Subtitles section Play video Print subtitles >> I want to spend the next 3 lectures, 3 classes, talking really closely about first order coupling and the reason is that there's so much to be gained by deeply understanding NMR spectra. As I said, a lot of what one is going to be doing is asking specific questions about stereochemistry and being able to ask those questions is intimately linked to understanding what's going on. Also just in general for solving structures, being able to read spectra, really read them at a level that goes beyond the level of sophomore organic chemistry, involves intimately understanding [inaudible]. So we're going to take a relatively slow path through this. In fact, we're going through the midterm exam only have 1-d spectra on our exam so that we really focus on understanding things. So I want to start by kind of making the bridge between last time's lecture where we talked about magnetic equivalents and we talked about non-first order systems and so last time was sort of the bad and today is going to be the good. So the bad is that I said a lot of the rules that you learned in simple sophomore organic chemistry really were oversimplifications. There are very few systems that truly behave in the way that you learned they should behave. These are the first order systems. So first order systems are anything like AX systems, AMX systems, A2MX. In other words, anything where coupled protons, protons within a spin system are far apart in chemical shift and if you do have 2 protons that are chemically equivalent like we have in A2MX system that those protons are both chemically equivalent and magnetically equivalent. We divided this and separated it from non-first order systems. [ Writing on board ] And these are systems in which you either have magnetically inequivalent protons that are chemically equivalent or you have protons that are similar in chemical shifts. For example, a non-magnetically equivalent protons we saw, for example, A, A prime, X, X prime systems and we talked about just how ugly those systems could be. Those were like the phthalate [phonetic] system where I said no matter how far apart, no matter how high a magnetic field you look at dioctyl phthalate or ortho dichlorobenzene is never going to get better than this complex pattern of lines and then I said we have other systems like AB systems where the protons are similar in chemical shift and ones that are related to this, for example, ABX systems. The good news about many of these types of systems is that many of these non-first order systems behave very much like first order and that you can start to apply some type of simple, rational understanding to them, which is more than I can say for an AA prime system, XX prime system or an AA prime BB prime system. Now sometimes these systems will look like first order, which is great because sometimes you can analyze these types of systems as first order and many times you can, but what I tried to show you last time was how there are ones that simply defy simple reduction. >> So what do you use X or Bs based on the distance or the separation of chemical shift not the actual distance between them? >> So let me show you exactly and let's take the AB system because I think this is a great starting point and what's nice is the AB system is going to be an archetype for many sorts of systems that although they're not first order we can apply first order analysis to and we can start to see the distortions that occur. So, a pure AX system is one in which you have a doublet so it's 2 hydrogens that are J coupled. Again, that's going to be the whole spin system so I'll just put on XX and YY to represent some other nuclei that aren't going to couple and not, of course, something with a hydrogen on it where it's J coupling. So you would have a doublet and then a big, big span between it and then another doublet. This little squiggly is just a break, break in the spectra. If those 2 doublets are far apart in chemical shift, then you're going to see them each as a simple 1 to 1 doublet. Now as the distance between them becomes smaller. In other words, either you have different substitutents that instead of having them be very far apart they're closer together in PPM or you simply went to a lower field spectrometer, now you start to see a distortion that we would call an AB pattern where the inner line, and so now instead of saying these are effectively very, very far apart now I'm saying they are far apart like so. In other words, this means, you know, 1 here and 1 way over there. Okay, now the typical way in which one characterizes this is the distance between these line sis the J value, the distance between these doublets and technically one takes not the dead center of the doublet but the weighted average because technically with a multiplet the position of the multiplet is not at its average but at its weighted average. In other words, since this line is a little bit bigger we take the center as just a hair over. It's the weighted average. In other words, if this line is 4 times as, if these lines are in a 4 to 3 ratio and they're separated by .07 PPM, we'd say, all right, you're .4 of the way over there; just a little hair. So, if we call this distance Delta nu, typically if Delta nu over J is much, much greater than 10. [ Writing on board ] We're in the situation like this and if Delta nu over J is less than or equal to 10 and those are approximations, then we're sort of into this AB situation. By Delta nu I mean the difference in position in hertz. So in other words, let's say the center of this line was at 7.30 PPM and the center of this line was at 7.10 PPM and let's just say here that our J value is let's say R, what will work out? What will work out