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  • >> I want to move on and start talking

  • about 2-D NMR spectroscopy and what we're going

  • to do we'll be using this as a tool very,

  • very useful for structure solving.

  • There's a whole sort of alphabet soup of different techniques

  • but rather than just unleashing a torrent,

  • I mean people do research in this area just

  • like they do research in organic chemistry

  • and so big thing is invent a new technique

  • to solve specialized problems, but rather than trying to sort

  • of talk broadly about everything we're going to focus

  • on getting a few tools in our toolbox and see how

  • to use these techniques to address different problems.

  • We'll start out with 2 tools in the toolbox that will be HMQC

  • and COSY techniques and then we'll add some more tools

  • and I'll try to put them into some sort of context.

  • There are 2 additional lectures that aren't specifically on 2-D

  • that will come in either possibly next time or the time

  • after that so we'll be talking specifically

  • about the Nuclear Overhauser effective, which applies

  • to 1-D NMR as well and we'll be talking about dynamic NMR

  • and dynamic effects in NMR spectroscopy,

  • but we're going to start.

  • Our next homework set will start to bring in 2-D and I'd

  • like to get you familiar with the tools.

  • All right theory I'm going to start really simple minded

  • and I think this is actually a good way to think about things.

  • So, in 1-D, we said the basic idea was your pulse

  • and then you observe, that's your 90-degree pulse.

  • The observe is your FID.

  • Have you now seen your FID on the spectrometers?

  • Have you seen the little wiggly, squiggly cosine wave

  • with a die off [phonetic].

  • So this is your FID and, of course,

  • what you've got here is an amplitude domain and then

  • over here you have time.

  • This is literally your signal dying off with time

  • and the cosine wave that corresponds to the periodicity

  • of the various nuclei.

  • So the whole idea in 1-D Fourier transform is this time domain

  • on the X axis ends up getting transformed

  • to a frequency domain and that's your parts per million

  • and so your spectrum still has amplitude on the vertical axis

  • and it has frequency in the units of PPM

  • on the horizontal dimension

  • and the reason we call this 1 dimensional NMR spectroscopy is

  • not because this is a 1-D graph, it's not,

  • you'd say this is a 2-D graph.

  • It's because you have 1 time dimension

  • and that gets transformed to a frequency dimension.

  • Now, in 2-D NMR, you get 2 time domains, 2 time dimensions

  • in the FID and they get transformed

  • into 2 frequency domains.

  • So I'm going to give you just

  • as I have given you my simplified version

  • of an NMR spectrometer, an IR spectrophotometer

  • and a mass spectrometer and so forth.

  • I'll give you my simplified version of a 2-D pulse sequence.

  • A 2-D pulsate sequence is going to be pulse weight pulse observe

  • and so what you do when you do this is you get 2 time

  • dimensions because the weight is you're waiting for some time,

  • you're going to vary the weight and then you observe.

  • So this first weight becomes time 1 and we'll call that t1

  • and the second weight becomes t2.

  • Now these are not to be confused

  • with the capital Ts we talked about for relaxation.

  • Remember we talked about Capital T1 is vertical is spin

  • relaxation where the magnetization returns

  • to the Z axis and Capital T2 is spin lattice relaxation

  • where the magnetization spreads out in the X, Y plane.

  • These are lower case ts and they in turn transform

  • when you do a 2-D ft they transform to 2 frequency domains

  • and so you get a spectrum that might look like this

  • where you have 1 domain here and this is called your f2 domain

  • and then another domain here and that's called your f1 domain.

  • Now what does this mean?

  • As you're varying, well, you understand here, of course,

  • in t2, you're collecting a signal

  • and it's dying off with time.

  • So you understand that basic transform

  • that if the periodicity of this signal is 1 cycle per second,

  • we get a line at 1 hertz and if the periodicity

  • of this line is 2 cycles per second, you get a line

  • at 2 hertz and if it's a composite of 1 cycle per second

  • and 2 cycles per second

  • and others you get a spectrum consisting of many lines.

  • Now similarly as you vary this t1 let's say starting

  • with hypothetically a millisecond

  • in the first experiment,

  • then the next experiment 2 milliseconds,

  • the next experiment 3 milliseconds, the next 4.

  • Another periodicity occurs.

  • In other words, your FID what you observe

  • in this time also shows variation that occurs in time.

  • Variation, amplitude, a periodic variation.

  • Those variations transform to the second frequency domain

  • and so you get a spectrum now that consists

  • of 2 frequency domains.

  • It is, of course, plotted 2 dimensionally

  • but it is really just as this is actually a 2-D graph this is 3-D

  • graph if you will and typically these days the way we express it

  • is as a topological map so you'll typically see a series

  • of contours that's just like if you've ever seen a topographical

  • map of the mountains each contour represents a

  • certain height.

  • So a very tall peak has many contours

  • and a short peak has fewer contours.

  • So it's 3 dimensions being represented being projected

  • in two, but again the reason we call this 2-D NMR is not

  • because there are 2 dimensions in the graph but rather

  • because there are 2 time dimensions.

  • All right that's what I want to say about sort

  • of the basic mechanics of the experiment.

  • There are 2 general types of 2-D NMR experiments.

  • One of these experiments is one of these families the one

  • that we'll be talking mostly about,

  • is correlation experiments.

  • Correlation means connectivity.

  • It means literally what's connected to what.

  • Another way of thinking of this is coupling.

  • It can be proton-proton coupling,

  • it can be proton-carbon coupling,

  • that's what correlation experiments give you

  • information on.

  • You've already been using this type of information

  • from coupling patterns and coupling constants.

