 Hello everyone, once again I welcome you all to MSB lecture series on Transparency Chemistry. This lecture is 57th in the series, of this one another 3 are left. So in this one let me begin discussion on NMR spectroscopy as I had mentioned. I am not going to the details of NMR spectroscopy but I would make you efficient in interpreting NMR data of multi nuclear system whether it is 1H, 13C, 19F or any other NMR active nuclei. So let us begin with first slide. So first slide I have given some reference books. If you are more interested in learning about nuclear magnetic resonance you can refer to some of these books. These are all very wonderful books and you can get wealth of information from these books. Please read and try to understand more about NMR spectroscopy. So NMR is the most powerful tool available for organic and inorganic synthetic chemists. Not only for organic and inorganic biochemists, material chemists, environmental chemists in fact all those who use chemicals NMR is the most powerful tool to characterize those compounds and also to determine the structure. It is used to study a wide variety of nuclei for example 1H, 13C, 15 Nitrogen, 15 Nitrogen, 19F, 31P and many others. This periodic table shows all possible elements which have NMR active nuclei, plenty of them are NMR active. In fact we can do NMR studies for most of these elements having isotopes which are NMR active. Of course they may vary all of them are not 100% but some of them are 100% and some of them are in trace quantity, some of them in 20, 30%. So what we have to understand first is which are the nuclei which exhibit NMR. So that means let us consider nuclei containing even numbers of both protons and neutrons have nuclear spin I equals 0 and therefore cannot undergo NMR. That means if you consider any nucleus having even number of protons and even number of neutrons they have nuclear spin I equals 0 and therefore cannot be used for NMR purposes. For example, if you consider 4 Helium, 12 Carbon, 16 Oxygen, R32 Sulphur, how about odd odd? That means nuclei with odd number of both protons and neutrons have spin quantum number that are positive integers. For example if you consider 14N, nuclear spin I equals 1, if you consider 2H, deuterium I equals 1 and also if you consider 10 boron I equals 3. So these are all having odd number of protons and odd number of neutrons and their NMR active. Rest of the nuclei having odd even or even odd combination of protons and neutrons have half integral values for nuclear spin. Example 1H I equals half and 17 Oxygen I equals 5 by 2. If you consider 19 half I equals 1 by 2, 23 Sodium I equals 3 by 2 and 31 P I equals half and many more. So that means here if you consider these are all even even NMR inactive whereas odd odd and all rest of them odd even or even odd combination of protons and neutrons or all NMR active and of course one should know how much such isotopes are present. So this one clearly gives you some idea about different type of nuclear spin we come across among elements in this periodic table. You can see all those indicated in red color have I equals half and those which are in yellow color have I equals 3 by 2 and blue 7 by 2 and then this orange one for lithium and nitrogen you can see and also those in green color they have I equals 5 by 2 and this in purple or violet they have 9 by 2. So that means you can see majority of elements do possess nuclei having NMR active I values. So what is nuclear spin? A nucleus with an odd atomic number or an odd mass number has a nuclear spin. Now it is very clear from my previous slide nucleus with an odd atomic number or an odd mass number possess a nuclear spin and the spinning charged nucleus generates a magnetic field that is true this information you must have learned from physics also any charged species generates a magnetic field when in its circular motion. So for example if you take if you consider nucleus as a tiny bar magnet and if you keep in a magnetic field they can see it can be aligned with the magnetic field or it can oppose the magnetic field. So that means spinning protons what happened the moment you put into they have haphazard way of arrangement the moment you put them in a magnetic field they are all aligned some of them are aligned with the field some of them are aligned opposite the field. So that means here a loop of current is generated here I. So that means when placed in a magnetic field spinning protons act like bar magnets I mentioned you it is in absence of magnetic field in the presence of magnetic field what happens some of them can be aligned in this way they have lower energy and more stable some of them can be aligned in this fashion and they have high energy and they are less stable. In fact you can convert this one to this one by supplying energy equivalent to the energy gap between these two levels. So you can see here two energy states you can think of alpha state and beta state in alpha state nuclei have aligned with the field whereas in beta state higher energy they have aligned in against opposite