and we have looked into certain iron complex examples where we actually see this kind of separation and we found this is class 1 class 2 class 3 kinds of mixed valence complexes. So these complexes are very unique for their multiple applications but one of the important aspect is can I find out whether it belongs to class 1, class 2 or class 3 and over here we need such spectroscopic experiment which can provide me the specific information on this oxidation state and MOSBA spectroscopy is one of them. So we are going to look into a few more examples. So this example number 9 that we are following. So over here I am going to take a 3 iron center complex, a trinuclear iron complex, 3 irons and they are coordinated with oxo species in the middle that is what we are actually seeing over here. And then any of the 2 iron is coordinated with acetate ion. So this is acetic acid which get the protonates and creates this acetate ion and proton and this acetate ion if I draw it that is how it looks like where the charge is getting changed between the 2 oxygen and we say it is mostly remaining in the form of and both this oxygen centers can bind metal so it can act as a bidented ligand and that is what we are drawing over here, we are drawing this system like this. So this is we have drawn over here and there are 2 of them are coordinated to that. Similarly, there is 1 over here will be 1 over here and this is how it is happening. So you can see each of the irons are coordinated with 2 acetate. So each of them are coordinating with 4 acetate all together all of them are bridging in nature. So it is not only bidented ligand but in this case it is actually acting as a bridging ligand which is interacting with 2 iron centers. And over there the iron coordination side you can see there are 5 coordination. So this acetate is a kind of forming this equatorial plane and 1 axial coordination is already done there by this triply bridged oxo species. So one of the bridging ligand is actually living over here this axial position. So this axial position is free which is coordinated with a pyridine motif and it is a ethyl pyridine. So this is the molecule we found it is a iron complex bound with bridging acetates all over there then there is this axial ethyl pyridine molecule 4 ethyl pyridine molecule. Now over here what is the oxidation state of this molecule? So over here what we found we have 2 iron centers is in plus 3 side and 1 iron plus 2 state. So all together you can say it is a plus 8 charge that we are looking into. And what is the overall charge provided by the ligands? So there are 6 acetate. So that give you 6 minus charge and 1 oxide in the center that is what we have 2 minus charge. So all together minus 8 charge. So this molecule is balanced with charge so the overall molecule is neutral. So 2 iron plus 3, 1 iron plus 2. Now we found this molecule is stabilized in the presence of solvents and there we get 2 different molecules. One we are saying molecule A which is this particular set of the molecule which is coordinated with benzene. So benzene is actually around this molecule and stabilizing it and there is another one Ca is 3 CCl3. So it is a methylated version of tetrachloromethane. So this is the 2 solvents we actually used and we get the compound of this site where there are 3 iron centers, 2 of them in plus 3, one of them in plus 2. Now our goal is to understand in this molecule how these irons are interacting? Are they interacting through this oxide so that we are getting a mixed balance complex or they are much more localized and keeping their plus 3 and plus 2 intrinsic charges. So let us find out what is the fate of the oxygen state with the help of MOSBUS spectroscopy. So we did this MOSBUS spectroscopy for this molecule for molecule A and molecule B which is nothing but molecule A means this full molecule stabilized in benzene and this one stabilized in Ca is 3 CCl3 and then we are going to showcase how the MOSBUS spectroscopy look for both of them and obviously we are going to show the X and Y axis for this and try to figure it out how it is actually shifting. So the benzene molecule so we actually see 2 different signals so there is one signal over here and then there is additional signals over here and we see this 2 sets of signal. So obviously you can see that there are 2 different metal ions are actually represented by them. One is with the red one, one is with this green one. So over here we are seeing this red one which is the iron plus 3 system and this is their quadrupole splitting. On the other hand the green one is the iron 2 plus system and this is their respective delta Eq. Now why we are saying 1 is iron 3, 1 is iron 2 for 2 reasons. 1st one look into the average values where is the delta values for iron 3 plus or iron 2 plus. So this is for the iron 3 plus and this is for the iron 2 plus and over here you can see the iron 3 plus is more on the negative side and which is expected as you have discussed earlier. Iron with less number of D electrons they have less shielding effect. So the S electron has more tendency to go to the nuclei. So the electron density on the nucleus, the S electron density will be more where I have less charge that means higher oxidation state and that increases that psi 0 square value because just to remind you the delta value isomer shift it is dependent on psi 0 square sample minus psi 0 square source and psi 0 square it means nothing but the S electron density in the nuclei and in the case we are talking about the 3 S electron density because there is the valence electron and getting mostly affected by this oxidation state. And this is multiplied with delta R which is nothing but the difference of the atomic radii between the excited state and the ground state and that is a negative number because the excited state of the nuclear state of iron 57 is actually smaller compared to the ground state. So it is a negative number. So you multiply that negative number with this number over here and psi 0 square of sample if it increases the whole system becomes more negative side comparatively negative side. So it moves towards the negative side and that is what we are actually seeing over here. So that is why we say it is actually a iron 3 plus system. Now also look into the ratio over here. So the ratio is actually 2 is 2 1 and that is what we expect over here from the original molecule which is actually having 2 iron and 2 iron in plus 3 and 1 iron in plus 2 state and that is what is shown over here. So that is what