Lecture 07. Introduction to NMR Spectroscopy

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Lecture 07. Introduction to NMR Spectroscopy
Concepts and Theory, Part 1
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Lecture 07. Introduction to NMR Spectroscopy, Part 1
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This is a graduate course in organic spectroscopy, focusing on modern methods used in structure determination of organic molecules. Topics include mass spectrometry; ultraviolet, chiroptical, infrared, and nuclear magnetic resonance spectroscopy.
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I dwelled on what I'd like to know what I'd like to do today in on Mondays to talk about anymore more spectroscopy and kind of Helena says prosecute works a caller concepts and theories and for me what I want to do is give my perspective on animal which is not a highly mathematical perspective in fact everything I write up here today is going to really be in terms of numbers is actually going to be simple arithmetic and most of it is more an embodiment of the idea rather than a specific calculation and you quote need to do sell where animals are begins is with the concept that a nucleus of certain sorts and I'll just write of protons finale has faded spin to and when you have a spinning charge it generates a magnetic dipole and if you apply a magnetic field we call that magnetic field being a lot then you have to different spin states the and we'll see examples of this in the case of nuclear quadruples but let's start with the case of a proton received 13 off you have to spin states that can exist quantized spin state the spirit of the players can either be spin off so if it's spin up In other words in the same direction as the applied magnetic field then this is going to be lower in energy so it could buy up I mean all lines aligned with a and if it's spin that now meaning aligned against the knot they were hiring managers and referred to throughout our discussions will referred to the lower energy state and regional state and to the higher energy state as the biggest thing now different types of nuclei have different spin properties rather than trying to start with generalizations about rules all come to those in a moment because at some point you'll be wondering in your project localized study of chlorine 35 or something like that let's just start with typical nuclei study so if you don't for example To the 400 megahertz cinema spectrometer in my building in natural sciences 1 you'll find that that instrument can study protons and going to write a couple of numbers fitting some going to write the atomic number in the mass prayer and it can study see 13 predicted study f 19 Americans study Ping 31 and user ,comma nuclei that are often studied by animal they're easy what did these nuclei have in common they have a one-and-a-half Indian and white forgetting their spin state what property and the black odd numbers of protons and neutrons or more specifically we can group them that their mass number is odd specifically that the sum of their protons and neutrons is on cell nuclei we've on mass number have a nuclear spin In the quantum characterization of nuclear spin is what's called a spin number and will call this the number of life doesn't really matter what the people they call it I and so that number is going to be 1 hand and that gives you all the ones up here but if we want a generalized more nuclei with an odd mass number will have a spin number of one-half the 3 hands were 5 hands etc. so that's the more more general idea the ones with 1 hand Our easy because they have what's called a nuclear died if you have 3 hands-off 5 halves 1 has will see in just a moment you have what's called a nuclear plant for Poland and those tend to be the hard harder so all the ones here or otherwise equals 1 half and spin states so we have the quantum number and then the to spin states they could have and so the spin states a plus or minus 1 so that's all of these H 1 see 13 books if 19 will come to nitrogen in just a 2nd a P 31 now a nuclear test with a Spain number of 3 hands can have spin states on plaster minus 1 half were plus or minus 3 hands and this is what you call a nuclear quadruple most of the time of many of the times nuclear nuclei with nuclear Quadra polls don't behave behave as if there in a more active In the next lecture will get to the concept of relaxation relaxation basically is how quickly flip between the 2 spin states work in this case between the forced states er Floridians and often they fled very quickly which means you can study
