Nuclear Magnetic Resonance Spectroscopy (NMR)

8/9/2019 Nuclear Magnetic Resonance Spectroscopy (NMR)

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Unit One Part 9:nuclear magnetic resonance

spectroscopy

Pink Floyd


2/79

Unit One

Part9

nmr spectroscopyequivalent hydrogensintegral vs. number of hydrogens

how do we know all

this info about

molecules shape,

interactions etc?


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Unit One

Part9

nmr spectroscopyequivalent hydrogensintegral vs. number of hydrogens

well a lot of our knowledge comes fromlooking at molecules using various forms

of spectroscopy. Today well look at nmr

and next lecture ir and mass

spectrometry...


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electromagneticradiation describes a

form of energy...


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it is a continuum of energy

that is arbitrarily divided into a

number of regions (the most

obvious being visible light)


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low energy EM radiation has a long

wavelength and low frequency (askthe physicists) whilst high energy is

small wavelength and high

frequency


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can electromagneticradiation interact with

molecules?

a stupid question...you

know what the answer

must be otherwise whyhave I introduced this

topic?


8/79yes!


9/79yes!

sunburn...simple

interaction of energy

and molecules...


10/79yes!

the fact you can see

this slide means EM

(visible light) must be

able to interact withour eyes


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whycanwe get

sunburnton acloudyday

but notwarm

?


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differentmolecules

interactwith

energy

different


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differentmolecules

interactwith

energy

different

...water in clouds blocks infraredenergy (which makes us hot) but

does not block ultraviolet energy

(which burns us)


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so how dowe use this?


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E1

molecule in energystate E1

E2if we take a molecule in its

resting state (low energy orhappily sitting around

chilling) and...


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absorption ofenergy

E1


E2

...irradiate the molecule

with energy some will be

absorbed causing the

molecule...


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absorption ofenergy

E1


E2

...to become excited or to

rise to a higher energy

level...

l l i


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E1

E2

molecule in energystate E2 or excited

the amount absorbed

will depend on what we

have changed within the

molecule...we canmeasure this...

l l i


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emission ofenergy

E1

E2

molecule in energystate E2 or excited

eventually the molecule will relax

and emit energy as it returns to

its resting state...we canmeasure this energy as well...


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what part of themolecule is effected?

...so, by measuring the energy

absorbed or emitted we can

see changes in the molecule.

So, what can we see?

l i l i ibl (UV)


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ultraviolet-visible (UV)

excites an electron

lt i l t i ibl (UV)


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ultraviolet-visible (UV)

excites an electron

high energy UV radiation

can excite an electron between orbitals

...this tells us about the conjugation within

the molecule (multiple bonds separated

by one single bond)

i f d (IR)


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infrared (IR)

vibrates bondsO

H

i f d (IR)


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infrared (IR)

vibrates bondsO

H

IR has slightly less energy andalters the vibration of bonds within

a molecule...this can tell us about

the functional groups within that

molecule

l ti


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nuclear magnetic resonance(NMR)

C-H framework

HC

CO

CC

H

H H

H H

H H

H H

l ti


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nuclear magnetic resonance(NMR)

C-H framework

HC

CO

CC

H

H H

H H

H H

H H

NMR uses low energy radio waves

to alter the spin of the nucleus of certain atoms...

this can give us a lot of useful information about

the position of C and H within the molecule. It is

by far the most useful form of spectroscopy we

routinely use


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what is nmr?

I t f k th


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Fox TV

I guess most of you know the

character House? and for those

of you that have watched it you

might recognise...


29/79magnetic resonance imagingPicture: Catherine E. Myers. Copyright 2006 Memory Loss and the Brain

...the use of MRI to

diagnose a variety of

ailments...

this beast is an MRI...it is


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identical to an NMR machine

except it spins a magnet

around a patient and an NMR

spins the sample...


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nobody likes theword nuclear

I guess its called MRI and not

NMR as no patient would allow a

doctor to insert them in a

machine with the word nuclear

in the title


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so what is nmr?

