 Good morning. So my name is Ashutosh Kumar. And in this section we will be teaching about high resolution NMR spectra of various molecules. So in the previous section we had looked that NMR is all about nuclear phenomena. There has to be a nucleus which has to have nuclear moment and one has to put that nucleus in magnetic field and then there will be energy like states created. You sign and radio frequency which will take the nuclei and nucleons from ground state to excited state and once they come back they give some kind of signal and that is recorded in NMR spectroscopy. Briefly that we had discussed in the previous section. Now we will move how to get the high resolution spectrum from all these nuclei. So as we see that in a magnetic field there is a nucleus which is shown here. Now once we put that nuclear in magnetic field. So there is a ensemble of a spin because NMR is not about a particular nuclear phenomena but however it is and about ensemble of a spin. So if you put it more will be aligned along the field and slightly less will be aligned against the field and this differential alignment of different spins create the energy state and then that is excited goes to excited state comes back and gives signal. So whatever the resonance frequency of this particular nuclei is there that is absorbed in NMR spectrum. Now that is what we are saying. So these energy states are created then you sign and light which is perpendicular direction that is called B1 oscillating field and because of that you get some resonance frequency and that display of this particular frequency of a particular type of nuclei is giving NMR spectrum. If you show different kind of nuclei gives different kind of resonance frequency that means hydrogen will have one frequency, carbon 13 will have another frequency, nitrogen 15 will have another frequency so and so far. Therefore in the NMR spectrum even in a given sample where there are different kind of nuclei like a proton, carbon 13 and 15, phosphorus 31 we get a different resonance frequency for each of these nuclei and that are non-overlapping. Therefore the information that can be extracted is phenomenal because of differential absorption line of different nuclei. Now so what kind of sample we can look at in NMR spectrum. So all sorts of samples ranging from the solid state, liquid state or gaseous state sample. Eventually theoretically we can look at all these samples solid, liquid or gas. So in solid what happens? Solid is a spectrum of powder sample generally gives broad and overlapping right. This is because the spins in solid are essentially arranged very close to each other and that give rise to different kind of spectrum. So suppose here we have a spins those are like this very closely placed and each of these will have a resonance frequency. So resultant resonance frequency will be something like this a very broad line and that gives some limited information. So therefore solid samples have a different way to do NMR spectroscopy and we in this course we are not going to particularly deal too much about solid state. We will give some idea about how to do solid state NMR but for subsequent course we can take up this as exclusively. On the other hand gaseous samples here molecules spin tumble fast but actually in a sample the actual number of spins are very less. So because of sharp fast tumbling they have a sharp lines however the abundance of spins present in the gaseous sample is less and therefore they have a low signal intensity. So this is also not very preferred way of doing NMR spectroscopy. Third case liquid state sample. Now in liquid the spins tumble very fast and because of that we have a sharp line and like a solid there are not too closely spaced. So therefore also the signal overlap is not that much therefore these liquid state samples gives the high resolution spectrum and for biologists and chemistry this is very important because they can get high resolution spectrum whereas they can study property of each of the spins. So in this course we are mostly going to deal with what kind of liquid state spectrum we get and how this is high resolution. Now let us look at what kind of nuclei gives the signal. So let us consider an atom the symbol of this atom is X and we have an atomic number which is Z and atomic mass which is E. So now let us consider few condition. So if A that is atomic mass and Z both are even. What I mean to say that Z is atomic number that means number of protons and number of electron. Atomic mass consists of number of proton and number of neutron. So if number of like if A atomic mass and Z atomic number is R even for an example carbon 612 oxygen 816 calcium these kind of nuclei they have spin quantum number as 0 and therefore these nuclei are NMR inactive nuclei therefore there is no signal from such nuclei. Let us consider condition 2 in the same formula Z is odd and A atomic mass is also odd. So for example H1 which is most abundant nuclei or lithium or boron or nitrogen 15 this is an isotope of naturally occurring N14 nitrogen. So all these nuclei where A and Z both are odd their spin quantum number is half integral N multiplied with half and these nuclei are NMR active so we can do NMR of these nuclei. Let us consider third condition where atomic mass is odd and atomic number is even like for an example carbon 13, 6, oxygen 17 or calcium 41. So these nuclei again are half integral nuclei and they are NMR active therefore these nuclei are also amenable for high resolution NMR spectrum. Consider condition 4 where A atomic mass is even atomic number is odd for an example deuterium this is an isotope of proton naturally occurring. So deuterium or N14 this is a natural isotope of nitrogen all these in all these case the spin quantum number is integral and they are also NMR active but their NMR requires little bit of tweaking and we will discuss that how to do NMR of these so called chloropron nuclei. If you look at all these nuclei essentially in the periodic table there are so many nuclei which are amenable for NMR spectroscopy therefore that is what we mean that NMR is amenable for all sorts of sample biological or material science or physics related sample. Now NMR gives signal for different nuclei different spins gives different signal and their signal depends upon something called gyromagnetic ratio so that means proton resonance frequency will depends upon the proton gyromagnetic ratio at the same magnetic field however carbon 13 resonance frequency will depends upon carbon 13 gyromagnetic ratio and if you have seen in