 So, welcome to today's class. Today again we will be discussing some aspects of high resolution NMR spectra of molecules and here after today's lecture we will end this chapter and we will move to next chapter. So, in the last class we discussed about energy levels and transitions that happens in various spin systems. So, we derived using quantum mechanical formalism the different energy levels and even we looked at what could be the transition probability. So, we derived for two spin system, strongly coupled system and weakly coupled system, strongly coupled system such as AB and weakly coupled system such as AX. So, in detail we looked at what could be the energy level for that and how the intensity will vary for these if they are weakly coupled or strongly coupled. Then we moved ahead and looked at the three spin system again for like a AMX system which will be all three spins are weakly coupled or AB system or ABX system. So, here AB will be strongly coupled and they are weakly coupled with X. So, looking at that we went ahead and looked at the patterns of lines. So, as we are saying that for a strongly coupled system the line depends upon that a factor sin 2 theta and that changes the patterns of the line and the intensity of the line. So, that we looked at how it will be for three spin system ABX system and then we also looked at the splitting pattern and complication that could arise because of a strongly coupled spin system. So, there we ended it and today we look at some of the interesting phenomena just to give you some glimpse like three spin coupled system that we are talking about energy states will be something like that. So, energy states here in three spin coupled system like all three will be here alpha alpha alpha state or two will be an alpha state and here in these two cases and here two will be in beta state. So, there will be three states like this and the final one will be beta beta beta states. So, we looked at the transitions in such cases will be single quantum. So, that means here the delta m will be 1 plus minus 1. So, these kind of transitions that we have looked 1 to 3 or 2 to 4 or 5 to 2 here 6 or 6 to 8 all will be single quantum transitions and here we are looking the first spin is flipping to beta. So, because of that if they are weakly coupled system will get like all equal intensity peaks and depending upon what is the coupling constant they will be splitted and they will be distributed in equal intensity ratio like this. So, this will be J, but for strongly coupled system as we discussed the line will be of unequal intensity and the central lines will be having more intensity than the outer lines. So, here if you look at the intensity here is less compared to the central line that is here and that as we looked at this is coming because of the strong coupling system. So, but in all these cases one thing will be true that delta m will change by 1. So, single quantum transition will happen. As we looked at there is a possibility, but there are forbidden in NMR that this is this kind of transition is called double quantum transition this can be achieved, but at the moment will stick to mostly single quantum transition. So, in these case either AB system or AX system or ABX system mostly we are looking at the single quantum transitions and the lines that arises because of these transitions. So, we will move ahead and today we will discuss some interesting aspects of the NMR that can arise because of the dynamic factor. So, as we know that molecules in solution are not static they have various kind of dynamics and this dynamism in system can influence their line shape and the position. So, what kind of dynamics have one of those that we are going to today look at is something called chemical exchange. So, chemical exchange of a nuclear species between two and more sites in a molecule that can give rise to dynamic effect in the NMR spectrum. So, what we mean by chemical exchange of nuclear spin? So, say spin is hopping two states. So, if it is hopping two states, so then there that that is called exchange between the two spins. So, if you spin is here or it is here. So, something if it is hopping between them or for an example here I show here amide bond NH is exchanging with water. So, this spin is hopping between the bulk water and the bound NH. So, such kind of phenomena is called chemical exchange or for an example here if the bond rotates C-R-C and bond rotates this C-H3 and this C-H3 their position can change and these effects in NMR will give various dynamic effects and their spectrum can change. So, that is what today we are going to mostly focus on that we call it dynamic NMR spectroscopy it is not for a molecule which shows only one kind of transition, but there is a change there is a hopping of the spins and that gives rise to various line shape or the resonance even resonance frequency. So, suppose a site having having different chemical environment and or the exchange of nuclei between two molecules is happening like here suppose this H is exchanging between two conformation one say is bound to this conformation another bound to this conformation. So, if the proton is exchanging between these two environment. So, what is happening here that chemical environment is changing and that is what