 Hello and welcome to this next segment of CD spectroscopy and MOSBA spectroscopy for chemist. My name is Arnab Dutta and I am an associate professor in the department of chemistry IIT Bombay. So, in the previous segment we are discussing about MOSBA spectroscopy. So, let us just recapitulate what we are actually discussing on that particular aspect. So, we have found MOSBA spectroscopy to be an interesting tool which can give us interesting information regarding the oxidation state, spin state and the geometry of the molecule. So, how is it happening? So, like any other spectroscopy over here we are talking about a change from ground state to an excited state. Now, which particular system we are changing from ground state to excited state? Over here we are changing the nuclear state of a system and when you talk about nuclear state to have an example we are taking 57 iron isotope which actually go from ground state of half to excited state of 3 half. And this particular energy is quite huge because we are changing the nuclear state and that required energy in the gamma ray region for electromagnetic radiation and for this particular change we require an energy in the region of 14.4 kilo electron volt. And how we can generate that much of energy of gamma ray and that we actually generate from a metastable state of iron 57 material which actually was staying at I 3 half and coming down to the ground state of I half and in reality it is actually developed from a radioisotope of cobalt which at the end producing the I equal to 3 half metastable state and coming down to this ground state of I half for iron 57. So, this is also 57 iron we call them the source and this is the sample and over here this particular states the nuclear states are just a little bit different because of the environment it is in what is the oxygen state, what is the ligands it is coordinating to what is the spin state of the iron all those things change the energy of the states minutely and that minute change we are going to capture where we actually put our source and sample separated and both of them we actually put them in a solid matrix or lattice why we are doing that because when this energy is actually coming out from the source to the sample because it is a huge amount of energy we expect some recoil energy on either side both on the source and sample the source is going to fill a recoil back because it is leaving that energy and the sample is also going to move to that side after it get heated by this huge amount of energy. So, we put them in the matrix so that we can make them almost close to 0. So, you can have a recoil less condition and I have discussed earlier that we can control the translation motion, but the phonon motion the vibration motion will be still active and that is why by temperature we can control that if we go to lower temperature we can actually have a better resolution of the MOSFET and along with that we actually put this source on a dynamic system and the sample on a static system. So, sample does not move source we can move it either forward or backward and over here we are using the Doppler effect by which this is going to move either towards the sample or from the sample out and by that our goal is to match the energy and match the perfect resonating condition. So, this is going to come as a curve like this where the unit of energy is given by this velocity unit because that is the Doppler effect that we are using and that is replicating the energy scale over here or I should say replacing the energy scale over here and this is the percentage of transmittance it start from 100 percent transmitter that means no transition at all and over there it began and here you have the maximum so that comes to the close to 0 and that is the signal we get and wherever we get on the scale of this v we call them the isomer shift not delta value. Now, along with that this excited state of 3 half it can be present in two different condition. So, what can happen that excited state of 3 half and also have the ground state of half the excited state can split up in two doublets one of them is the plus minus half one of them is the plus minus 3 by 2 and this remain in the plus minus half because that is the only state possible for the ground state and why it is happening that is because if my system has a quadrupolar moment and if this is actually present this excited state is going to split up and I can have two different transition this is how it is going to look like in the real life. So, say this is that system where I am not showing any quadrupolar moment at this moment but let us say I actually have a quadrupolar moment then this line is going to split up into like this and if I take the average of these two I will get the over here. So, the delta value is with average and the splitting between these two line is known as the quadrupolar splitting and this is actually happening if we have an electric field gradient present over here and that has two different components one is the lattice and one is the valence lattice is when it is coming from its coordination geometry that it is actually asymmetric or the valence is how the electron is actually oriented in that particular molecule if it is asymmetric or not if it is asymmetric then you will get the valence contribution if it is asymmetric around the coordination it is the lattice contribution. So, this is the basic of the MOSBUS spectroscopy we are following so far and we have taking a look into different applications or different examples of MOSBUS spectroscopy that is giving us an idea what is the oxidation state, spin state and the coordination geometry of iron. So, we will continue our journey on that and over here we are going to take other examples of that. So, the example number 7 we have covered 6 examples so far. So, seventh one is on ferrocene so a little bit on ferrocene so far. So, ferrocene is the following molecule where we have this anion cyclopentadienyl anion which is coordinated into iron and we have two sets of cyclopentadienyl anion. So, what