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  • >> 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.