we are saying chemical environment change of the nucleus between two molecules or here we can say two site exchange like the previous example here if you look at this C-H3 because of rotation of this bond can be rotating in two states. So, now because of these rotations the spin will be exchanging between two chemically different site. So, one maybe here the chemical environment is different than here. So, if this proton is exchanging that means they are exchanging at two different site and therefore as we know that the their resonance frequency for any proton depends upon the chemical environment. Now this spin is swiping between two states therefore their resonance frequency can also be different and now this proton is swiping between two different frequency or for an example let us take hydroxyl ion in alcohol. So, OH can exchange between bulk water. So, now this OH being levied can go to the bulk water and again it come back. So, something like here is H2O molecule and suppose in some of those case like here OH can exchange with the bulk water. So, hydroxyl proton in alcohol exchanging with bulk water such kind of phenomena also comes under purview of chemical exchange. Now, if the exchange is happening slow that means this spin is able to show C2 different frequency. So, because one chemical environment like we had seen in the previous slide. So, one chemical environment that also spins is here and then another chemical environment is seeing from here. So, if the exchange is slow the here for case of ethyl alcohol hydroxyl proton has enough time at each position. So, there will be two peaks one at the position number one another at the position numbers two. So, because now two chemical environment are distinctly absorbed by this hydroxyl proton in slow exchange there will therefore there will be two separate lines for the two chemical environment. So, say chemical environment number one chemical environment number one A and chemical environment number B and now if they are slow exchange. So, now this proton has enough time to see both of these and therefore, there will be two separate peak, but if they are swiping fast. So, if exchange is fast now this proton does not have enough time to see the both the chemical shift distinctly therefore, there will be a resultant chemical shift which will be average of these two chemical shift. So, rather than AB to distinct we have an average which will be average of A state as well as B states. So, this dynamic phenomena also depends upon how the exchange is happening whether it is fast or slow or intermediate time scale. So, those can be classified at various NMR time scale. So, something fast exchange we call it if it is happening the exchange between those two proton happening fast if the time scale is somewhere in this domain like nanosecond to picosecond time scale and slow will be somewhere here millisecond to second time scale. So, we can call this as a slow exchange and this will be fast exchange. So, in fast exchange as we saw that there will be average peak here and in slow exchange because now proton can see or the spin can see two distinct chemical in moment A and B there will be two peaks here. Now, in this case this is a average peak what will happen to intermediate time scale that is the interesting phenomena to see. So, intermediate time scale what is happening now it is not fast enough to have a unique sharp peak at intermediate state it is not slow enough to see two distinct. So, therefore it will be at average position but lines will generally be broad and that that is actually comes because of this intermediate exchange. Those intermediate time scale comes in the regime of microsecond at the NMR time scale. So, microsecond time scale at compared like compared to NMR phenomena this is called intermediate time regime domain. Intermediate time regime. So, now so as we say if you if the exchange is fast we are seeing here the one average peak. So, exchange rate say K is exchange rate is 10000 hertz means like if you see 10000 per second hertz is per second. So, if something exchanging very very rapidly with speed of 10000 per second that will be fast exchange and if it is 10 hertz. So, it can see both of these states and that is slow exchange. So, as we see here in slow exchange regime we have a two distinct peak and as we move to fast exchange regime we have only one peak. But interesting phenomena is happening here. So, as the motion in the exchange increases between these two states we see lines in line intensity gets decreased and at some position like here 10000 hertz gives a broad line and then it becomes sharp and the intensity increases. So, for all intermediate exchange rates the line will look fairly complex and that is what here we see. So, this is kind of a fairly complex line because of intermediate exchange regime. Now, suppose if you take this molecule 3 methyl amino 7 methyl 1 2 4 benzo triazine this molecule like this here and here the NMR spectrum were recorded for at different time scale motion. So, this what happens that this bond as we saw this can rotate CN bond and because of this rotation the peaks for CH3 can come differently. So, what happens now if it is slowly very slowly rotating and that we can achieve by lowering the temperature. So, at the