  • When you see a triplet here, you say, oh, that's a methyl group

  • and then it integrates the 3 hydrogens you say, oh,

  • that's a methyl group that's next to a CH2 group.

  • When you see a quartet here and it integrates to 2 hydrogens,

  • you say, oh, that's a methyl group that's next

  • to 3 hydrogens.

  • Maybe it's next to a methyl group and correlations give

  • that same type of information.

  • When you see a 17 hertz coupling in a trans alkene, you say, oh,

  • that 17 hertz coupling must have a partner somewhere.

  • Ah, here is its partner that also has a 17 hertz coupling.

  • So you're already using connectivity information

  • in helping to deduce your structures.

  • Two-D experiments provide that information

  • in a more general term.

  • The other type of 2-D experiment that we'll be talking

  • about are Overhauser effect experiments.

  • We'll be talking more about the Nuclear Overhauser Effect

  • in a couple of lectures.

  • Those give rise to information on spatial proximity.

  • [ Writing on board ]

  • These can be very useful for information

  • about stereochemistry and conformation.

  • All right my philosophy on teaching 2-D NMR spectroscopy

  • as I said before there's a whole alphabet soup

  • of techniques out there.

  • My philosophy is not to bombard us but to give us a small box,

  • a small tool box of what I'll call core techniques.

  • In other words, techniques that if we are good at we can use

  • to solve a variety of problems and then if you're good

  • with those techniques you'll be able to say oh here's a whole

  • in my tools where I have a very specialized problem

  • that isn't being solved by these tools and you can go

  • to Phil [phonetic] or go to the NMR manual and say, oh,

  • I'm encountering this particular problem with a COSY

  • and A Toxi [phonetic] isn't helping me out

  • but I remember him saying something

  • that there was some type of technique called a relay COSY

  • and saying I can add that to my toolbox.

  • So, okay, the first 2 tools that we'll be talking about are COSY,

  • which was really the first main 2-D technique.

  • It stands for correlation spectroscopy.

  • So this is typically proton-proton

  • or let's just say homo-nuclear coupling

  • and then the second technique that we're going to add

  • to the toolbox is HMQC and this is heteronuclear correlation.

  • Well, I should say something.

  • So we're learning about the modern versions

  • of the experiments.

  • HMQC uses something that's inverse detection.

  • That means on the f2 dimension you're detecting proton

  • and on the f1 dimension you're detecting carbon.

  • The older, less sophisticated version

  • of this experiment was called het core [phonetic].

  • I'm going to put it in parentheses

  • but that's not really, it's not the same thing.

  • Het core was heteronuclear correlation spectroscopy

  • and now that's what you'd call HMQC.

  • Het core was an experiment where you would collect carbon data

  • on the f2 dimension and proton data on the f1 dimension

  • and it was a slower, less-efficient experiment.

  • So we're going to start with these 2 techniques

  • as our initial starting point for building our toolbox

  • and we'll see that they're extremely powerful.

  • We're then going to add in Toxi [phonetic].

  • Toxi is what stands for total correlation

  • and I'll put that in quotes.

  • It's like a super COSY that gives cross peaks

  • with all other nuclei in the spin system.

  • I'll show it to you today but you won't have the, you won't

  • yet have the experience to see where it's useful.

  • We'll bring in some problems later on, but I don't want

  • to bombard you with too much and HMBC is sort

  • of a long range het core.

  • In fact, that's the version

  • of the experiment that it used to be.

  • It is basically these two experiments are conceptually

  • more complicated because initially you're going

  • to say what do I need them for and it gives you a ton of data

  • but when you start to encounter specific problems of overlap

  • in the case of the former and in the case of the latter fragments

  • that you can't put together they'll be very helpful.

  • So all of these are correlation techniques

  • and then we will also throw into the mix

  • of core techniques NOSY and ROSY.

  • These are both Overhauser effect experiments.

  • [ Writing on board ]

  • They both give rise to information on proximity.

  • NOSY is good for molecules that are small and molecules

  • that are very large, but there's a whole right in the middle

  • of medium-sized molecules that don't work well in it

  • and ROSY ends up working well with medium-sized molecules.

  • [ Pause ]

  • All right.

  • Let's start with COSY and HMQC

  • and let me just show you the general gist

  • of the 2 experiments.

  • So let's start with COSY.

  • Imagine for a moment that you have propanol and so

  • if you think of your H1 NMR spectrum

  • of propanol you'll probably think

  • of something that looks like this.

  • You'll see a triplet with a 1 to 2 to 1 triplet

  • for the CH2 that's next to the oxygen.

  • You'll see a singlet for the OH typically unless you're very

  • free of acid or very free of water and the singlet is going

  • to correspond to the OH that's going to be exchanging rapidly

  • and not coupling unless you, as I said, are very acid free.

  • You'll see something that looks kind of sort of like a sextet

  • in a 1 to 5 to 10 to 10 to 1 ratio.

  • I guess that's not the prettiest of sextets.

  • Let me make my outer peaks a little smaller.

  • Then you'll see something that looks like a triplet

  • in a 1 to 2 to 1 ratio.

  • As I said, you already know correlation.

  • You know that when I see this triplet here downfield it's

  • telling us that I have 2 hydrogens, it's telling us

  • that I have a CH2 next to a CH2 and when I see this triplet

  • up field I see, I know that I'm having a methyl group and I need

  • to go off 3 hydrogens.

  • I'm having a methyl group and it's next to a CH2.

  • When I have this sextet here,

  • you know that I'm having one methyl group is it's 2 hydrogens

  • and by being a sextet I know it's coupling

  • with equal coupling constants to find different hydrogens.

  • So for this simple problem you're very good

  • at reading this.