them by relaxation is affected by properties like symmetry as well and I'll get to that in a moment with another example but if I give you an example of a nucleus with spin state of 3 have of on there too 2 different isotopes to be 10 and be elected in the 11 has I think involved during the the 11th has 80 spin state of 3 halves and if you look at the animal or spectrum of born hydride from this 1 what you see in the H 1 animal Farm Jesus see formal lines equally spaced and of equally equally high due to the hydrogen coupling with the nuclear while the pope it's very unusual because normally we think about splitting in 2 ways doubling it off your thinking a Triple-A 1 2 2 2 1 trip Buddha quartet a 1 2 3 2 3 2 1 trip with but what's happening here is the hydrogen see boron and they see the warrant having been stated negative 3 hands were negative 1 of her negative positive 1 positive 3 and say See the force in states in rise for a while and alright but so let's look at some other nuclei with on mass numbers on 1 very important nucleus and biomolecular Panama is 15 nitrogen 15 has a spin number of equals 1 and indeed any 15 is often study most nitrogen it's not in 15 we talked about this when we talked about mass spectrometry we said at the natural abundance of 1815 the 0 . 3 8 per cent and that's really really love isotopic abundance of C 13 stay inactive is 1 and a half per cent is 1 . 1 per cent and you know that ,comma banana Morris not very sensitive you need to have a reasonable sample size more than you have for proton typically and sometimes or often collect data for much longer mobile the time here down 10 to up . 3 8 per cent studying natural abundance is pretty hard and so often you do this With isotopic labeling two-dimensional and 15 based techniques are a mainstay of protein and Amara spectroscopy and in general since most proteins are expressed these days what you do is you simply grow up here Ecole within 15 ammonium chloride and they absorbed that use it to make up the amino acids and then you can get a fully in 15 labeled protein which is very useful In 15 is starting to become more important in on in some natural product structure determination alkaloids as you may have seen for example in your stock have lots of nitrogen in them and so being able to figure out the positions of those nitrogen's can be very important in the case of something like an alkaloid or synthetic project you might not be able to put in 15 in animal spectrometers are becoming more sensitive and so it becomes not completely nuts to think about using and 15 techniques in urine at the end of the course I may talk about some two-dimensional techniques within 15 natural abundance that that people were doing on just because I think it's an it's useful but that won't be until until the end of November and December another nite nuclear another ,comma knew well liked ,comma another nucleus is oxygen almost 17 remember we said his only role natural abundance it's only very low I it's only . 0 4 per cent an oxygen 17 has a sufficient number of online is equal to 5 hands so that's the nuclear 2nd have 7 can have 6 states negative 5 halves negative 3 has negative 1 have positive 1 of 3 five-setters etc. and so it has start doubly damaging and so it's not generally studying I'd said that takes care of our nuclei With on mass numbers now the next class of talk about is if you have any even menace number ended even atomic number so that's easy those in those nuclei light see 12 16 and the answer is very simple those have a spin number I equals 0 they have no spin and those of Panama in on this since you don't have different spin states you can't have quantized transitions between spin state so there's no way they can be studied by and more spectroscopy so the last class then becomes nuclei with and even mass number but not atomic number so that would include nuclei deuterium nuclei light and 14 I guess that would be the common ones we encounter in organic compounds these have
all have a nuclear quadruple remember quadruple was anything that doesn't have a dipole just spit up spin down side despite almost one-half so these have all have a nuclear Quadra Poland been number ii equals 1 2 3 etc so for example if you take deuterium you have a spare number I equals 1 and so you have to really spin states available to it and you know the direct manifestation of this that many of you have seen with your own eyes who's fantasy 13 most of the what Selvin years chloroform right the 1st solvent most of us reach for because it's pretty chief solvents as solvents still in pretty good at dissolving organic chemicals it's cheap because it doesn't have that much deuterium and right you you only have 1 deuterium for all that weight of chlorine you need the deuterium to to get the deuterium what for an Aymara spectroscopy what do you always see when you run an a spectrum durable at triple at anniversary interesting sort of started fled 1st the ACL parade in the city 13 NMR you see 0 1 still wine 2 1 trip was centered at 77 people it's rearrange together those separation between the lines His 32 hops in other words the distance between these 2 alliances 32 hurts the distance between these 2 lines is the parts if you're running your spectrum under 500 megahertz spectrum at that means the carbon Adamov is running at 125 . 