to the applied field and the energy difference is given by H nu equals to delta E and if you supply the energy equivalent to this one this can go to this one here the magnetic field of the spinning nuclei will align with either the external field or against the field. A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. So it is very simple you try to imagine the spinning nucleus like a top you must have played with in childhood or someone may be playing even now. So this is something like this the top is there and if it is rotating like this what happens when it is rotating steadily and looks almost static okay when it is a perfectly balanced top it will be rotating and it appears like it is standing sometimes they say top is sleeping okay yes in that case what happens and now let us assume the applied magnetic field is in this direction along this axis of this top and then you apply another energy in this direction energy so for example you start playing a top and then try to touch it at like this perpendicular to that one what happens it starts wobbling so when the frequency of this one is suitable then what happens it will turn something like this it will eventually it will wobble wobble and it becomes like this that is what exactly happens in that case okay so this is called flipping that means that much energy it already gained when we supplied in the form of a photon in the radio frequency region. So now let us look into the relationship between this energy gap and the magnetic field strength so this is energy difference is proportional to the magnetic field strength the energy gap is proportional to the magnetic field strength that means energy gap can be increased by increase in the magnetic field strength it can be decreased by decreasing the magnetic field strength. Now that is given by this equation delta E equals h nu equals this is gamma or rho h over 2 pi into B naught. So this term is called gyromagnetic ratio it is the ratio of its magnetic moment to its angular momentum. I am referring with respect to the nucleus. So now gyromagnetic ratio is a constant for each nucleus it is characteristic of each nucleus it will vary from one nucleus to another one and then for proton this is 26753 per second per gauss for hydrogen and a 60 megahertz proton is required to flip a proton that means a 60 megahertz photon is required to flip a proton in a 14092 gauss field that means if you keep a hydrogen or a proton in a magnetic field strength of 14092 gauss field to flip it the spin like this to become like this or to go to higher energy you have to supply a photon equal into 60 megahertz that is what this equation is telling you. So what is now we have to look into another term important term called magnetic shielding what is it? If all protons absorb the same moment of energy in a given magnetic field not much information could be obtained that means if you consider a molecule such as ethyl alcohol or isopropyl alcohol or acetaldehyde or anything if all hydrogens absorb the same moment of energy to go to excited level then probably NMR would not have been useful tool for determining the structure that means how it is different so but protons are surrounded by electrons that shield them from the external field so that means when you place a molecule in a magnetic field of course if we have NMR active nuclei what happens the flipping of nucleus takes place when it is taking place under the influence of a certain magnitude of magnetic field strength you should remember that we are not putting naked proton or naked nucleus without any electrons since they are surrounded by electrons they also generate a magnetic field. So that means the magnetic field generated by surrounding electrons varies from one nucleus to another one as a result what happens they require a different amount of energy for flipping that is called precision frequency the circulating electrons create an induced magnetic field that opposes the external magnetic field as a result what happens that magnetic field experience by nucleic would vary so magnetic fields that must be increased for a shield proton to flip for the same frequency for example let us say applied magnetic field strength is B naught and because of the electrons circulating surrounding the nucleus under magnetic field they also generate a magnetic field so that opposes to applied magnetic field as a result that net magnetic field experienced by nucleus drops in that case what happens if the radio frequency is fixed then what you have to do is you have to supply whatever the induced magnetic field generated by electron equivalent to that one so in that case what happens again resonance occurs we should remember so that means in order to flip that one at the same frequency okay there are two options are there I can decrease the radio frequency for flipping on the other hand we keep that one constant instead what we do is we will try to match the loss of magnetic field by supplying extra magnetic field so that is what we do so magnetic field strength must be increased so B