lower temperature actually at 223 Kelvin we see distinct two peaks. But as we go increasing like here temperature is getting increased and therefore rate is also changing. So, at some temperature like minus 10 degree 263 Kelvin the rate is 225 per second and now one can see a very broad line at the central position of these two and as we increase come at the 0 degree where rates of the exchange is 500 per second and we have an average peak like this. So, two site chemical exchange as we saw it depends upon the rates of exchange and at the slow exchange rate we have a two distinct line at intermediate we have a complex line and at the fast exchange we have a sharp relatively sharp line but at average chemical shift. Now, let us take some another example of like absorption of the bridge proton that also depends upon the temperature. So, if you like start here, so if you see here k infinity that rate is infinity one can see the spectrum for protons are extremely sharp. So, here you get doublet for each of these two protons. So, four peaks doublet because of the coupling but as the speed like as the exchange rate increases at different temperature. So, here we are reducing temperature the exchange is getting slower down and at the same like at some of the rates like 106 per second at minus 119 degree centigrade one can see a broad line. So, this is intermediate exchange regime as we go down and if the rate is like exchange rate is very very slow at very low temperature like minus 180 38 degree centigrade again you see the lines are like where lines are broad but now it has become more complicated because now motion is restricted and you see more complicated line like some of those here looks doublet of doublet because now proton can see other proton for very long time and that is why some couplings are active. So, temperature plays a role in case of exchange. So, at the lower temperature exchange can be reduced. So, this kind of phenomena is used for many of the experiment where the proton is exchanging between two sites to record a sharp spectrum and many of the organic molecules where one see that exchange chemical exchange is happening very fast experiments are typically record at lower temperature to see if this proton is really exchanging between two sites or so. So, by lowering temperature where scientists can identify the protons that are in exchange. So, temperature dependence experiment in such case can be very useful. Now, same thing can be also seen if we go for slightly bigger molecule say if you are talking about proteins. So, proteins are also very very dynamic molecule and proteins are also very dynamic molecule and say if we want to study some phenomena where proteins is exchanging between two conformation. So, protein conformation say if you like landscape of a protein here there is one conformation of protein here there is another conformation of protein. If protein is exchanging between these two states exchanging means slightly conformation is changing or maybe a loop is opening closing something like happening, but it is not enough to see that lowly populated states actually by changing the landscape by lowering the temperature one can capture such states. So, that is what people do they either populate this state or lower the temperature to find it out the alternate conformation that protein can exchange. So, this kind of phenomena is very helpful in understanding the exchange between two sites or protein conformational exchange and so and so forth. Now, coming to the next chapter if exchange is happening what happens to multiply pattern as we see that because of the J coupling we see a spectra are getting splitted and that we looked at in details how they split. So, what happens if the those protons that causes the splitting are exchanging. So, say here same example we take here the proton is in one conformation and here protons are in another conformation these two have a two distinct chemical side. So, that is what we looked at because of these two chemical sides if they are exchanging exchange is slow we see two lines, but if exchange is fast enough we see an average line and if it is intermediate we see a broad line for two different chemical side. But for a spin the resonance frequency changes from one value to another value in its multiplied state structure and the lines due to spin flip of the other spin which is J couple what happens to the splitting pattern that is what question we want to ask. Let me refresh this. So, say this spin is coupled with this spin both have some coupling. Now what happens if this spin is flipping what happens coupling with this. So, actually the same argument can hold for whatever we just looked at the flipping rate or the exchanging rate of this spin is how much is it fast slow or intermediate. Like for an example we can look at again the same molecule if OH proton from the alcohol is exchanging with water what happens if it is fast exchanging what happens if slow exchanging or what happens intermediate exchanging. So, same thing one can hold if it is fast exchanging you see a generally sharp peak but a unsplitted peak when it is slow exchanging you will see the two peaks just for like here an example I will give it collapse of multiple spectrum. So, what is happening here