7 megahertz or come back to that in a 2nd which means 1 ppm is 125 herds which means the lines here are separated by about 3 intense other ppm and that some of the roughly 200 ppm scale so as James said those lines are really close together and manifestation is it's 0 1 2 1 2 1 Triplette because 2 a first-order approximation a 3rd of Europe do do the are in spin state negative 1 a 3rd if you're due to answering spin states 0 and the 3rd if you do the islands are in spin state of positive and would see in a moment that there mood still minuscule differences in the population the spin states and that that's really really important also see in a moment that that number 32 comes back when we see something else on most of the time so deuterium is kind of special among nuclear Quander polls is that most of the time nuclear Quadra polls nuclei with quadruples undergo rapid relaxation with deuterium a special it's relaxation is long and others say to put it in a predator in simple terms it is effectively life among the meaning to so many of the nuclei with nuclear quadruples like clearing 35 and chlorine 37 How do we know that those I have while we right I will take that back we can't know whether they have nuclear dipole on nuclear quadrupled but we know there's been number 1 hit for 3 have survived cancer 7 have they happened to have the higher ones so we never see J. coupling we never see spin spin coupling to chlorine 35 requiring 37 if we did your carbon spectrum here your see 39 a more spectrum would actually be much more complicated because you'd be seeing splitting from scoring OK so nitrogen 15 I'm sorry nitrogen 14 also has a nuclear quadruple it has a been number of ii equals 1 and so normally you have rapid relaxation so for example if we come to that and did what we were dealing with the flow when I ask you about the only or spectra and if we look at the animal or spectrum of this and of course most of your nitrogen 99 . 6 2 per cent of your nitrogen virtually all is N 14 In here and we don't see J. couple to this protons so as I said the fact that we do CJ coupling between the deuterium J. coupling just means spin spin coupling to the sea 13 is because deuterium is the oddball hearing that it often doesn't under guard stations but most nuclei we then that nuclear quadruple don't see I don't show but nuclear coupling because we have racked and relaxation as I was saying earlier with my example aboard a hydride symmetry is the oddball on are highly symmetric species end up being oddballs in that you have slow relaxation sober hydride BH for minors has territory he grows symmetry coupling from the ball on to the hydrogen is a case that you may see an eyesore 1st by accident 1 of those cases where you simply dissolve up a
compound in a solution and you get an animal or spectrum and so this is ammonium chloride India alright ammonium chloride has any work plus CL minors In the ammonium has stepped symmetry and the 1st time I happened to accidentally have this in a simple and took banana more spectrum as I said India must the 6 so I saw him more spectrum with peaks that were so far apart that you barely could tell they went together except the odd thing was they were all the same height and this spacing was the same as that has like what's going on all the way back to Europe nitrogen so this is your work 1 NH in other words 1 bond coupling between the nitrogen and hydrogen I don't remember what the coupling constant ears but it's fake James J. said yet Jesus is the term that we used to refer to spin spin coupling not just Proton lot so we would describe this as J-1 CDs People's 32 and later on when we start to talk about to the techniques like M Q C & H BC terms like 1 CH J 2 C H & J C 3 CH in other words 1 bond to bond and 3 bond carbon hydrogen couplings will become very important structures termination all right so that only the last last left their spinning nuclear dipole he was spinning in the presence of an applied magnetic field that I said there were 2 states the alpha state and data state in the office state was lower in energy and made it so I can make a little diagram he and I can show just like you learn in electronic structure where you learn for example you have CPI-or battles and piste orbitals and you have populated God electrons in year 2 orbitals here we can think about populations of nuclei it's a little bit different sense we're talking over the entire sample but if we have are applied magnetic field being Nolan and we have a alpha state In beta state member