>> -- and get good at recognizing

and identifying the multiplets

and extracting a couplet in constants.

So the things that we are talking about and the question

that was raised: Is that coupling is really 700 Hertz?

What makes it 700 Hertz?

So let's talk about factors that J depends upon.

And I've tried to sort of group out maybe five factors

and give us some points.

So let's say one is the magnetogyric ratios

of the nuclei involved.

[ Writing On Board ]

And - so remember how when we started talking

about magnetogyric ratio, I gave you two examples?

We talked about the C13 satellites of chloroform

and protiochloroform, right?

So when you see the H1 anomer of CDCl3,

of course you don't see any of the deuterium chloroform that's

in there; but you see that 0.2% or 0.1% of protiochloroform.

And you see a peak that's very big at 7.26.

But then, symmetrically disposed about it,

you see these two satellites.

And so this is your CHCl3,

or more specifically your C12HCl3 peak.

But then your satellites come from your 1% --

1.1% -- of C13 CDCl3 - CHCl3.

And the spacing of those two lines, the distance here

between these two, is 208 Hertz.

And that distance happens to be 1.6.

Is 1.6 the - is 1 over 6.6 the distance that we see

in the C13 anomer of CDCl3, right.

In CDCl3, you do, indeed, see the C13, the 1% C13.

And you see this one-to-one-to-one triplet

centered at 77 ppm.

And the distance between the lines here is 32 Hertz.

And that ratio of 208 to 32 is equal to 1 over 6.5,

which is the magnetogyric ratio of proton

over the magnetogyric ratio of a deuteron --

meaning that when everything else is equal,

if you have a magnetogyric ratio nucleus

with a magnetogyric ratio that's big, you get big coupling.

If you have a nucleus with a magnetogyric ratio that's small,

you get small coupling.

For heteronuclei, we said that fluorine, for example,

has a big magnetogyric ratio.

So fluorine is going to have big coupling.

We said that carbon has a relatively small magnetogyric

ratio, so carbon in general is going to have small coupling.

So the gamma of the nucleis [phonetic] is important.

The number of bonds: So generally --

generally, generally -- we see coupling through one or two

or three bonds, sometimes four or five;

that's called long-range coupling.

So if we have some system w, x, y, z --

and we look at it if we are talking about coupling

through one bond, coupling through two bonds,

or coupling through three bonds --

and I need to make some wildly general generalizations,

I could say our J1's are on the order.

And I - you can just sort of look

at the appendix I handed out.

And if you want to keep a number in your head, I'd say -

let's say 30 to 300 Hertz.

Kind of one-bond couplings are big.

They are on the order of 100 Hertz give or take, you know,

a factor of half a log unit.

We saw you can have a 700-Hertz coupling in the phosphorus.

But generally, yeah, like 100 Hertz would be a good number

to keep in mind.

J2's are generally on the order of 0 to 20 Hertz.

And again, I'll give you examples that fall out of there.

But if you want to keep a number in mind, you know, 10 Hertz,

20 Hertz, something like that.

And J3's are generally on the order of 0 to 20 Hertz.

So in other words, one-bond coupling is huge;

two-bond coupling, three-bond coupling is smaller.

All right, other factors involved: geometry, particularly

in three-bond couplings

where good overlap leads to big coupling.

[ Writing On Board ]

Remember, coupling is carried by electrons in bonds,

where a nucleus feels it - another nucleus

through polarizing the pair of electrons making up the bonds

so that the one with its spin up is near the one with its -

nearer to the nucleus that spin down and so

that travels along the line.

And if you have good orbit overlap --

for example, an anti-periplanar relationship

or a syn-periplanar relationship --

you'll have a big coupling constant.

If you have bad overlap --

for example, a 90-degree dihedral angle --

you'll have essentially no coupling constants, no -

zero-coupling constant.

And if you are sort of at a low angle like a gauche angle --

you know, something close to 90 degrees, 60 or 120 --

you have still relatively poor overlap;

so your coupling constants are small.

That was what we were talking about the other day

with cyclohexane, where I said, "Yeah, for a tran -

for an axial coupling on the order

of 10 Hertz, 8 to 10 Hertz."

For axial equators, that's axial-axial.

For axial equatorial, or equatorial-equatorial

where your dihedral angle is

about 60 degrees, yeah, 2 or 3 Hertz.

All right.

So geometry also matters.

Hybridization.

[ Writing On Board ]

And a general rule is, more S character leads to bigger J's.

[ Writing On Board ]

And other factors: electronegativity.

[ Writing On Board ]

You know, hybridization is like Y in trans-alkenes.

We see a J of 17 Hertz, typically, versus in a one -

in a diaxial interaction on a 1-A axial-axial coupling

on a cyclohexane, which also has 180-degree dihedral angle just

like a trans-alkene; we only see a coupling constant

on the order of 10 Hertz.

In other words, you have more S character in an sp2 bond

and in a bond involving an sp2 carbon

and a bond involving an sp3 carbon.