well? Let's say that our J value equals 17 hertz. Now, imagine for a moment you're on a very low field spectrometer. Imagine you're on a 100 megahertz spectrometer what's Delta nu at that point? [ Pause ] 730 hertz. Everyone agree? >> Delta [inaudible] 20 hertz. >> Delta, 20 hertz. So at 20 hertz these guys would be hugely close together. In fact, we'd have a situation that looked. [ Pause ] Like this. At this point Delta nu actually will be just a hair further apart because it's the weighted average. I'm going to shift it over just a hair. I'll make the outer lines just a little bit bigger. This would be a situation where Delta nu over J is very small where Delta nu is about 20 hertz and J is about 17 hertz. If we had the same system at 500 megahertz, what would the difference in, what would Delta nu be for 500 megahertz? [ Pause ] A hundred hertz, right? So at 500 hertz, 500 megahertz, Delta nu is equal to 100 hertz. So you'll look at this situation and at 500 megahertz you'd be more like this and 100 megahertz you'd be like this. So this is your AB pattern and if they were ever closer they'd be like what I sketched out before where the inner line would be huge and the outer line would be very tiny. What? That might, it would be like a 60 megahertz spectrometer like one of the freshmen or sophomore and we actually have like a 100 or maybe it's 60 in the sophomore lab it would be like this or imagine the situation that instead of having substituents that put these apart at .2 PPM, imagine they were separated by .1 PPM, but the main thing to keep in mind is for any given doublet no matter what the center of this peak whether I looked at it at a 500 megahertz spectrometer or at a 100 megahertz the center of this peak is going to be 7.30 and the center of this peak, again, weighted average center is 7.10. Now 17 hertz is more characteristic of a trans alteen [phonetic], which was actually what I was doing when I was drawing this. For something like this we'd be more like about 7 hertz for a J value. Thoughts or questions at this point? [ Inaudible question ] Okay, will the center move, so, if you improve the equipment? So here we've gone from this is our 100 megahertz, this is our 500 megahertz and the point is the center of this peak for this whatever hypothetical compound this is, the center of this peak is always at 7.3 PPM whether I'm at 500 megahertz or 100 megahertz, but the distance between the peaks because the number of hertz per PPM is much smaller at 100 than at 500, the distance between the peaks here is very far, it's 100 hertz apart or relatively far, and over here it's only 20 hertz apart. [ Inaudible question ] The inner one? The closer they are together the more they tent into each other and that really is the difference between the AX. [ Inaudible question ] The center is related to the ratio of the bigger. [ Inaudible question ] Absolutely. Absolutely. Well, here the bigger one is at 7.29 PPM or 7.29 PPM and the smaller one is at 7.31 PPM and by here we've got these 2 lines and one of them is at 7.2 PPM and the other is at 7. whatever the number is. Now what's valuable about looking at AB pattern and understand it is it really becomes an archetype for all sorts of systems that behave very near to first order. So we were talking before about phenylalanine and I guess the example I gave when we were talking about spin systems was acetyl phenylalanine methyl amide. [ Pause ] And I pointed out that we had 1, so this is like a spectrum in chloroform solution so I'll say NCDCL3, and we decided that we had 1 spin system over here and the multiplicity of this proton of the NH is a doublet because it's split by 1 coupling partner. Each of these protons they're non-chemically equivalent so they split each other but they're going to be similar in chemical shift, they're similar in the environment so they'll both be at about 2 and a half, 3 parts per million. Why do I say about 3 parts per million? Well, they're off of a phenyl group so if we were methyl group off of a phenyl I'd say 2 parts per million. It's an ethylene so that pushes it to like 2 and a half parts per million. They're beta to a couple of electron withdrawing groups, they're beta to a nitrogen, they're beta to a carbonyl, so then it's going to shift them downfield by about another half a PPM. So we'd expect them to both be at about 3 parts per million but probably not to be on top of each other. So each of these is going to show up at as a DD and that DD is going to be part of what looks like an ABX pattern because this is part of an ABMX system. M is something that's far apart from either A and B and C and so forth and X. So we have 1 proton that's going to be way downfield and nitrogen protons are typically at about 7 parts per million. One proton that's going to be moderately downfield because it's next to an electron withdrawing group and it's alpha to a carbonyl and beta to a phenyl group so this is going to be about 4 and a half parts per million. Then these guys that are both going to be close to 3 parts per million. So we have far apart from this and H far apart from alpha and the alpha is far apart from the beta. So this guy here is going to be split by 3 different protons. So he's going to be a DDD if all of the Js are different or a TD, and we'll talk more about these or DT, if 2 of the Js are the same. Or a quartet if all 3 Js are the same within the limits of experimental error. [ Pause ] So the one I really want to draw our attention to then is these 2 hydrogens here because now this type of AB pattern really can serve as an archetype for more complex patterns that are non-first order but are close to first order. So an ABX pattern is something where you have the AB pattern in which each line is further split.