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  • >> All right, today we're going to talk

  • about two different things.

  • We talked about proton chemical shifts and today I want

  • to talk briefly about C 13 NMR shift.

  • We'll talk more about them later on but then we're going

  • to start, we're going to spend a reasonable amount

  • of time talking about spin-spin coupling and in order

  • to understand this we really have to understand the concept

  • of chemical equivalence which ties into concepts of symmetry

  • and stereochemistry and confirmational analysis

  • and it's really beautiful, chemical equivalence

  • and so we'll be talking about chemical equivalence

  • and spin-spin coupling.

  • We're actually going to be spending a good deal of time

  • because there's a lot to understand

  • and as you can already see from the problems people are saying,

  • hey what's going on here?

  • And these problems these very simple molecules have all sorts

  • of cool issues of spin-spin coupling and all sorts

  • of cool issues of stereo chemistry

  • so we're actually going to spend a number of lectures on them.

  • Next time we're going to develop a concept called magnetic

  • equivalents which is an amplification

  • on chemical equivalents but it's too much to take in on one

  • and then we're going to spend a couple of times talking

  • about details of spin-spin coupling.

  • All right, what do I want to say

  • about carbon NMR spectroscopy first of all?

  • All right, if proton NMR was difficult

  • because you have a very small population between your alpha

  • and beta states carbon NMR is even worse and first you know

  • that carbon 12 doesn't have an NMR spectrum.

  • It's not active.

  • It doesn't have a magnetic dipole

  • and we only have one percent

  • or more specifically 1.1 percent C 13, so most of your molecules,

  • your small molecules don't contain any C 13.

  • For small molecules some of them contain one C 13.

  • Now things get worse.

  • The magnetogyric ratio for C 13 is only a quarter of that

  • of the magnetogyric ratio for a proton.

  • Now remember what the implications are of that.

  • That means that you get roughly a quarter, I'll use tilde

  • to say approximately a quarter of the Boltzmann difference,

  • say difference in Boltzmann distribution in alpha

  • and beta states, so already we've got fewer nuclei

  • that can flip up.

  • To put it another way a 500 megahertz spectrometer gives you

  • a carbon spectrum at 127.5 megahertz, a hair over a quarter

  • because the magnetogyric ratio is a hair over a quarter.

  • Things get worse than that.

  • Your dipole is only a quarter as strong.

  • Guess what?

  • If you want to generate an electric current

  • like a generator you want a big hunking magnet

  • to spin in a coil.

  • Proton is a little magnet but a carbon is a tiny magnet

  • because it's got a quarter of the magnetic dipole.

  • So you're damned again and then you further get damned

  • because the procession rate is also a quarter

  • since for a proton at that same 117,500 gauss magnet you're

  • processing at 500 times per second.

  • For carbon you're only processing at 125 million times,

  • did I say thousand, million times per second.

  • So that also gives you a quarter as much electricity in the coil.

  • So you've got 1.1 percent and a quarter of a quarter

  • of a quarter which means you're only 1/5,800th as sensitive.

  • Question.

  • >> Is the procession, are you talking

  • about the limiter frequency?

  • >> The limiter frequency, yeah.

  • So if you take a magnet and just spin it in a coil,

  • if you spin it faster you get more voltage.

  • If you spin it twice as fast you get twice as much voltage.

  • If you take a magnet that's twice as big you get twice

  • as much voltage and so for all of these reasons and you've got

  • in addition to a smaller magnet you've got fewer of them

  • because you've got, even

  • if you give a 90 degree pulse you get only a quarter

  • of the magnetic dipole from having only a quarter

  • as many nuclei going down into the X, Y plane.

  • >> Is that a quarter of all or is

  • that a quarter of what's protons?

  • >> It's a quarter-- compared to protons.

  • So carbon is much less work, much less sensitive technique

  • than proton NMR spectroscopy.

  • Now there are few redeeming features

  • so one thing that's redeeming is we typically do proton

  • decoupling, so normally a carbon would be split by all

  • of the protons so for example,

  • the carbon in ethanol would be split into a quartet

  • in the carbon and the methyl group of ethanol, would be split

  • into a quartet by the three hydrogens that are attached

  • to it and then it would be further split by the hydrogens

  • over on the methylene carbon.

  • But what we do is we irradiate the proton

  • so all the carbon you're going to see,

  • virtually all the carbon you're going

  • to see is called proton-decoupled carbon.

  • That flips the spins of protons rapidly

  • which means the carbon doesn't see them as spin up or spin

  • down so the carbons appear as singlet.

  • Well that's good because that means all your carbon signal is

  • gathered in one peak so that gives you more intensity.

  • Now the other thing is when you do that, so that leads

  • to singlets which is good, sharper,

  • bigger and the other thing it leads

  • to is what's called the Nuclear Overhauser effect

  • and we'll talk more about this as a technique

  • but the basic principle of the Nuclear Overhauser effect is

  • that by perturbing the alpha and beta states

  • of the protons you end up enhancing the difference

  • in Boltzmann population between the alpha and beta states

  • of the carbon, so that too gives you a bigger signal,

  • so all of this leads to a better signal

  • than you would otherwise get in a proton non-decoupled carbon.

  • All right anyway, suffice it to say now days it's easy

  • to collect a carbon NMR spectrum.

  • It typically will take more sample

  • so you can collect the proton NMR sample on strychnine

  • and use a milligram of material or even tenths of a milligram.

  • For a carbon you might want to put 30 megs

  • in the NMR tube if you have a chance.

  • You could do it at ten.

  • You could do it at a milligram but it takes a lot more time.

