Subtitles section Play video Print subtitles >> All right what I'd like to do today and on Monday is to talk about NMR spectroscopy and kind of how NMR spectroscopy works. I'll call it concepts in theory and for me what I want to do is give my perspective on NMR 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 that you quote need to do. So where NMR begins is with the concept that a nucleus of certain sorts and I'll just write a proton for now, has a spin to it and when you have a spinning charge it generates a magnetic dipole. And if you apply a magnetic field, we'll call that magnetic field B naught, then you have two different spin states or more and you'll see examples of this in the case of nuclear quadrupoles but let's start with the case of a proton or a C 13. You have two spin states that can exist, quanti-spin states. The spin of the nucleus can either be spin up, 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 I'll put, by up I mean aligned with B naught and if it's spin down meaning aligned against B naught then we're higher in energy and we'll refer to throughout our discussion. We'll refer to the lower energy state as the alpha state and to the higher energy state as the beta state. Now different types of nuclei have different spin properties. Rather than trying to start with generalizations about rules I'll come to those in a moment because at some point you'll be wondering in your project well could I study chlorine 35 or something like that, let's just start with typical nuclei studied. So if you go for example, to the 400 megahertz NMR spectrometer in my building in Natural Sciences 1, you'll find that that instrument can study protons. I'm going to write a couple of numbers for these. I'm going to write the atomic number and the mass number. And it can study C 13 and it can study F 19 and it can study P 31. And these are common nuclei that are often studied by NMR. They're easy to study. What do these nuclei have in common? They have a one-half indeed and what, forgetting their spin state what property on the blackboard do they have in common? >> Odd numbers of protons and neutrons. 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 odd. So nuclei with an odd mass number have a nuclear spin and the quantum characterization of nuclear spin is what's called a spin number and we'll call the spin number i. It really doesn't matter what we call it but they call it i and so that number is going to be one-half and that gives you all the ones up here but if we want a generalize more nuclei with an odd mass number will have a spin number of one-half or three-halves or five-halves, etcetera. So that's the more general idea. The ones with one-half are easy because they have what's called the nuclear dipole. If you have three-halves or five-halves or one as we'll see in just a moment you have what's called a nuclear quadrupole and then those tend to be harder. So all the ones here of i equals one-half have spin states so we have the quantum number and then the two spin states they can have and so the spin states are plus or minus one-half. So that's all of these H 1, C 13, oops, F 19 we'll come to nitrogen in just a second and P 31. Now a nucleus with a spin number of three-halves can have spin states of plus or minus one-half or plus or minus three-halves and this is what you call a nuclear quadrupole. Most of the time, many of the times nuclei with nuclear quadrupoles don't behave as if they're NMR active. In our next lecture we'll get to the concept of relaxation. Relaxation basically is how quickly you flip between the two spin states or in this case, between the four spin states or three in some cases and often they flip very quickly which means you can't study them by NMR. 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 I an example of a nucleus with a spin state of three halves, of boron there are two different isotopes. There are B 10 and B 11 and B 11 has, I think they both do but B 11 has a spin state of three-halves and if you look at the NMR spectrum of borohydride from this one what you see in the H1 NMR is you see four lines equally spaced and of equal height due to the hydrogens coupling with the nuclear quadrupole and it's very unusual because normally we think about splitting into a doublet or if you're thinking a triplet or a one to two to one triplet or quartet or one to three to three to one triplet, but what's happening here is the hydrogen C boron and they see either the boron having a spin state of negative three-halves or negative one-half or positive one-half or positive three-halves and so you see the four spin states and that gives are rise to four lines. All right but so let's look at some other nuclei with an odd mass number. [ Silence ] So one very important nucleus in biomolecular NMR is N 15. Nitrogen 15 has a spin number of i equals one and indeed N 15 is often studied. Most nitrogens, not N 15. We talked about this when we talked about mass spectrometry we said that the natural abundance of N 15 is 0.38 percent and that's really, really low. The isotopic abundance of C 13 spin active is one-and a half percent is 1.1 percent and you know that carbon NMR is not very sensitive. You need to have a reasonable sample size, more than you have for protein typically and sometimes often collect data for much longer. Well by the time you're down to .38 percent studying it at natural abundance is pretty hard so often you do this with isotopic labeling. Two dimensional N 15 based techniques are a mainstay of protein NMR spectroscopy and in general since most proteins are expressed these days what you do is you simply grow up your e-coli with N 15 ammonium chloride and they absorb that and use it to make up the amino acids and then you can get a fully N 15 labeled protein which is very useful. N 15 is starting to become more important in some natural product structure determination. Alkaloids as you may have seen for example in Neil Gard's [assumed spelling] talks have lots of nitrogens in them and so being able to figure out the positions of those nitrogens can be very important. In the case of something like an alkaloid or a synthetic project you might not be able to put N 15 in and NMR spectrometers are becoming more sensitive and so it becomes not completely nuts to think about using N 15 techniques in your NMR. At the end of the course I may talk about some two the dimensional techniques with N 15 at natural abundance that people are doing just because I think it's useful but that won't be until the end of November or December. Another common, well not common, another nucleus is oxygen, O 17 remember we said is only low natural abundance. It's only very low I should say. It's only.04 percent and oxygen 17 has a spin number of i is equal to five-halves so that's a nucleus that can have six spin states, negative five-halves, negative three-halves, negative one-half, positive one-half, three-halves, five-halves, etcetera and so it has sort of doubly damned and so it's not generally studied. All right so that takes care of our nuclei with odd mass numbers. Now the next class I'll talk about is if you have an even mass number and an even atomic number so that's easy those are nuclei like C 12, O 16 and the answer is very simple. Those have a spin number of i equal zero. They have no spin and those are NMR inactive. Since you don't have different spin states you can't have quanti-transitions between spin states so there's no way they can be studied by NMR spectroscopy. So the last class then becomes nuclei with an even mass number but not atomic number so that would include nuclei