So you have a bigger coupling constant.

So the other - another factor, let's say -

I'll just say other factors such as electronegativity.

[ Writing On Board ]

So for example, a hydrogen that's next to an oxygen that's

on a carbon with an oxygen will have slightly smaller J values,

particularly if there are two oxygens on there.

All right.

Let's - let me give you some typical J values,

and then we'll look at some examples.

And I don't want --

[ Erasing Board ]

And I want us to have a - you - I mean these sort

of generalizations that I just put up are pretty useful.

But unless you are doing a project with particular nuclei,

you may not end up keeping all the numbers in your head.

But let's just look at sort of some typical examples,

and these are actually in the appendix here.

So let's take a look at a four-O alkane.

So your J2HF -- and this is all in the appendix --

is really, really big.

It's, like, 44 to 81 Hertz, so even bigger than a sort

of generic two-bonds coupling that I talked about.

Fluorine has a large magnetogyric ratio,

so you have got very big coupling.

Your three-bond HF, your three-bond coupling,

is on the order of 3 to 25 Hertz.

[ Writing On Board ]

And fluorine is so, so good at coupling, right?

So this is two-bond coupling.

This is three-bond coupling.

Fluorine is so good at coupling

that you can even sometimes see a little four-bond coupling:

J4HF is on the order of, let's say, 0 to 4 Hertz.

[ Writing On Board ]

[ Inaudible Class Question ]

Part of it's the polarization, but part of it is the orbitals

that fluorine is used - using in bonding.

So if you have a hydrogen, right, we talked about two-bond

and three-bond H-ings [phonetic] coupling.

Right? So a typical two-bond H-H coupling might be -- I said --

14 Hertz, sort of, in an unperturbed system.

And a typical three-bond H-H coupling I said,

"Let's use 7 Hertz as a typical number."

So that hydrogen is contributing a 1s orbital.

But the fluorine is going to be contributing orbitals

that are going out further,

because it's using the second shell in coupling.

So we are in the 2 - you know, sp3 technically.

So your coupling is going to be bigger with fluorine.

So I don't think it's just -

and it probably involves some electric.

Yeah, actually, you know, it would involve electronegativity

as well, because it is going to be pulling those electrons

in really tight and that's going to give more interaction

of the nuclei with the electrons.

So yeah, I guess it's both of those.

Other questions or thoughts on this?

[ Inaudible Class Question ]

Of one over the other?

>> Yes.

>> No. I mean think about your Newman projection

of a perfect chair cyclohexane.

If I draw it as a Newman projection --

[ Writing On Board ]

-- I'll just draw half of the cyclohexane.

So this is axial.

This is axial.

This is equatorial.

This is equatorial.

And in a perfect chair cyclohexane,

this dihedral angle is 60 degrees;

this dihedral angle is 60 degrees;

and this dihedral is 60 degrees.

So it really should be about the same unless you flatten the ring

out or do something to perturb it.

[ Inaudible Class Question ]

Ah, OK. Do you ever see coupling that's essentially

through space?

One can, so outside of the realm

of covalent chemistry that we are seeing.

You can, for example, see coupling or cross-hydrogen bonds

and other situations where things are held

into close proximity with each other.

So for example, you can do ing-15 [phonetic] coupling

across DNA, for example, from one nuclear base to another

through the hydrogen bonds.

But just for a normal ordinary organic molecule where,

you know, it was in some confirmation,

not that I know of any examples.

[ Inaudible Class Question ]

Nothing where the molecules aren't locked together.

I mean I suppose you could come up with some example

where you take some poor molecule

and bang two methyl groups into it so hard that they are banging

in at van der Walls radius.

And you might.

I don't know the answer.

That's the sort of thing that one would try experimentally.

Other questions?

Benzenes are interesting with fluorobenzenes.

And you will have -- I think it's

on this coming Monday's homework set --

you'll have some fluorobenzenes.

And it's a little bit counterintuitive.

The problems are pretty easy.

They are out of Chapter -- what is it?

-- 5? 4? What's the one that I gave you to read that -

the coupling involving other nuclei?

Six. OK. Anyway, so keep this in mind.

So your ortho coupling, your J3HF,

is on the order of 6 to 10 Hertz.

And so that's kind of what you'd expect.

And what's sort of surprising: I mean in the case of hydrogens,

you have meta coupling, but it's usually on the order of a couple

of Hertz -- you know, 2 or 3 Hertz, 1 to 3 Hertz.

In the case of fluorine, your meta coupling is bigger

than you might otherwise expect.

It's on the order of 5 to 6 Hertz.

And then because we know that fluorine is very good

at coupling, you even see some para coupling,

and it's on the order of 2 Hertz.

And these are all kind of approximate.

[ Pause ]

Yeah, I mean the good news on all of this --

that Appendix F is just such --

[ Pause ]

-- such a treasure trove.

And so the good news is Appendix F puts a lot