  • And remember if you have one milligram

  • versus ten milligrams it's going to take a hundred times as long

  • to collect the same signal to noise ratio which means

  • if you're in your research lab

  • and you have some sample it actually makes sense

  • if you're trying to collect a carbon spectrum

  • to weigh your sample or at least be cognizant of how much you put

  • in your NMR tube because you want

  • to get your spectrum quickly and you want

  • to get good signal to noise ratio.

  • It also makes sense when you're filling your NMR tube

  • to only fill it with the appropriate amount for the coil.

  • The coil on our [inaudible] is three and a half centimeters

  • or requires a three and a half centimeter high sample.

  • That's a half a mil, so if you dissolve your sample don't

  • dissolve it in a mil and a half.

  • Don't try to be clever and dissolve it in .3 mils

  • because then you miss up the shims on the spectrum

  • because you get flux lines at the end of the sample.

  • So anyway that's the way to get good spectrum.

  • All right, I want to talk about where the peaks show

  • up so carbon NMR spectrum,

  • the carbon NMR spectrum has a big range typically

  • from about zero to about 200 or 200

  • and change parts per million, 220, 240 parts per million.

  • Aliphatics show up at about 10 to 40 so on that big range

  • of 200 or so ppm that's in the up field region.

  • Carbons next to an electron withdrawing atoms show

  • up down field but it's not quite as pronounced so carbon next

  • to a halogen you might even see it in this range,

  • carbon next to a nitrogen.

  • It's going to be sort of at the end of that range

  • but by the time you're next

  • to an oxygen is an electron withdrawing group I'd say 50

  • to 70 for a carbon next to one oxygen

  • so that's sort of stands out.

  • Alkines aren't that common but remember I said there's

  • about two and a half parts per million,

  • 2.2 parts per million maybe for a typical alkine CH.

  • For an alkine carbon it's about 70 to 80 ppms

  • so that kind of stands out.

  • All right, alkenes and aromatics whereas

  • in proton NMR the alkenes show up a little bit more up field,

  • 5 to 6 and the aromatics a little more down field 7 to 8,

  • alkenes and aromatics are all sort of lumped

  • in at about 110 to 150.

  • Beyond about-- and of course these are all typical values.

  • If you put in oxygen on an aromatic like anisole or phenol

  • where you put an oxygen directly on a carbon that carbon might be

  • at 160 parts per million.

  • If you have a very electron donor--

  • we'll talk more about specific shifts but you could easily

  • if you have high electron donations say an ortho carbon,

  • you could easily be a little below, 115 ppm.

  • All right carbonyls, esters

  • and carboxylic acids you get a little bit of a resonance affect

  • so they show up a little bit less far down field than some

  • of the others let's say about 170 to 180 parts per million.

  • Aldehyde show up further down field, let's say about 190

  • to 200 ppm and ketones I'll say are RCOR prime for ketones,

  • let's say about 205 to about 220 ppm.

  • Bless you.

  • I just want to give you one little handout.

  • By the way, if you ever miss your handouts

  • or misplace them I put them up on the web

  • with the video part course so you can always go ahead

  • and download the handouts.

  • All right so just as I had my-- does anyone else need one?

  • Just as I had my little pigeon drawing of my take of what

  • to look at in reading a proton NMR spectrum I have my little

  • pigeon drawing of what to look

  • at when reading a C 13 NMR spectrum.

  • In other words, this type of region is aliphatics.

  • Here you have a carbon next

  • to a significantly withdrawing group like an oxygen.

  • Here's your alkene aromatics.

  • Here's your esters and acids then you go

  • to aldehydes and ketones.

  • So to a large extent it's sort of like H 1 NMR

  • but with maybe a factor of 20 on the scale.

  • In other words most of what you're going to see

  • on a proton NMR is from zero to ten.

  • Most of what you're going to see

  • in carbon NMR is from zero to 200.

  • Carboxylic acid will be further out.

  • Ketones will be further out but that kind

  • of gives you my read on things.

  • All right this should give you the basics to start

  • to use carbon NMR in helping to analyze some

  • of the homework sets that we're going to get.

  • So one thing that you can get is a reading of things

  • like carbonyl groups in there and another thing you'll be able

  • to do is count up and see how many different types of atoms

  • because whereas in proton NMR you may have overlapping

  • resonances most often because the carbon spectrum is more

  • dispersed and peaks typically show

  • up as singlets most often you will be able to see one peak

  • for each type of carbon.

  • All right I want to talk now about spin-spin coupling.

  • And if I had to give you a very, very general way of thinking

  • about it the way I'll describe it and we're going to amplify

  • on this in today's talk, the way that I would describe it is

  • that protons that peaks are split

  • by adjacent protons that are different.

  • We'll later on amplify on the concept of adjacent talking

  • about two bond coupling, three bond coupling

  • and long-range coupling.

  • But today what I'd like to amplify on is the concept

  • of same and specifically different.

  • So let's start with an example that's very, very,

  • very intuitive, very, very, freshman,

  • sophomore rather chemistry.

  • Let's take the H1 NMR spectrum of chloro ethane

  • and so my little pigeon sketch of this spectrum

  • of chloro ethane would look something like this.

  • We have a one to three to three to one triplet somewhere

  • down field of three ppm or just a hair down field of three ppm

  • and then we have, I'm sorry, one to three to three to one quartet

  • and then somewhere just hair down field of one we have a one

  • to two to one triplet.

  • The triplet comes of course from the CH3 and I can say

  • that in this particular example CH3s are split

  • by the adjacent CH2s and I can say because the three hydrogens,

  • well let's talk about the CH2s.

  • The CH2s are split by the CH3.

  • The three protons of the CH3 group are the same