is ferrocene? Two cyclopentadienyl anions coordinated iron center and iron is specifically in the plus 2 oxidation state when you developing this ferrocene. So, how it is done? So, the reaction actually start from this system cyclopentadienyl you can say two alkene groups so that is the diene and you can say it is a 5 member ring so that is why cyclopentadienyl so that is why cyclopentadienyl is over there and over here one of the unique property of this cyclopentadienyl is over there you have two protons which is actually you can say the sp3 hybridized carbon or the saturated one these are actually the unsaturated ones and over here if I take one of the protons out what will happen? See if I take one of the protons out over here so it will create a carbon ion and this will create different motion over here so you can have this this can come over there this can come over here so let me just move this part so you can see it is moving around and getting resonating and not only that it is having now 6 pi electrons so all together I am creating a aromatic system which we can write like this the anion charge is actually moving around all the 5 carbons so it is a aromatic system this cyclopentadienyl we start with and now in becoming an ion over there this particular aromaticity is actually going to favor this particular deprotonation step over here and that is why the pKa value of this carbon hydrogen or this CH2 group will be lower compared to what we expect for a other alkene because it wants to deprotonate so that it can achieve the stability through this aromaticity and that is why it has a much more lower pKa value and this particular system the ferrocine is actually developed from this cyclopentadiene we deprotonate that create this cyclopentadienyl anion and once we create that so let us say I am doing this reaction and typically we can do that in organic solvent even by adding koH because it has a very low pKa value so I create this cyclopentadienyl ion and then if we add half equivalent of iron plus 2 salt it actually binds with 2 of this such anion and creates ferrocine how it actually looks like so over here what actually happens this system binds with the iron in this particular geometry where it is actually interacting with all the carbons present over here and the carbon plane is actually remaining horizontal such a way that all the p orbitals that is participating in the aromatic formation of the cyclopentadiene and react with the d orbitals of the iron same thing happens for the bottom one and this particular interaction we call them as hapticity and over here all the 5 carbons simultaneously interacting with the iron so we call them the hapticity is 5 which is written as eta 5 eta is the Greek term and we use it for showing this kind of interaction. So, both of them are interacting in this particular way and which is bringing the stability to the complex over here and over here we actually write this cyclopentadiene anion as short from a Cp so we write it is iron Cp2 this particular molecule over here and this particular molecule is actually very stable and this is bright orange in color and when we do this particular synthesis cyclopentadiene anion we first prepare react that with iron and then we get this ferrocene this is the name of the system which is nothing but iron connected to 2 cyclopentadiene anions and this is actually as we said sandwiched between 2 cyclopentadiene anion which we write as Cp between 2 Cp rings we call the term rings because it is actually a aromatic ring and that is how the structure actually looks like and this molecule actually has a very unique property that you can sublimate it and through the sublimation we actually purify this molecule from the reaction mixture and we get a pure form of ferrocene and this ferrocene has very unique properties this is actually very nice electron transfer agent so we can use it as a mediator for electron transfer and secondly it is very well soluble in organic solvents and typically it does not react with any of the other molecules so it remains as a molecule which can be present in the system without affecting the reactivity of the others so that is why this particular ferrocene molecule is used as a control as an internal control for monitoring the potentials applied potential that we are actually putting and most of the time when you are talking about a organic medium reaction we typically show them that this reaction is happening at this particular potential versus ferrocene because what we do when you are looking into electrochemistry system and we are say putting potential this is the positive direction means oxidation this is the negative direction means reduction and over here when we run ferrocene it typically show a very nice reversible graph where the iron plus 2 goes to iron plus 3 one side and this is the iron plus 3 is coming down to iron plus 2 and this particular potential that you are seeing it is actually put it such that the potential of iron cp2 going plus 2 and 0 it is written as 0 volt so why we are writing iron cp2 plus and 0 because over here iron charge can be plus 3 or plus 2 but it is coordinated with 2 cyclopental dinel and ion where each of them has a charge of minus 1 so that is why the overall charge will be plus 1 when it is plus 3 state or 0 when it is plus 2 state so that is what we are getting from the electrochemistry and as we said that in any other organic solvent it kind of remain constant so we can use it as an internal standard so that we can screen a lot of molecules and normalize them with respect to this particular potential of ferrocene and that is one of the important usage of ferrocene so now we know ferrocene is important and once the ferrocene actually goes to iron plus 3 condition we call them the ferrocenium ion so ferrocenium and ferrocene where the iron is in plus 3 or iron is in plus 2 state and ferrocenium is an ion we are writing because it has a charge of plus 1 ferrocene is a neutral molecule so that is the background on ferrocene now let us take a look how we can