 Once again, I welcome you all to MSP lecture series on interpretive spectroscopy. This is the fourth lecture in the series. In my previous lecture, I started discussion on chemical shifts. The signals, we call it as chemical shifts. Let us, you know, go back to those few points. In NMR spectrum, we get NMR signals. What these signals say about the molecule? So that means the number of signals shows how many different kinds of protons are present and then the location of the signals shows how shielded or deshielded. This is very, very important information. And then the intensity of the signals show the number of protons of that type. So one is different type of signals and where they are appearing and then how many protons are there in each signal. And then the signal splitting, you know, what is the splitting pattern, whether two lines are there, three lines are there or four lines are there, one line is there. So this signal splitting shows the number of protons on adjacent autumn. That means how they are connected. As I mentioned in case of ethanol, we have CH3, CH2, OH. And all these points will tell you where CH3 is located, where CH2 is located, where OH is located and then how many different type of hydrogen atoms are there and each one, how many are there hydrogen atoms and where they are appearing. And then the splitting would tell you whether CH3 is close to this one, something like this. So all this information, again, comes very nicely without compromising whatever the information we get from the bonding concept, okay, valence bond theory or molecular orbit theory, whatever we get, exactly in the same way it proves the presence of these signals, these groups that appropriate place. So this is where the importance of NMR comes, okay, in characterizing molecules. So we call these signals as chemical shifts, they are measured in parts per million and ratio of shift down from TMS to total spectrometer frequency in hertz will give you parts per million in PPM. And then as I mentioned, so this is when we measure chemical shifts in parts per million, this is independent of magnetic field strength. That means whether you measure chemical shifts at 60 MHz, 100 MHz, 300 MHz or 500 MHz or even in 1000 MHz, the value remains same as far as denoted these chemical shifts in parts per million. So then we use the scale called delta scale. So for when we measure in parts per million, we use delta scale and then just make you familiar with chemical shift terms. I am going back again to the basics what I discussed in my first lecture. So this one, induction of opposite magnetic field. So if you just see here, M equals plus half, orientation of nucleus, orientation of nucleus is like this. And then precision of electron density, so precision of electron density surrounding this proton and induced magnetic field, induced magnetic field due to circulation of electron density opposing here, opposing the applied magnetic field strength. So that means applied magnetic field will cause the electron to persist about B naught which in turn causes circulation of electron density and a generation of a small magnetic field B i. You can see here, this is B i here in green one I have shown. So as per Lenz law, the direction of an induced current is such as to oppose the cause of producing it. So because of which it induced current that would always oppose the root cause of that one. Hence direction of B i is always opposed to the applied magnetic field. Now the net magnetic field experienced by the nucleus is B net equals B naught minus B i. So the magnetic field experienced by the nucleus is less than the applied magnetic field and hence a lower frequency. Now in order to cause transition, you have to apply a less frequency in a direction perpendicular to applied magnetic field. So whatever you are applying here. So that means the electron density surrounding the nucleus produces a shift to lower frequency. So as a result what happens? You can compensate whatever the loss by reinforcing this one so that you can keep the normal frequency constant or you have to bring down the magnetic field strength of the B 1 that applied orthogonal to B naught that should corresponds to now the present frequency. So this is where the terms low frequency, high frequency, low field shift or field shift all those things comes into picture. So that means this can also be represented by B net, net magnetic field experienced by the nucleus B naught into 1 minus sigma. Here sigma is referred to as the shielding constant. So if we keep the Larmor frequency constant we need to increase the magnitude of B naught to achieve resonance as I said. So equivalent to whatever B i is there, whatever is there corresponding to that one you have to apply. So that B naught is strength and now to neutralize whatever the effect of this one. It is called up field or higher field shift, that is the reason it is called up field or higher field strength or also called diamagnetic shift. Why we are using term diamagnetic later we will be coming with another term called paramagnetic. So diamagnetic shielding and paramagnetic shielding will come. So as a result of diamagnetic shift all signals will be shifted to lower frequency. So whereas de-shielding is referred