say this OH group is exchanging with the water group. So, now for this there is a alpha state and here is a beta state for this. Now what happens of this water proton how does it affects of the methylene proton in alcohol. So, as we are seeing here it is alpha state and in water it is beta state. So, now the chemical shift if it is alpha state is like this and if it is in beta state is at other chemical shift. Now if the exchange is happening very fast then one can see an average chemical shift at this state. So, that is what we are saying in ethyl alcohol if there is a little bit of acid. So, because of this acid the proton OH proton can exchange and then that because this acid can catalyze the exchange process. So, results in equilibrium between alpha state and beta state for OH proton. Now this OH proton is connected to the alkyl R proton and water proton. So, this leads to the collapse of the methylene scalar coupling with OH and that we will see in the next slide. So, this was we are talking about. So, now here is our OH proton which is exchanging this OH proton is exchanging with water proton. Now, so because of exchanging it will have a singlet peak otherwise it could have also had a triplet peak because of the splitting of this CH2 proton. Now this exchange also affects this splitting of CH2 because now OH proton is not causing any splitting to this only splitting will come from CH3 which will be triplet. So, this methylene proton will have triplet splitting because of this CH3 not because of OH because now OH is exchanging. So, that is what we say collapse of multiplate if it is in exchange. The another phenomena that can cause the collapse of multiplicity is if there is a electron, unpaired electron present with the nuclear spin. So, in this case suppose this is a radical which is called tempo radical if there is a unpaired electron. So, what will happen to these protons? So, now these protons has actually OH this proton has a electron free electron. Now, this free electron causes fast relaxation of these protons here these protons and because of that they will not see the J coupling and they will result into collapse of multiplate structure because of the fast dynamics that is happening of this electron proton coupling. So, electronic spin also can cause the collapse of multiplate structure. So, if that is the case then what happens to the J value of such exchanging protons? So, as we saw that in case of multiplate collapse the rapid exchange is happening. So, if protons are exchanging between two states this rapid exchange can cause the collapse of multiplate structure. So, that can also cause the averaging of the J value. So, like for an example I will try to explain it here. So, suppose this A molecule because of the coupling there are like two resonances for this A1 and A2 and they are coupled by J state. So, this is suppose this is a proton here whatever attached wedge this has a coupling because of something is split into two. Now, the same proton goes to B state and here the chemical shift is different and there will be two transitions B1 and B2 because of the different chemical in moment the JAB is there. So, now what happens if the A state and B state exchanges with each other at some time scale. So, what then if you look at now A. So, here A and B are coupled and that is how we are seeing two splitting here and here A2 and B2 are coupled that is why we see splitting here. So, because of coupling we see splitting and because of exchange what will happen that they will give a J average here. So, this will be J average. So, that will be they will called an averaging of the chemical shift average J value because A site and B site is exchanging with each other. So, that one can give it the weighted average of A site and B site depending upon how much population it spends in A state how much population it spends in B state. So, and that will be contribution coming from the both states. So, this will be J average PA of say JA plus PB of JB. So, that will be average J of these two sites that one can see for the average chemical shift. So, in this chapter we have started from the chemical shift. We looked at the chemical shift depends upon the environment of molecule then we will went ahead and explained the another important parameter which is called coupling constant. Coupling constant and chemical shift together is very helpful in determining the chemical moiety structures and the small molecules can be identified using these two parameters. Then we went ahead looked at little more complication of the spin-spin coupling. If they are weakly coupled or a strongly coupled how the line shape patterns sorry how the intensity of lines change or how where the transitions how they are moving there and here. Then we looked at the dynamic phenomena of the NMR because of chemical exchange if the protons are exchanging between those two sites. And then we looked at what could be the parameter that will be affecting the exchange like the temperature is 1 and if it is exchanging between two sites the electron presence of electronic spin can influence the exchange process. Then we looked at the averaging of the J value. So, we will end up here for this chapter and then we continue with the next chapter next week. Thank you very much.