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.