to as down field, lower field or paramagnetic shift all shift to higher frequency where the induced magnetic field will be reinforcing the applied magnetic field. In that case what happens, net magnetic field experienced by the nucleus will be much more than the magnitude of B naught. So that is called paramagnetic shift or paramagnetic de-shielding shift. So the extent of magnetic shift is determined by the amount of electron density surrounding this nucleus. These points are very very important, these three points I have shown here. The extent of diamagnetic shift is determined by the amount of electron surrounding the nucleus, the ease with which electron density can circulate about B naught and the magnitude of B naught. That means the amount of electron surrounding the nucleus and also the ease with which electron density can circulate about B naught. That means how many electrons are there in the valence shell, whether they are spherically symmetrical or unsymmetrical field and also the magnitude of B naught. At constant B naught the shift is dependent only on the amount of electron density and the symmetry of the electron density. It is not only the magnitude of the electron density, the symmetry of the electron density is also very very important in generating opposite field or applied field, in generating diamagnetic shielding or paramagnetic de-shielding. Let us consider 13 C nucleus here I equals half. Here nucleus is surrounded by 1 S 2, 2 S 2 and 2 P 2 electrons. The valence shell in case of carbon we have 2 S 2 and 2 P 2, so 6 electrons are there. So that means here compared to 1 H the electron density is more and hence the magnitude of induced BI will also be greater. So if you compare the nucleus of hydrogen kept in the magnetic field and 13 C nucleus kept in the magnetic field in both will be generating induced magnetic field. The magnitude of the induced magnetic field generated by carbon is much more than that of hydrogen because their electron density is on the 1 S 1 whereas here we have 2 S 2, 2 P 2. So now the electron density surrounding the hydrogen is spherically symmetric whereas in case of 13 C electron density is due to 1 S 2, 2 S 2 symmetric but the same due to 2 P 2 is not spherically symmetric. So this is important that means in case of 1 H still we have 1 electron is there and then the 1 S orbital is symmetrical, spherical as a result what happens the electron density is spherically symmetric whereas in case of 13 C. So electron density due to 1 S 2 and 2 S 2 is spherically symmetric whereas that of 2 P 2 is not spherically symmetric. So only way to obtain spherically symmetry is by having 1 S 2, 2 S 2 and 2 P 6. So that electron density is equal in all the 3 axis if you imagine P x, P y, P z are orthogonal to each other if all of them are completely filled then you can have electron density equal in all the directions x and y and z direction as a result probably there will be spherical symmetry that means we are talking about C 4 minus ion if not with electronic configuration of 1 S 2, 2 S 2, 2 P 6, 2 P 2 it is not spherically symmetric and it is very different from that of 1 H generated induced magnetic field here it is there. So I can write here this is called whatever the reference is there right side whatever it comes low frequency of field shift and then higher field high field and shielded shielding. So here high frequency because in this case what happens it is reinforcing so it is high frequency shift and this is low field shift and high field is low field shift and then shielding de-shielding. So this is how we designate with respect to reference here. So that means the lack of spherical symmetry for the electron density surrounding the 13C nucleus results in diminished circulation of electron density and hence lower bi. So that means if the spherical symmetry is not there the induced magnetic field would be having a small value as a result what happens the shielding effect is minimum. I read again the lack of spherical symmetry for the electron density surrounding the nucleus 13C nucleus results in diminished circulation of electron density and hence lower bi value. So thus shielding constant sigma can be divided into so that means after considering the nature of the electron density whether it is spherically symmetric or unsymmetric the sigma the term can be divided into two components sigma D diamagnetic shielding and sigma P paramagnetic de-shielding. So this one can be divided into two that means basically it is a combination of sigma D and sigma P where sigma D is diamagnetic shielding and sigma P is paramagnetic de-shielding. The sigma D results from the unhindered circulation of electron density which produces bi opposes b0 and hence is always positive that results in shielding. So that means sigma D results from the unhindered circulation of electron density unhindered circulation of electron density happens when it is spherically symmetric which produces bi and that always opposes b0 and it always positive then the net effect is shielding. So sigma P results from the hindered circulation of electron density when the electron density is unsymmetric then we that results in hindered circulation of electron density under the influence of magnetic field b0 