the alpha state status aligned with the magnetic field we can think about some nuclei being in the office and some nuclei being in the beta state and there is an energy gap between these 2 states and we can think about the energy deflect a nucleus from the alpha state to the beta stage as the energy of a full time in other words and energy quantum In the electromagnetic spectrum and that Delta III is going to be a major new in other words the energy the difference the frequency of a photon 2 flat a nucleus From the alpha state to the beta state is going to be dictated by the difference in energy such that calls in ancient Delta created equal station now what sort of energies are we talking about what we're talking about 500 megahertz for Proton so we're talking about radio frequency alarming just give you a calibration here so if you think about you believe an dilatory so maybe if I think about UV and I think about it ,comma Fulop maybe I think about my Mercury line at 254 animators from my TLC lamp that I think about it ,comma for work say containing benzene ring around the toxic benzene ring and maybe a sale right if we just take 254 100 meters and I go ahead and clog the interior members sequels land and looser that's our wavelength and you calculate knew the frequency and then you calculate equals 8 snow the new program with planks constant you get that Delta III corresponding to a photon and UVA is 113 kcal and then you stop and think like an organic chemist and say OK wait what's 113 kcal per mole was the difference between a pioneer pies stock it's a little stronger than the strength of a carbon-carbon single bond that was stronger than the strength of a carbon hydrogen bond in other words the energy differences in the UV spectrum corresponds to the string of bar and now if you think about services are you if we think about doing this I wrote you idea and if we think about later on and I think about a typical stretch well we've been talking a lot about carbon meals carbon yields observed at about 1700 wave numbers we said that wave numbers was centimeters per wave which made major wavelength is 117 hundreds of a centimeter in that slander and then you calculate your frequency out it's in the infrared range and you plug into equal dealt equals a chair and you find out that Delta III is equal before Oct . 8 7 calories promote and you say OK that kind of makes sense I know that infrared is lowered energy than you very slow on energy than visible I know that we don't have sufficient energy to break bonds in the are indeed all you're doing is picking them up a higher vibrational state you remember your energy curves with your vibrational states it takes many jobs before you get to the point that the dissociating well if we do the same for Panama let's say we take 500 megahertz and we clogging and again plug-in equals H then Delta III E is equal to 0 . 0 0 for 7 7 but it's not kcal per mole it's so the 1st thing when you see CMI MRI spectroscopy is you're getting deemed bad light because the technique involves very little energy absorbed another words when you absorb measuring the UV spectrum it's very easy for a detector to detect the energy of a photo and when you're measuring and I always spectrum it's very easy and already you're detectors have been much what more sensitive and it's going to get worse From there or not so we talked about dealt the equals age old what's new for a it's not planned terrible what's new role for a nucleus new is dictated by being a lot Over the 2 OK well so far so good I said we know what is the applied magnetic field so if you look at this say all of this kind of makes sense bigger applied magnetic field means bigger difference in energy right dealt the equals age times gamma being a over to apply that kind of makes sense all right let's just take a look at that 1 is that need there's a linear proportionality so if I again plugged into this equation by debt that 7 that's TSE's so another words if I just go ahead and plug in to this applied to this equation all come come back to of I had to a diamond the 2nd we find out that if we apply as 75 thousand on a 70 thousand 500 gallons magnetic field that leads to 300 megahertz for 1 h therefore go to higher magnetic field that leads to a higher frequency it's going to be in a linear fashion so if I apply 117 thousand 500 gallons magnetic field now we're at a 500 megahertz and more respect from her if you make a 300 megahertz Annamari spectrometer you have an electromagnet like this maybe this big superconducting magnet this big where you have a coil of wire with electricity passing through it in liquid helium in the wiry superconducting city electricity flows and flows and flows without any resistance a diminution and you get a strong magnetic field murder to build the technology to get a