So chemical shift is the idea of very quickly

that was introduced, which I saw with James sort

of a light bulb go on, that the frequency

at which a proton resonates is going to be proportional

to the applied magnetic field.

So, for example, tetramethylsilane

at a 70,500 gauss magnet undergoes procession,

the protons under procession or flip their spin

at 300 million cycles per second.

If we take that same molecule of TMS and put it

in 117,500 gauss magnet, then TMS undergoes procession

and flips its spin at 500 million hertz,

but what happens is, okay, so if we now have just sort

of a plain vanilla methyl group, so not TMS, not a methyl group

on silicon but a methyl group on acyl chain,

the methyl group is going to undergo procession

at approximately 300 million, 300,

later on I'll be saying it's closer to 300,000,270,

but we'll just use 300 million for round numbers,

at 70,000 gauss magnet

and at the 117,000 gauss magnet it's going

to undergo procession at 500,000,500 hertz.

So rather than saying, oh,

at a certain magnet we're 300 hertz downfield of TMS

and a different magnet we're 500 megahertz downfield of TMS,

we can just normalize and say in both

of these cases we are 1 PPM downfield,

downfield means higher frequency than TMS.

So that normalization allows us to compare the frequencies

of protons regardless of the magnet that we're using and,

of course, if we go ahead, the math is really simple here.

So if I tell you that the methyl group

in methanethiol undergoes resonance 600 hertz downfield

of TMS and I asked you how many hertz would it be

on the 117,000 gauss magnet how many would it be?

>> One thousand.

>> One thousand, exactly,

but in both cases it would be how many PPM?

Two PPM. So when you look at the X axis of an NMR spectrum

and remember I said we transformed our time axis

in the FID to a frequency axis, you now know 1 PPM,

the span from 0 to 1 or 1 to 2 or 2 to 3,

corresponds to 300 hertz on 300 hertz,

300 megahertz NMR spectrometer.

It corresponds to 500 hertz on a 500 megahertz spectrometer

and conversely since coupling constant is independent

in frequency and we'll get to that later on,

versus of the applied magnetic field that triplet

of say a methyl group and ethanol is going

to look tighter, it's going to look more close together

on the 500 megahertz NMR spectrometer

because that triplet is still going

to be 7 plus 7 is 14 hertz wide, but 14 hertz wide

on a 300 megahertz spectrometer is 14-300ths

of a PPM whereas 14 hertz wide

on a 500 megahertz spectrometer is 14-500ths PPM.

So, instead, here you'll spend 2-100ths of a PPM

and if I'm doing the math right in my head

and here we'll spin a little less than 2-100ths of a PPM

so the peaks will be tighter and more dispersed

in a higher field spectrometer.

All right chemical shift depends on the electronic environment

that the protons are in and this is what the physicists were

so upset and why they gave it this contemptuous name.

If you have an element

that pulls electron density away from the protons.

So, for example, sulfur is a little bit electron withdrawing.

It's a little bit electronegative relative

to carbon and so you pull electron density

and then hydrogens, which are shielded by the electron cloud

around them, the electrons oppose the applied magnetic

field, have less electron density

and so they feel a stronger magnetic field

and hence resonate at a higher frequency.

So TMS the silicon is a little electron donating it shows

up upfield lower frequency.

Here in methanethiol it shows downfield at higher frequency

and there really is a nice relationship.

You can see this in the case of the halogens.

So if I take methyl iodide,

it shows up the methyl group obviously at 2.10 PPM.

If I take methyl bromide, it's at 2.70 PPM.

If I take methyl chloride, it's at 3.05 PPM

and I'll just put PPM here and if I take methyl fluoride it's

at 4.30 PPM and if you look at the electronegativity,

the pulling electronegativity of the halogen, of course,

as you go down the periodic table you become less

electronegative and so by the time you start, well,

you start with fluorine and the electronegativity is 4.0,

the electronegativity of chlorine is 3.0,

that of bromine is 2.8 and that of iodine is 2.4.

So you can almost see here there's almost a direct

proportionality or a linear relationship.

The more it's pulling electrons away

from the carbon the more you're going ahead and deshielding.

So, more electronegative.

[ Writing on board ]

More electronegative substituent is more electron withdrawing.

[ Writing on board ]

And that's more deshielded.

[ Writing on board ]

Now what's cool and what's significant is

that these effects really end up being reasonably additive

and so see if you can spot the trend

and make some predictions in your head.

We start with methane and the chemical shift.

By the way Delta is a term that's often used

to mean chemical shift in PPM.

There was an older scale, tau, that was used in the 60s.

The two scales were competing and they were opposite.

Delta started at 0 for TMS and by the time you got

to like an aldehyde you'd be at 10.

The tau scale it was completely reversed.

You started at 10 for TMS and by the time you got

to like an aldehyde CH it would be at 0.

And, in fact, I don't talk about this anymore.