the previous class there is a relation between gamma H divided by gamma 13c and this is 4. So on the same magnetic field proton resonance frequency will be roughly 4 times more than the carbon 13 resonance frequency. Similarly the omega for N15 resonance frequency for N15 will depend upon the gyromagnetic ratio of N15 and this is actually 10 times less than the proton resonance frequency. So on the same magnetic field the resonance frequency for N15 will be 10 times less. So for an example we are recording an spectrum on 800 megahertz which corresponds to say 20 Tesla proton will resonate or at 800 megahertz whereas nitrogen will resonate at 80 megahertz. So that is what we mean at the same magnetic field the different type of nuclei will have different resonance frequency and those are non-overlaping resonance frequency because they are too far. So now what happens for the same kind of nuclei like for an example proton. Now for proton so do all proton in a molecule absorb same resonance frequency? No they will not. In the last last slide we saw that for a resonance frequency of proton at say 20 Tesla magnet is roughly equivalent to 800 megahertz but do all proton resonate at 800 megahertz no. The reason behind this because these proton nuclei are not bare nuclei there is something around it and what is that it is an electronic cloud. Now this electronic cloud is also have a spin if you put that in magnetic field this electronic spin will interact with the magnetic field and they can generate a local field and that local field will be different for different kind of proton nuclei and that gives rise to a phenomena which is called chemical shift. So difference in the resonance absorption frequency for particular kind of nuclei depends upon their local surrounding. So if I take a molecule say let us take a methanol here we have one proton here and one proton here but local environment for this proton and this proton is different. Therefore the resonance frequency should be something different. Now so if this is the different what kind of information that we can deduce because of different resonance frequencies for a particular type of atom and that gives various interesting phenomena one is called chemical shift. This chemical shift arise because of the different chemical environment for a particular kind of nuclei at the same magnetic field like as for an example we give CH3 resonance frequency is going to be different than OH resonance frequency in methanol. The another that we are going to discuss in this chapter is spin-spin coupling. So these two protons are also somehow coupled through bond and they will affect their resonance frequency this is called spin-spin coupling that will come later. The next thing if you see here we have a three proton and here we have one proton. Now this here three protons are contributing to signal and which is different than one proton contributing to signal. So intensity of CH3 protons should be three times larger than the proton than the OH proton and the intensity of resonance line gives enormous information about the number of proton that contribute to particular kind of signal. The fourth one that we looked in the previous chapter is relaxation time and line width. So line width for some proton can be very sharp for some proton can be broad and this gives information about the dynamics at a local site and these are all these four important information can be deduced to understand the structure the chemical environment their neighboring effect also the relaxation property of a particular kind of nuclei. Now so the chemical shift which is the first one the difference in the resonance frequency because of the differential chemical environment was first discovered in 1951. Actually the first time what they did they took a ethyl alcohol, ethyl alcohol if you remember your chemistry it is a CH3 CH2 OH they took this ethyl alcohol put in a 40 megahertz NMR spectrum and then they recorded this spectrum of that and that time recording was done using oscilloscope. So what they found that instead of one line they are getting three lines and these three lines triggered that there is a chemical environment different in protons of ethanol ethyl alcohol. So here look at three lines one corresponds to methyl, methylene and OH proton. So this is for methyl one here then this is for methylene and this is for OH and if you look at carefully this intensity ratio of this looks three times more than this and methylene is two times more than OH. So clearly using this spectrum we can essentially quickly we can recognize that here the contribution is coming from three protons, here contribution is coming from two protons and here it is one proton. Now looking at their resonance frequency and looking at the intensity we can identify that this corresponds to CH3, this corresponds to CH2 and this corresponds to OH, this corresponds to OH. So that was first discovered by Arnold Dharmati and Packhardt, Professor S.S. Dharmati who has a postdoc when he discovered this. This discovery opened its application of NMR spectroscopy in chemistry. Now each and every molecule that chemistry people synthesize they record an NMR spectroscopy to find the number of proton contributing to signal and also what is the chemical environment and what is the coupling pattern and that gives high resolution spectrum for a chemical moiety. Now I was mentioning about Professor Dharmati. So he was a postdoc in 1951 when he discovered it he came back to India and then early like early 60s in 61 basically actually with a generous support from government of India they established the NMR facility in TIFR Mumbai. So if you look at Indian NMR has a long history it started right after like discovery of important phenomena like chemical shift and nuclear moment and since then NMR legacy is carried over in India and many people are doing high quality research in this field. So you can see Professor Dharmati and Javarlal Nehru inaugurating the NMR facility in TIFR. Now more about chemical shift so what is the origin of chemical shift? As I said electronic cloud around the nucleus causes the shift in the resonance frequency for a particular nuclei. So this electronic cloud as we see that as we see that there are different kind of electronic cloud around a particular nuclei. If you look at the 12th class chemistry we know that there are various kind of nuclei like if you know electronic configuration for say proton it is 1S1 so S kind of orbital is there. If you look at carbon we have 1S2, 2S2 and 2P2 right. So that is the