uniform 100 17 thousand 500 Dallas magnetic field you need a cattle about this bigger across about this high-tech how's the superconducting magnets the liquid helium and the shims and so forth and finally if you get to say 800 megahertz and of course it's all 1 proportionality you're going to have a 188 thousand hours magnetic field and that is closest to his biggest can currently be made uniform so now you have a magnet that Hester's even bigger and needs to have its own room in order to houses in flux lines that Calvary far out and the limits on commercial instruments these days are about 900 megahertz of the thing costs hours cost about for the whole whole thing about 2 and a half million dollars and by the time the 900 megahertz it's many many millions of dollars in medieval 1 of the things 1 gigahertz out there but we really for now at least seem to what think so we really seemed to have just pushed the limits of technology for how much how much electricity you can put a superconducting coil without it just ripping itself apart father sells the other quantity we have in this equation is Gamez called the Magneto job ratio sometimes you hear it referred to as the Giro magnetic Riccio This is a property of the individual nucleus the bigger the Giro magnetic ratio bigger than Magneto Giri gray she'll effectively the bigger the nuclear spin the bigger the magnet that the new focuses protons actually are good they have 1 of the biggest Magneto Jaric ratio of any nuclear I studied 26 thousand 700 in 50 and 15 3 1 month and year end so just to to put this into context at 70 thousand at 117 thousand 500 gallons in other words the relatively large magnet so at 117 500 doused you have the nucleotide flips it stands at 500 megahertz if we work at sea will get a job or magnetic ratio all of 2 out of 6 thousand 700 in 28 and that corresponds to absorbing energy at a frequency of 125 . 7 4 Major heard on this 117 thousand hours magnet so 1 of the implications remember I said you were dealing with a very small energy differences 1 of the implications is the energy differences are even smaller for carbon then for now you're getting Double-A damned for carbon because the natural abundance of C 13 is only 1 . 1 cent so most of Europe carbons on Stephen C. 13 indeed with small molecules most of your molecules don't even contained 1 C 13 In them we saw that mass back will you see the sea 13 isotopic murky and for the small molecules that we were looking at that he is small compared to the same 12 isotopic but you're getting again and again because it's small Jairam it's small Magneto Ji recreational leads to smaller energy absorption now the other thing you have to remember is even now you're recording you're see 13 NMR spectrum on "quotation mark 500 megahertz and Amara spectrometry you're not reporting carbon Meggett your carbon atom at 500 megahertz if you will you be that lucky person not in France but maybe on Mars who has access to a 2 gigahertz animosity that trumpeter and there ain't no such animal rights are flowing 19 isn't so bad is Magneto gyrate ratio was 20 thousand 25 thousand 179 so that corresponds with the same spectrometer to 470 thousand 470 . 5 8 megahertz usually it takes certain types of probe technology will talk more about that later but certain types of Quayle technology to to a higher frequencies and certain types of Quayle technology to tune in to lower frequencies so often if you want a really good proton NMR you will use a special parole where the protocol Quayle that's tuned for time he is in and close to the sample and the 2008 "quotation mark let's tuned for other nuclei is bigger and further away from the sample that that sort of won't be is good for carbon 13 because you have the Quayle's further away from the sample the quality of its good some 13 conversely if you find that dot that fell Dennison has put a broadband probe in the spectrometer will be the nucleus of the lower frequency it is inside in the coil you may find that the pope John and collects is not as good a is not a sensitive words not as sharp and Welsh him because the choir was further out fluorine is interesting because often you can use the same coils for fluorine and for code of and and for Proton phosphorus also has a smaller Magneto generic ratio it's 10 thousand 840 non-member fluorine and phosphorus have all of their naturally occurring nuclei as at 19 and all of the naturally occurring nucleotides P 31 so these are not dimmed by the Wall isotopic abundance the way phosphorus is another nucleus that sometimes study is deuterium a deuterium we talked about the nuclear quadruple you also have your locks Quayle in there the Magneto Gyula ratio for deuterium is 4 thousand 107 so that means lock frequency and the spectrometry is at 70 thousand 76 but is at 76 . 