naught must be increased for a shielded proton to flip for the same frequency so for example if you take here this is a 60 megahertz it absorbs for flipping under the influence of B naught equals 14,092 Gauss this is a naked proton and if it is shielded proton what happens net magnetic field experience will be less because the magnetic field generated by the circulation of electro density would oppose so then B naught will be B naught minus B I net magnetic field experienced by if I say the induced magnetic field say is B I resultant magnetic field experienced by the nucleus will be B naught minus B I so now whatever the B I is there equivalent to B I we have to supply magnetic field so that resonance frequency remains same so that is what we do in each case and in some cases what happens in case of D shielding we have to decrease it or we have to increase the frequency this is how the shielding influences so magnetic field strength must be increased for a shielded proton to flip for the same frequency stronger applied field compensates for shielding so that means whatever the balancing should be added so that the absorption frequency remains same so here I have given this radio frequency for different magnetic fields 14,160 megahertz this is respect this is with respect to hydrogen and 23,500 is 100 so this way we have 60 megahertz NMR 100 megahertz NMR 200 megahertz NMR 250 400 500 600 or even we have 1000 megahertz NMR now so now what would happen protons in a molecule depending upon their chemical environment are shielded or D shielded essentially they will be shielded by different amounts for example if you consider a simple molecule such as methanol if you see here the electron density on hydrogen is pulled you know that H is now almost like H plus less electron density is there it is less shielded so observed at a lower field whereas here they are more shielded they observed at a higher field so now this is how they have shielding and D shielding makes the signals appear at different places as a result what happens each functional group or each group is unique so that NMR is very helpful in determining the structure. So now the number of signals indicate different kinds of protons present in a molecule if you take ethanol for example ethyl alcohol we have CH3CH2OH we get three signals that indicates three different kinds of protons are present in the molecule okay the location of the signal shows whether the proton is shielded or D shielded one is it gives number of signals indicates number of a set of protons available and then it will give the location of the signal indicates whether they are shielded or D shielded accordingly we can arrange them in the sequence or the intensity of the signal shows the number of protons of the type now the intensity of the signal also tells in each group how many protons are there and signal splitting shows the number of protons and adjacent atoms now the splitting would tell you how many are there in the neighborhood in the next position or in the adjacent on adjacent atom so that means here it is this NMR signal would give you all this information first it would give you the number of different kind of protons present and the location in would indicate whether the proton is shielded or D shielded again the intensity would indicate number of protons of that type are present for example CH3 CH2 OH if you consider ethanol in CH3 three are there and CH2 two are there and OH1 is there that information comes from intensity and then splitting splitting would tell you CH3 is next to CH2 and CH2 is in between CH3 and OH and OH is that other yet next to that one only CH2 is there so something like that this information all information comes from NMR signals this is how it becomes very easy to interpret the structure and determine the structure ultimately this is the NMR spectrometer so this is the magnet magnet okay B naught whatever we tell so sample is inserted here and then a detector will be there and radio frequency will be applied here and then the signals would come a detector is there and it will be you know then whatever the detector detects and then absorption will be shown on a recorder plot like this and this one this is the B naught B we say B is the applied magnetic field so it is always in the increasing applied magnetic field is plotted in the direction and absorption will be in this one in this axis so let us consider methanol graph here you can see this is how methanol NMR spectrum appears like this and now this one three these three protons are represented by this one here and then here the small one is due to H so that means if the moment you see in this one you can see more shielded higher field low frequency shift for these things whereas this one less shielded this H is less shielded and lower field high frequency shift that means you can classify so when it is D shielded we call it as of course here if we have used the TMS as a standard I would tell you it is 0 with respect to that one these signals are assigned the chemical shifts so this is less shielded