which produces bi aligned with b0 or reinforces b0. So in this case what happens in this case if this is b okay b will be like this bi whereas in this case this is applied so in this case this will also be like this bi. So net is b0 plus bi so this is always negative so that means the shielding constant is the sum of sigma D total effect of sigma D and sigma P at a given time whichever is dominating would decide whether the signal is shielded or de-shielded. So now let us consider two examples of H2 and F2 ideal examples to look into these aspects sigma D and sigma P components in total screening constant shielding constant. So now if you just take diatomic species and applied magnetic field you can have all possible orientations of this diatomic species but you can consider the average of all possible orientations that falls into two categories one is perpendicular to the applied magnetic field one is parallel to the applied magnetic field so that I have shown here. So this one a diameter molecule with orientation in perpendicular here and then in this one parallel here with respect to the applied magnetic field. So here cylindrical symmetry relative to b0 is there so that means the electron density around autumn a is affected due to its bonding with another a that means earlier when we had simple a that is a different environment now we have a a bond is there and a a bond has two electrons now the electron density around autumn a is affected due to its bonding with another a even hydrogen autumn in H2 no longer has a spherical symmetry because of the distortion of the electron density by the other nucleus but in the parallel orientation of H2 the magnetic field in this magnetic field the electron density has cylindrical symmetry in the direction of b0 cylindrical symmetry is there in the direction of b0. So this is not unimpeded and the paramagnetic component is 0 so here paramagnetic component is 0 in this case but in the perpendicular orientation the electron density around each hydrogen is not symmetrical with respect to b0. However so delta p is negative and delta so since hydrogen is only surrounded by one less electrons and S are which are spherically symmetrical as a result what happens it is still the electron density is spherically symmetrical as a result the magnitude of sigma p component that generates because of hindered electron circulation that is negligible and bi only and bi is only 15 percent so sigma p term for H2 is negligible. So sigma p is so low and its contribution is about only 15 percent as a result what happens sigma p term for H2 is negligible that means it is only sigma d diamagnetic shielding what matters in case of H2 molecule. Now let us look into F2 here for the F2 molecule the analysis of parallel orientation and perpendicular orientations are very similar to that of H2 each F atom has a closed shell of electron density if you take F. So we have something like this so something like this we have 88. So in this case basically what happens in the perpendicular orientation due to the presence of p electrons and the incumbents of the circulation of electron density about b0 due to the FF field bond produces paramagnetic shielding. So in this one electron density is more and also we are talking about p orbital as a result what happens circulation of electron density there is a incomparance hindrance is there as a result produces a considerable amount of paramagnetic d shielding that means sigma p term is quite large. So actual shielding observed is average of the shielding in all possible orientation if you ask me question why I am considering only perpendicular you know parallel and perpendicular yes. These two are taken after considering averages of shielding in all possible orientations when you consider all possible orientations in all angles you have to take two extremes one is perpendicular one is parallel only those two are considered they are averaged one. So except for so because of this reason except for one H and alkali and alkali and earth metals where we come across only s electrons in the valence shell in all cases paramagnetic term dominates and hence d shielding is there we should remember if you consider one H NMR, sodium NMR or any other alkali and earth metal NMR, lithium NMR in all these cases what happens paramagnetic term is not there and shielding is there but apart from this one if you go to any other metals any other elements where we have p orbits are there p orbits will be having valence electrons the paramagnetic term dominates. So fluoride ions show a chemical shift of 400 ppm compared to bare fluorine nucleus whereas in case of F2 the paramagnetic d shielding increases its chemical shift from 600 ppm relative to fluoride an ion. So you can see as a remarkable contribution coming from sigma p term because of hindered circulation of electron density as a result of p electrons. So this will give you some idea about paramagnetic d shielding and diamagnetic shielding and also why we have in case of one H alkali and alkali and earth metals a short chemical shift region whereas in case of other nuclei we have a long range of chemical shifts. This is the delta scale chemical shift in ppm I am telling again shift down from PMS in case of one H and 13 