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