it is appearing left side and it is lower field that is also called down field shift and then high frequency shift whereas here once it comes on the right side it is more shielded these three and higher field and then low frequency shift. So now let us look into tetramethyl saline so this is the structure of tetramethyl saline this is called TMS it is added to the sample why it is added to the sample this is added as a reference since silicon is less electronic it within carbon TMS protons are highly shielded they are highly shielded because of so much electron density so signal defined as 0 whatever the signal it shows that is calibrated to 0 value and now we are recording NMR spectrum 1 H NMR for all molecules using this as a standard so organic protons absorb down field to the left of the TMS signal most of the organic protons they absorb down field to the TMS signal that means 0 if it is here everything will be coming towards positive side 0 to 10 or sometime more. So what is chemical shift chemical shift is measured in parts per million I shall tell you why it is measured in parts per million why not in Hertz ratio of shift down field from TMS to total spectrometer frequency will give you chemical shift in parts per million or the displacement of magnetic resonance frequency of a sample nucleus from that of a reference nucleus for example this is the same value when you determine chemical shift in parts per million it is independent of magnetic field strength so that is the reason we measure in parts per million whether you measure in 60 megahertz 100 megahertz or 300 megahertz it does not change this is called delta scale whenever we write we use this term delta. Now this is the delta scale I have given I have again defined here what is chemical shift chemical shift in PPM is called delta either you have to write delta or PPM there is no need to write both the moment you write delta it is understood that it is in PPM or if you write it PPM is understood it is delta scale this is the ratio of shift down field from TMS that means whatever the value that NMR spectrum shows that one in Hertz divided by spectrometer frequency will give you chemical shift in parts per million for example you can see here 10 PPM for 600 megahertz divided by 10 so here it goes like that and similarly in case of this is in case of 60 megahertz if you go for 300 megahertz also you can see it is a 10 300 3000 divided by 300 or here 600 divided by 60 so this is how for convenience to keep the NMR signal value constant irrespective of in which magnetic field we are measuring so it is given in delta scale or PPM so location of signals now so I have given some more electronegative atoms D shield more and give a larger shift values for example you can see here values are given and then effect decreases with distance effect decreases with distance distance between two groups additional electronegative atoms cause increase in chemical shift for example you can see here alkane CH3 would appear around 0.9 and alkane CH2 will appear around 1.3 and if you go for CH tertiary carbon appears at 1.4 and in case of this one when I have a CO group it appears at 2.1 and alkane proton appears at 2.5 if you have a halide then CH2 appears around 3 to 4 and if you have something like this this one appeared 5 to 6 and then if you have a double group and then CH3 it appears around 1.7 and aromatic groups appear around 7.2 a phenyl a methyl group on phenyl group appears around 2.3 aldehyde group hydrogen appears around 9 to 10 carboxylic proton appears 10 to 12 and alcohol value around 2 to 5 and aryl alcohol H will be around 4 to 7 and amine will be 1.5 to 4 so this the range is given again there is a significance in giving the range so now aromatic protons why they are so much D shielded let us look into it the value is around 7 to 8 if you assume this benzene group say it is placed like this and with respect to the applied magnetic field then what happens induced magnetic field because of circulation of this delocalized electrons would generate a ring current that generates a magnetic field that is aligned with the magnetic field if it is aligned with the magnetic field what happens it is more D shielded and it appears at higher frequency so that means here we call it when it is aligned with say induced magnetic field is reinforces the external field as a result it is D shielded the net magnetic field experienced by the nucleus is B naught plus B I naught B naught minus B I B naught plus B I as a result what happens the radio frequency has to be increased in this case either you have to decrease the magnetic field if we cannot do it we have to increase the radio frequency there is a reason they are appearing at D shielded region we call high frequency shift for the same reason in case of mineral protons same explanation can be given they appear on 5 to 6 so induced field reinforces but induced field generated here is in magnitude less than what we saw in case of acetylene now in case of acetylene proton what happens it comes around 2.5 so induced one