C spectrometer frequency in megahertz. So here as I said if you measure chemical shifts in ppm this is independent of magnetic field strength. So to compare that one you can see here I have taken 60 megahertz and 300 megahertz and then if you take the chemical shift here 1, 2, 3, 4, 5 whatever you have taken it does not change it does not change it remains in ppm study whereas in Hertz it is changing but in ppm it does not change. So this is the advantage for the same reason always chemical shifts are given in parts per million. So that when you take in different magnetic field strength NMR spectrum so it should not really matter. So now the location of signals. So after understanding this chemical shift what is chemical shift and what is shielding what is de-shielding. Now let us look into the location of the signals and location of the signal is given by chemical shift as I mentioned. If you just look into NMR signal chemical shift of methane it comes around 0.2 delta r 0.2 ppm and then if you look into methyl chloride it comes around 3 and then if you just compare between CH4 and CH3Cl this delta coordination shift the change in the coordination shift is 2.8 and then if you add one more chlorine dichloromethane if you take it is 5.3 again it is 2.3 and if you take chloroform CHCl3 it is 7.2 and again difference is 1.3 that means addition of each chlorine atom ok changes the delta of remaining protons by 2.3 ppm. So that means it is additive here you can see here every time we are replacing one hydrogen by chlorine the difference is increasing by about 2.3 ppm that means it indicates nearly additive. So you should be able to check whether this is holds good in case of all or this is an exception you can examine yourself. So that means more electronegative atoms deshield more and give larger shift values. So that means here we have 3 chloride 3 chlorine atoms are there and more shielding is there more deshielding is there because the electron density is pulled towards them and also as a result chemical shift is larger. So then effect decreases with a distance. So here these chlorine atoms are 2 bond apart yes if I write like this ok 2 bond apart 1 and 2 bond apart. So effect decrease as the distance between these will move. So effect decreases as the electronegative atoms are moving further from the nucleus we are considering. So additional electronegative atoms cause increase in chemical shift if I keep on adding more and more electronegative atoms. So more and more deshielding will be there and then the chemical shift value will increase. So here I have given chemical shift values for different type of protons here you can see here methyl groups in alkane comes around 0.9 and in alkane again if you look into CH2 groups it comes around 1.3 and CH will be coming at 1.4 and then if you have a CH3 group next to a carbonyl group it comes around 2.1 and if it is next to acetylene it comes around 2.5. Next what happens if CH2 is next to halogen or oxygen it will be in the range of 3 to 4 and next in the olefin sudden jump is there it is 5 to 6 and then when we have a methyl group in a olefin it is again 1.7 and then if you consider OH group in phenol phenol it is there. So it comes around 7.2 and then in this one CH will be red one I have shown here 2.3 and then in aldehyde it comes around 9 to 10 in acid it comes around 10 to 12 in alcohols it comes around 2 to 5 and then in aromatic alcohols it comes around 4 to 7 and of course here this is incorrect. So this is for this aromatic hydrogen atoms it comes around 7.2 whereas here for aromatic alcohol phenols H will come around 4 to 7 and amines it comes around 1.5 to 4 and sometime it can also go much lower. I have given some nucleus again and also I have shown their natural abundance and also the nuclear spin values and frequency with respect to hydrogen at 100 megahertz I have given and also the standard references used while measuring NMR of various nuclei also I have given here this again an important table. You can see 1 H 100 megahertz the standard we use is tetramethyl silane we call TMS and same thing is to in case of 13 C also TMS we are using and frequency will be 1 fourth 25.1 and natural abundance is 1.1 and then 19 F 100 natural abundance and frequency is 94. This is CfCl3 we are using as a standard in case of 29 silicon again we are using TMS and in case of phosphorus we use 85 percent phosphoric acid we are using and in case of 77 selenium we use no dimethyl selenium we are using in case of 103 rhodium again 100 percent abundance very interesting and frequency is 3.2 times and then rhodium metal itself is a standard there and then 117 tin natural abundance is 7.7 and frequency is 35.6 and then tetramethyl tin used as a standard and again we have another 111 19 it is 8.6 in this case we use the same standard and Xenon is there XCOF4 is the standard and tungsten we are using Na2WO4 and then in case of platinum we are using Na2 PTCL6 sodium hexachloroplatinate and in case of mercury we are using dimethyl mercury as a reference and corresponding frequency with respect to 100 hertz megahertz 1HNMR is given here. So, let me stop here and come back with more interesting discussion on NMR spectroscopy. So, have an excellent time reading. Thank you.