learn a little bit more of that from the mosbar spectroscopy so before going to mosbar spectroscopy one of the thing that we want to learn is about its symmetry so now when you are drawing ferrocene we are drawing in this particular way where you can see the rings we can draw in two different ways one such a way that the rings are actually eclipsing each other so what do we mean by eclipsing each other it is a 5 member ring and now if I take a look from the top of this molecule from here we try to see how it looks like from here and how it will be looking like like this and then the other molecule it will be on the backside like this almost similar position eclipsing each other so we call them the eclipsed ferrocene the other possibility is let me draw it over here is this where the other ring is oriented in the opposite direction so that if I again look from the top how it will look like like this for the top one this is how it will look like on the bottom one so now the question is what is the symmetry of either of this molecule and this particular molecule in this condition it is known as the staggered ferrocene so let us find out what is the point group so I hope you remember the drill what we perform to understand what is the point group of a molecule so we just ask them a question so the question asked to ferrocene are you linear answer is no are you belong to any cubic groups that means tetrahedral or octahedral answer is no do you have a cn and answer is yes cn is there over here you can see it is a c5 rotation so it actually goes through here center of the cyclopentadienyl anion through the iron and here is a c5 present over there so next question is are there 5c2 perpendicular to your c5 and the answer is again yes so it goes through over here through the vertices of each of the carbon over here so yes they are present so it belongs to dihedral group now the next question is do you have a sigma h and answer is yes sigma h is going through here which is the perpendicular plane in this particular drawing which is going through the iron so this two cyclopentadienyl anion is actually reflecting each other whereas iron is sitting on the plane which is learned better on this top view you can see the two cyclopentadienyl anions are actually reflecting on top of each other so this molecule belongs to d5h point group now what is the point group of this molecule staggered conformation so let me draw it a little bit larger so that we can follow it properly and there is the back ring so that is also present now the question is where the other symmetry remains present on this molecule so let us go ahead with the same questionnaire is this molecule linear obviously no does it belong to any qb groups tetrahedral or octahedral that is also no does it have any cn and answer is still yes is at the same position you still have the c5 through here so although the front and background cp rings cyclopentadienyl rings are oppositely oriented but the c5 is still there if you rotate over here 72 degrees each of the vertices on the top changes position and similarly these ones are also doing the same on the background so it retains its c5 next question is does it have 5c2 perpendicular to c5 and then we found yes the c2 is still there but compared to the previous one now it is actually a different position it is now going through this particular cross points between the two cp rings plane and you have five of them where it is actually crossing through and you will see there are five of them so it is still present which is actually crossing between so I am just drawing the rough part of that so it is actually going through this particular portions so these are the portions where it is actually having that c2 crossing over so when you rotate it and this c2 is actually going through the center part on the iron so when you rotate the c2 over here 180 degree because you are actually rotating through the cross points this will rotate 180 degree and come to present in the black the top position and the black one is going to the bottom to this dotted rate position so they are going to to a indistinguishable and superimposable structure so that is very important try to envision that and try to practice that a little bit to find out where the c2's are so it is going through there now the question is do I have a sigma h and now the answer is no previously we have that sigma h through there over here because the two cp rings actually reflect on top of each other now we do not have it because now they rotated a bit so if I ever reflect it that is going to go backwards and there is nothing there at a similar position and this will come on the top and there is nothing there on the top position so that is what is actually missing sigma h is absent there is a big difference between the eclipsed and the staggered ferrocene so this is the eclipsed one okay sigma h is not there the next question and you ask do you have 5 sigma d's because you are already in the d point group because it has a cn and n number of c2's perpendicular to cn if there is a sigma v it will be sigma d so if there is a sigma v over here and if it is there where is it and the answer is yes there is sigma d and that is actually present through here if you look through there so let me draw that one more time for the sigma d's and that is actually present going through each of the vertices so if you look into each of the vertices when you are going through these are actually crossing the position of the two c2's so that is why we said there are 5 sigma v's answer is yes so this staggered ferrocene the spelling is wrong over here ferrocene has the point group of d5d it was d5h in the eclipsed d5d in the staggered so that is what we got so far and now we will conclude this segment over here and in the next segment we will start for the MOSBA spectroscopy of ferrocene or ferrocene derivative molecules and try to understand how we can use MOSBA spectroscopy to understand its oxidation state, spin state and the symmetry around the iron centers thank you thank you very much