is considerably less compared to those 2 groups here in case of aldehyde protons it is even more because of electronegative oxygen atom present here as a result what happens it appears around 9 to 10 it shifts is much more D shielded and it is high frequency shift and then OH and NH signals it is very important chemical shift here depends on concentration and hydrogen bonding in concentrated solutions D shield the protons so signal is around 3.5 for NH and around 4.5 for OH this is very very important if hydrogen bonding is there how the chemical shift is affected so that means you due to the hydrogen bonding in concentrated solution that D shield the proton so that it goes around NH proton would appear around 3.5 and OH proton would appear around 4.5 so proton exchange between the molecules broaden the peak another example is for example this these are very acidic they can readily exchange so take this one and keep it in deuterium solvent and if it completely disappears and you may not see next day when you run NMR OH signal that indicates that you have a OH so that means you take this one freshly record NMR in CDCL3R D2O and then leave it for 24 hours after that one all OH have become OD in that case this signal due to this H disappears because in case of OH you have OD so that indicates your compound as OH same thing is to in case of NH also so if sometime confirming the presence of NH is very critical you can do this exchange process record freshly prepared sample and identify where NH signal is coming leave it for 24 hours all NH becomes ND and then take NMR this is missing so that indicates you have NH and of course you can also confirm these things from IR spectroscopy by looking into OH stretching frequency that should be different OD will be there now so now carboxylic acid proton would come around 10 so you can see here in this one this is for CH3 and this is for OH is shown here and number of signals you can see how many signals are there 1, 2, 3 signals are there because we have 1, 2, 3 are there and relative chemical shifts also shown here for example this one is around 2.25 and this is 3.41 and this is 3.36 that means you can say relatively to what extent they are deshielded with respect to this 0 value of TMS. Now let us come to the intensity of signals if you consider this molecule here we have these 3 are equivalent so we have 9 protons and here we have 3 protons so when we get 2 signals so signal due to this one should have intensity 3 and then intensity of this one should be a 9 so now that means it is 1 is to 3 you can see here the space taken is 2 spaces 2 and then 6 spaces it is taken that means 1 is to 3 1 is to 3 means it is 3 1 is to 3 is there so this is how the intensity of the signal will tell you how many identical or equivalent are there then and now we can say without any problem yes there are 3 methyl groups are there and they are all on the same carbon atom so that means this quaternary carbon can be identified here the other one is also shifted little bit if oxygen is next to CH3 group so you should be able to tell it is an ether something like that so intensity of the signal I told you show by integral trace this is called integral trace that will give you so number of equivalent protons present in a given molecule. So now how many hydrogens when the molecular formula is known each integral rise can be assigned to a particular number of hydrogens that is what I showed you here you can see here for example here 1 2 they are all these 2 are identical 4 type of protons are there then we have 4 signals are there and the ratio if you consider this is 1 is to 2 is to 3 is to 6 that means here 0.5 is to 1 is to 1.5 is to 3 so that means basically very nicely it will tell you how many similar type of protons are there then segregating them as groups and then adding at appropriate place to arrive at the correct structure to understand it so would be very easy. So now let us come to spin-spin splitting so non-equivalent protons and adjacent carbon have magnetic fields that may align with or oppose the magnetic fields so now let us look into for example if we take CH3 CH2 OH CH3 is there and CH2 is there and OH is there and CH2 is adjacent carbon and 2 hydrogens are there and how these adjacent hydrogen atoms influences the signal of CH3 and same way how CH2 get influenced by CH3 protons that we have to see that means we should remember when we are looking into the signal around CH3 CH2 also generates a magnetic field those 2 protons and they can also be aligned with the magnetic field or oppose the magnetic field and depending upon that one this signal will be split so this magnetic coupling causes the proton to absorb slightly higher down field when the external field is reinforced and slightly up field when the external field is opposed that means now instead of giving one signal it may show 3, 4 or 5 signals and then we have to see what are the intensities and then after understanding one or 2 molecules what we should do is we should try to make it a generate general so that every time