electronic configuration around carbon 12. So here contribution is coming from S orbital as well as pre orbital and P orbital has a different shape. Now electronic cloud around S orbital created by S orbital and pre orbital can be very different. Similarly if you look at the D orbital it is a dumbbell shape and here is electronic cloud is very different than the S and P and similarly F is more complicated. So different kind of orbitals create a different kind of shielding effect and extent of screening will be different and that will change the resonance frequency and that will be obviously different for different nuclei. So this effective field around that is different. So local site like if you have a magnetic field and here is my nuclei and here is electronic cloud. So electronic cloud can be dense or sparse and that is going to affect what is the local magnetic field absorbed by a particular nuclei. So that will be different from H0. H0 is our main magnetic field. However experienced magnetic field is different than the main magnetic field because of something called local effect and this is called actually screening constant or shielding constant because this is creating kind of a screen that main magnetic field can absorb. Now this sigma local can either be positive or negative depending upon how they are arranged. So positive value implies that this is creating a shielding and negative value implies this is creating a D shielding. So that means the main magnetic field in either case shielding or D shielding it is going to be different than the main magnetic field which is H0 and the experienced will be H local. So this shielding constant which is essentially local gives the contribution from the diamagnetic as well as paramagnetic. What are diamagnetic? So diamagnetic contribution is sigma D and paramagnetic contribution is sigma P. So if you look at heavier nuclei, heavier nuclei has a complex orbital right. If you look at any of the heavy metal nuclei or F orbital nuclei they have SP, DF and they are mixing of orbital because of this mixing orbital the electronic cloud are in particular direction and directionality of the electronic cloud. So that creates basically different kind of contribution of the shielding constant for heavier nuclei. For a smaller nuclei if you look at like proton there is like diamagnetic effect and for a smaller nuclei like carbon 13 or proton this is diamagnetic contribution which contributes mostly to the local magnetic field. Now diamagnetic contribution is generally positive and paramagnetic contribution is negative. So if you look at if you go back and try to analyze this here we are saying this is positive. So if this is positive for diamagnetic nuclei that means the local magnetic field absorbed by the proton is going to be less than the main magnetic field. However if this is negative then local magnetic field absorbed by the proton is going to be more than the main magnetic field. So if you look at the paramagnetic or diamagnetic contribution that is what we are saying. Here the diamagnetic field is spinning along the magnetic field and therefore it is adding up and here paramagnetic is against it. Because of that now their resonance frequency is going to be very different than the local magnetic field. So H local for proton will be H0 1 minus and that if this is positive so H local will be less than H0. However for paramagnetic thing H local will be more than H0. So far so good and now we know that what kind of shielding and screening is created by different kind of nuclei. Again go back to example of CH3, CH2 and OH mostly here we are dealing with a proton chemical shift. So all three are proton. Now we are saying all of them have a diamagnetic contribution to the chemical shift. However the contribution because of CH3 is different than the CH2 and different than the OH. Therefore their resonance frequency whatever we saw and that was seen by professor Dharmati is different. So here it is more shielded therefore the resonance frequency is here OH. OH is connected to an electronegative group it is desilded and therefore you have resonance frequency which is coming here. So till now we looked at different kind of nuclei we will have a different resonance frequency what I mean that proton will have a one kind of resonance frequency, carbon 13 will have another kind of resonance frequency and 15 will have another kind of resonance frequency. Therefore they give non-overlapping spectra. Next we look at even in the proton depending upon their chemical environment they can have a different resonance frequency. So and we also looked at their resonance frequency which is omega H depends upon the gamma H and here if you look at here it depends upon the magnetic field H0 or whatever B0 we write gamma H. So that means at one field we are going to have one resonance frequency for proton if we change the field there will be one resonance frequency for the proton. So like on 600 megahertz or that corresponds to 14 tesla we have a resonance frequency for proton which corresponds to 600 if we change the field like we make it 20 tesla we have a resonance frequency which is 800 megahertz. Now this is a problematic why because all the time we have to write on what magnetic field one recorded the spectrum and that is going to be very problematic. So what people thought why do not we devise and a strategy where we get rid of this field dependent in representing the chemical shift and that give rise to a concept which is called the PPM value. So PPM value actually get rid of the chemical shift on what field it was recorded whether it is 20 tesla or 14 tesla. So essentially what it says that it is a difference from the main magnetic field and then you divide by the main magnetic field value. So the value so resonance frequency that we get in megahertz but difference in the CH3 and CH2 and OH resonance frequency that is in hertz. So if you divide hertz by megahertz that comes to be it comes to be part per million PPM and now chemical shift is explained or is denoted in part per million PPM that gives get rid of the field dependence representation of chemical shift. So I will take the next class in explaining in detail the how PPM value was derived and how the contribution is coming the contribution of the field is getting neglected when we define in PPM value. So I will stop it here and I will look forward for your question in the next class and in looking to have a interactive session where you asked lots of questions and we try to answer all of those. Thank you very much.