Recently a former student from my spec class came to my office

with a paper for his research

and was asking me about this scale.

It's like, wow, I haven't seen that in a long time.

He had pulled a 1960s paper.

Anyway Delta PPM, 0.23 for methane.

If we just look at the chlorinated hydrocarbons,

chlorinated methanes, and we add 1 chlorine,

we already saw we're at 3.05.

So, in other words, we shift down 2 and then some PPM.

So you go to dichloromethane and it shouldn't surprise you

that you go about another 2 PPM.

You're running out of electron density

so you don't pull away quite as much with the second but, again,

you jump from about 3.05 to 5.32

so that's another 2 and then some PPM.

You go to chloroform, where's chloroform show up?

Seven point 27 or 7.26 right in the middle and now, again,

you go about 2 more PPM.

So you can start to use these ideas in your head to say, oh,

I can have a reference value for 1 peak and then perturb it

and just as I was saying with IR spectroscopy it's worth having a

base of knowledge in your head.

There's a huge amount of information

in Silverstein [phonetic], there's a huge amount

of information in Pretch [phonetic],

but just like you have of vocabulary

and then sometimes you go to the dictionary,

you'll have a vocabulary of IR

and you have a vocabulary of NMR.

So let me give you the way I think about IR,

about NMR spectroscopy.

[ Writing on board ]

So sort of reference frame I keep in my head

and I can do a hell of a lot with the numbers that I'm going

to give you in just the next few minutes.

So, the number I like to keep in my mind for sort

of a plain vanilla methyl group is .9 PPM.

That's why I said when I used 1

on the first example it was an oversimplification.

Point 9 PPM is a methyl group that's not near any electron

withdrawing or electron donating methyl group and end of a chain.

A plain vanilla methylene group, ditto,

not near any electron withdrawing

or any electron donating group, about 1.3 to 1.5 PPM.

A methine group, again, not near anything

in particular is about 1.5 to 2.0 PPM.

So, in other words, the difference between a methyl

and a methylene group let's call it about .4 PPM.

The difference between a methylene and a methine group,

let's call it about .5 PPM.

Why is methane so low?

So it's a very electron-rich environment.

Part of the reason you end up deshielding here is

that the steric crowding is actually pushing electron

density away from carbon because you'd say, oh, I would think

of let's say you take isobutane you'd say I always heard

that a methyl group is electron donating so why is the methine,

why is the methine is isobutane actually shifted downfield

and one way to think of it is

that the electrons are basically pushing into each other

and pushing away here.

So methane, and we're going to talk

about how you rigorously calculate what's called

empirical additivity relationships and most

of the empirical additivity relationships use methane

as the starting point.

They use .23 as the starting point whereas I

because we don't normally take spectra

of methane my reference frame

in my mind's eye really becomes these 3 values here

and you can build a hell of a lot from that

and that's what I'm going to show you now.

All right.

So, a little knowledge may be a dangerous thing

but a little knowledge is also a very valuable thing.

So, we already have a little knowledge

that chloromethane is at 3.05 PPM.

Now let's consider the methylene group in chloroethane.

So where do you expect the methylene group to show up?

[ Inaudible response ]

Three point what?

>> Zero nine.

>> Okay. How do you get 3.09?

>> Because going from a methylene, from a methyl group

to a methylene is about .4 and then I would say it's additive

because there's a plus and the chlorine brought it

to about 3.05 so I just added.

>> So you add point -

>> -- the being on methylene brings it, oh, I say 3.45.

>> 3.45. Okay.

Don't worry.

I screw up simple arithmetic on my feet all the time

and you would be darn close to right.

It's actually 3.47.

See a little knowledge is not a dangerous thing.

Let's take isopropyl chloride and let's try

that same logic with that.

[ Pause ]

3.9. Great.

And the actual is 4.14 and guess what?

That's good enough for reading a spectrum because now you look

at a peak and you say, oh, that peak is about 4.0 PPM.

That's probably not a methyl group next

to something electron withdrawing.

It's probably something that we're already further downfield.

I want to give you a couple of other base values

and then we'll have some fun with them.

All right.

So all of these examples that we're looking at are alpha

to an electron withdrawing group.

We can see that in general alpha means

on the carbon directly attached.

We can see that being alpha

to an electron withdrawing group shifts you 2 or 3 PPM downfield

with respect to the base value.

So, for example, things I'll keep in mind I like to keep

in mind, I don't know why I keep it in mind but I happen,

but you could say I'm going to keep methyls in mind.

I happen to keep methylenes in mind because I see a lot

of methylenes next to an oxygen.

So a methylene and an ether group is approximately 3.6 PPM

and that kind of makes sense, right?

Oxygen is a little more electron withdrawing than chlorine.

It's a little further downfield.

Honestly, if you said 3 and a half nobody would fault you,

but from that then you can go ahead and say, oh,

if it were a methyl group, we'd be closer

to 3 parts per million, maybe 3.2 parts per million.