we should not calculate the moment we look into your molecule we should be able to tell how many lines will be there how many signals will be there and how it is split that we call it as spin-splitting so all possibilities exist so signal is split here. So let us look into one example quickly so now let us look into 112 tribromomethane so non-equivalent protons and adjacent carbons are there now these 2 are different from this one so now let us look into the signal of these 2 signal of these 2 will be influenced by this one that means let me write here this is the applied magnetic field so B naught I am looking into this H B, H B is there and H B yes it should give a signal and now what happens if H B is something like this now we have H is there H A how it is going to influence H A would influence in this fashion for example it can be aligned in this way with the this one okay this is our together and now R it can be aligned something like this so as a result what happens one is here one is here this is low so we get one signal and one signal in one is to one so this is how it is on the other hand so basically it would be something like this it can be aligned with the magnetic field this one when we are talking about H B signal or it can be opposite so that means now it is influenced by two protons one with the field one the due to the neighboring one so with this one will give one signal this one will give one signal one is to one we get a doublet here now this is a triplet how it is coming here so now we have something like this two are there both of them can be aligned with the magnetic field now one can be aligned like this one can be aligned like this and this is true for this one also here we can have two arrangements now this can be aligned like this and this can be aligned like this and now both of them can be aligned in this fashion now what happens now one is to two is to one so three this is coming this is one is to two is to one so this one is for this one so now this one under the influence of these two here what happens it will be split into three with the intensity ratio of one is to two is to one so this is how we can understand now it becomes very easy to write for example if the formula is given simply say C2 H3 Br3 if it is given and if the spectrum is given here and you should be able to write the correct structure here so this is how we can use NMR very effectively with molecular formula we should be able to write structural formula of looking into this one and analyzing and interpreting as I mentioned here I showed you HA reinforces field this one for these two one is in this direction this H is aligned with the magnetic field this signal and now this one is opposite aligned in the direction opposite to the applied field because it can be it can have spin like this or it can have spin like that when you are looking into the signal due to this one so two signals have come now if you see the other one now when we are looking into signal of this one these two have two small magnets they can be aligned like this here they can be aligned like this here now the field is increased so high field strength now one is like this and one is like this and other combination is like this so and now both of them are down so this one signal is coming here so intensity is 1 is to 2 is to 1 so this is how NMR understanding is simplified by looking into the spin-spin splitting then what happens it becomes very tedious or laborious job to calculate how many signals are coming for each one every time you see sometimes you may have 6 equivalent protons 9 equivalent protons or 15 equivalent protons then what happens it will be little bit cumbersome to understand or calculating every time for that one we have a simple rule here a general rule that is called N plus 1 rule if a signal is split by N equivalent protons it is split into N plus 1 peaks so that means number of equivalent protons causing splitting 0 means 1 signal in the neighboring you do not have anything means you get a singlet and if you have 1 if you get a doublet if you have 2 you will get a triplet if you have 3 you get a quadrate if 4 or there you get quintet 4 plus 1 5 so 5 plus 1 6 6 plus 1 7 7 we get then to understand the relative ratio of these signals in a multiplied we have to go for Pascal triangle Pascal triangles will be unique for each nuclear spin so this one is for i equals half the same Pascal triangle one cannot use one should not use for i equals 1 or i equals 3 by 2 i equals 3 you have to write separate Pascal triangles first by writing the intensity of the first signal and you can continue writing now this one is for nuclear spin i equals half so this is the Pascal triangle that can tell you 7 lie separate means the intensity of this 7 lines will be 1 is to 6 is to 50 is to 20 is to 15 is to 6 so if you look into isopropyl alcohol we can see that one I will start my next lecture in repeating the NMR spectrum of isopropyl alcohol or isopropanol so until then have excellent time understanding NMR thank you for your kind attention.