we are seeing over here and we found this discharge which is actually bound by the benzene is mostly on the localized side. Now if we go further this is actually recorded at 298 Kelvin we will go further and record the same data again and again to further lower temperature and what we see. So we continue to see that it is actually staying as it is at 298 Kelvin. So this is at 240 Kelvin then we go further down 280 Kelvin and it remains the same and even if we try that at 120 Kelvin it remains the same. What happens to this iron 2 it also remains the same with the ratio of 2 is 2 1 compared to this iron 3 plus signal. So they remain as it is. So in all possible condition the molecule with benzene is actually remain in localized states that we are actually observing from the MOSBus spectroscopic. Now if we go further with the CST-CCL3 molecule what do you observe? So for that when we do MOSBus spectroscopic we actually saw only one set of data only one set of doublet which is probably quadruple shifted and that is what we recorded at 298 Kelvin then we wanted to go a little bit lower oxygen, lower temperature the pigs becomes broader but mostly laying at the same place at 240 Kelvin. Then we went further down and there we started seeing two sets of pigs that is at 120 Kelvin and if you go further down there we see nicely splitted two sets of doublets. So this larger one is the same one we observed over here for the iron 3 plus whereas the smaller one representing the iron 2 plus and this is what we are seeing at 80 Kelvin. So at 80 Kelvin temperature at very low temperature we found that yes my molecule is now localized. However at high temperature is very difficult to see at the 120 Kelvin temperature it is kind of coming there whereas this iron 2 signals are also showing up and if you look closely to the previous one so that is actually further before it can actually even splitted out so it is remaining much more localized state to delocalized state in the higher temperature. In the higher temperature we are seeing only one set of signals which we can say it is a iron 2.67 plus state which is nothing but the average state between 2 3 plus and 1 2 plus so that is why this particular number and this is over there and this peak slowly splitted into iron 3 and 2 as we go to lower temperature. So we can say in the presence of CH3 CCl3 it is actually delocalized especially at the elevated temperature. So with that thing in mind now the question comes why this benzene and this molecule has this different kind of observations. So that is coming from the intermolecular interaction. This molecule when you look into it we can see that when it is actually forming the lattice over there if we have benzene molecule which is actually have this aromatic ring this particular aromatic ring actually stacked with the pyridines present in this molecule. So this pyridine we are discussing about so the benzene actually come and stack with them which actually provide them the intermolecular force to stabilization and once this intermolecular force are there it is kind of keeping the molecule static it is kind of holding its position because that is the stability is giving. So this molecule is less dynamic and as it is less dynamic it has less chance that this irons can have orientations that will support their exchange of electrons because these irons are already being tilted. So they have to make a few further changes so that it can interact with the oxide based bridging molecule and through this hospital they can exchange electron but if it is static through this interaction with the benzene so the irons are not able to move very much. So at that condition the iron may not avail the perfect orientation that would support their electron exchange and that is exactly is happening when we are talking about this benzene over here it is creating this interaction with this peripheral pyridine rings and that is kind of making it less dynamic the irons cannot move everywhere because this piper stacking is a kind of strapping it up and that is why it is becoming less dynamic and less chance of exchanging electron. So that is why it is having this localized formation whereas CH3-CCL3 there is no such interaction so molecules are more or less on its own the isolated molecule and over here you have enough dynamics that this iron 2 and 3 can take up particular orientation and exchange electrons so that at the end I am going to get a iron 2.67 plus charge system which is somewhere in between an average system. So this is a more delocalized system and that is exactly we got over here. So that is the hypothesis we can get when you are talking about this particular molecule when you are discussing the delocalization. So just to wrap it up this is actually a trinuclear iron complex bridge between these unique oxo species which is holding 3 irons in the place and every supported by 6 acetate ions which is 2 of them are shared between each of the iron centers and then this particular molecule when we try to take a look into its MOSBA spectra what we are actually seeing is the following in the benzene based system when you actually put this molecule in benzene based solvent we are seeing fully localized system from room temperature to lower temperature whereas in the molecule which is soluble in CH3-CCL3 we are first seeing a delocalized system which is slowly moving to a localized system as we are going through this MOSBA spectra. So why it is happening? This is happening because of the benzene actually induce some stacked interaction with the pyridines which is actually stabilizes it whereas CH3-CCL3 cannot do that and that is at lower temperature only when the molecular motion is slow enough so that I can see the different state we are able to see that localized state otherwise it is mostly delocalized and delocalization is saying that the electron transfer quite faster at this particular temperature compared to the time of the resolution that you can get in the MOSBA spectroscopy. So that is what is happening over here and it is a very nice example how MOSBA spectroscopy can tell us what is happening in the molecular level and what is the effect of a solvent molecule which is coming from the periphery or outside of the actual core of the molecule but how it can still affect and this particular minute effect we can also detect with MOSBA spectroscopy. So with that we would like to conclude for this particular segment over here and we look forward for more examples in the coming segments. Thank you.