76 maker on the other With a quiet so the client probe technology is really wonderful what they've done in the quiet probe technology is they have cooled the probe and it's stupid guy I guess it's not a superconducting probe but what it is is a very low noise pro and because of the electronics of the probe recalled she don't get much electronic noise and the result is it's very high sensitivity and we were fortunate they had just when we bought it they just developed technology that had both carbon and proton sensitivity and basically was special coiled technology so that instrument is super good for Proton it's got a huge just incredible signal-to-noise ratio better than even the 800 megahertz spectrum it's also stupid of course I won't come back to these Magneto generic ratios because you've seen this with your own eyes now we already talked about the coupling the J-1 CD coupling chloroform and remember I said You see this 1 2 1 2 1 triple in the sea 13 Aymara and the separation of the lines is 30 to her well if you have a lot harder declares in the proton NMR so here we have our DEC coupling H 2 coupling but if you've ever looked hard at the chloroform peak in the proton NMR what you see is something like emitting pig where you're seeing the agent of course when you're looking at is quite form but you also see 2 peace here the better the sea 13 satellites and those correspond so this is your seat 12 people and those correspond to the J. coupling To the Sea 13 in other words what you're seeing here is a double-edged and the separation of those 2 lines is 209 hurts and the mathematical relationship between 2 0 0 9 and 32 is the same as the mathematical relationship the ratio between the 2 hot 2 thousand 26 thousand 753 and 41 0 7 no other words it's 6 . 5 in other words the Magneto Giri gray Schiele is 6 . 5 times bigger for proton Linford don't lie and we see that directly in the Jake coupling the effect of the magnet that the deuterium has in splitting the comment is 1 6 . the effects that the comment has been splitting apart time because coupling is neutral but the last thing I want to talk about I've talked about how damned we are by energy being loyal in the case of carbon about isotopic abundance but now the really damning thing ends up being the Boltzmann distribution that is the population all of the spin states In the question in the case of a benzene levels have your molecules are in the ground electronic state In the case of a key tone all of the Cardinals are in the ground vibrational state but in the case of nuclei the energy difference between the alpha and beta states is so small that both are populated and if you think back to your peak economic and you calculate the number in the beta state versus the alpha state that's going to correspond to the difference in energy Delta III over Katie team were carried is the Boltzmann can't and then if we just remember that got the eating equals age was equal to h times download times being Over to apply and then we say OK let's just take at 70 thousand 500 Dallas that's our 300 megahertz let's plug in NBA draft divided by an alpha is equal to unify plug-in that to the negative 6 . 6 3 times 10 to the negative 34 times the Magneto generic ratio 2 6 7 5 3 time 70 thousand 500 if applied magnetic field over to Part II divided by a planks constant of 1 . 3 8 times 10 to the negative 23 and let's say were saying at 2 98 Calvin so I say to 98 here and when I work that all out that I get is this number comes out to a quotient that's very very very close to 1 . 9 9 9 5 2 former lines and 5 2 corresponds to the ratio in the beta state over the ratio in the office today in other words we have 48 more protons out of 2 million so were all of your carbon nails are available to absorb the photon because remember when you apply a photon it can either pick a nucleus up from the ground state to the 1st to the from the office state to the beta state laws down from the beta state to the office space so it's only that differential population only that 48 out of 2 million that are available to observe if we apply a higher magnetic field it only gets when you your own mostly nearly better and 117 thousand 500 Dallas member that serve 500 megahertz then we only get to an invade over it in another words a relative population of Kuwait again from lines 9 9 9 1 9 in other words it always gets a little bit better it's only 81 protons now arms 2 million so we're damned by the low energies we're damned by the low differences in population and this is why animals are compared to other spectroscopic techniques is very insensitive and why it took a long time to develop next time we'll talk about how the animal a spectrometer works how we absorb our energies and and how we translate that into a spectrum and also talk talk a little bit maybe about some of the aspects of this