...but seriously...what

does an NMR (or MRI)do?

h d


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hydrogen

atom

ve

+ve

a hydrogen atom comprises of

one proton and one electron. for

nmr we are primarily interested

with the single proton of the

nucleus...

h d


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hydrogen

nucleus spins

the positively charged proton is

spinning and this creates a

magnetic field (is it the left or

right-handed rule?)

sample


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sample

random orientation

in a molecule (or collection of molecules)

we have lots of protons (or hydrogens) all

spinning in various directions...but if we

apply a magnetic field to the sample...

sample


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sample

aligned

-spinstate

-spinstate

sample


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sample

aligned

-spinstate

-spinstate

...then all the protonswill align with the

field...

sample


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sample

aligned

-spinstate

-spinstate

if we give the protons energy

we can excite them so that they

oppose the field (these are the

only two states they can exist

in once in the field)

sample


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sample

excited

-spinstate

-spinstate

sample


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sample

excited

-spinstate

-spinstate

the energy required to do this

is in the radio wave frequency(so not much)

sample


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sample

relaxes

-spinstate

-spinstate

take away the energy source and

the protons will relax (become

aligned with the field once more)

sample


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sample

relaxes

-spinstate

-spinstate

record energyemitted

when they do this they emit

energy as a radio wave...which

we can measure

sample


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sample

relaxes

-spinstate

-spinstate

record energyemittednow, the cool bit is...

the amount of energy they will

emit depends on the strength ofthe magnetic field that they

experience...

hydrogen


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hydrogen

atom

ve

+ve

each proton has an electronspinning around it...this is a

spinning charge so it too

produces a magnetic field...

sample


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sample

relaxes

-spinstate

-spinstate

record energyemitted...so, the amount of energy emitted is

influenced by the electrons around the

proton or the chemical environment. As

bonds are electrons we now haveinformation about bonds / structure of

the molecule

NMR machine


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NMR machine

...a radio in

a magnetpicture: Blackman et al. Wiley

basically, NMR (MRI) is

just a two-way radio in a

magnetic field (just bigger

and more expensive)


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so what does thenmr tell us?


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caffeine

N

N N

N

O

CH3

H3C

OCH3

H

CH3

SiCH3

H3C

H3C

TMS

NMR tells us about

the individual H in a

molecule...


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caffeine

N

N N

N

O

CH3

H3C

OCH3

H

CH3

SiCH3

H3C

H3C

TMS

in caffeine there are

four different kinds of H(or proton) depending

on where they are...


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caffeine

N

N N

N

O

CH3

H3C

OCH3

H

CH3

SiCH3

H3C

H3C

TMS

so the NMRspectrum has four

peaks...one for each

kind of H


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H

aromatic

C

H

H

alkenyl

X C

H

X = O, Nor halide

C C

H

allylic

C C

H

alkyl

8 6 4 2 0

low field high field scale

shieldeddeshielded

low electrondensity

highelectrondensity

we can predict roughly

where a peak will be found

in the H NMR spectrum as

it is related to the

concentration of electrons

near each H


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H

aromatic

C

H

H

alkenyl

X C

H

X = O, Nor halide

C C

H

allylic

C C

H

alkyl

8 6 4 2 0


shieldeddeshielded

low electrondensity

highelectrondensity

if there are lots of

electrons near to an H

then it is set to be

shielded...


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H

aromatic

C

H

H

alkenyl

X C

H

X = O, Nor halide

C C

H

allylic

C C

H

alkyl

8 6 4 2 0


shieldeddeshielded

low electrondensity

highelectrondensity

if there arent many

electrons near the H it is

deshielded...

electron withdrawing


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H

aromatic

C

H

H

alkenyl

X C

H

X = O, Nor halide

C C

H

allylic

C C

H

alkyl

8 6 4 2 0


shieldeddeshielded

low electrondensity

highelectrondensity

groups pull electrons away

causing objects to be

deshielded and once

again...


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H

aromatic

C

H

H

alkenyl

X C

H

X = O, Nor halide

C C

H

allylic

C C

H

alkyl

8 6 4 2 0


shieldeddeshielded

low electrondensity

highelectrondensity

...proving it is useful to

know about polarisation of

bonds etc


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H

aromatic

C

H

H

alkenyl

X C

H

X = O, Nor halide

C C

H

allylic

C C

H

alkyl

8 6 4 2 0


shieldeddeshielded

low electrondensity

highelectrondensity

correlation table


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C CH3C

H2C Ph

C CH2

C

H2C C C

H2C I

C CH

C

C CH

C

C CH2C Br

CHCl2

H3C C CC

H2C Cl

Ph H

H3C C O H2C OC C

O

H

H3C N H3C O

C C

O

OH

Ph CH3 Ar OH R OH

C C HC

H2C F

Type of

hydrogen (ppm)

Type of

hydrogen (ppm)

Type of

hydrogen (ppm)

0.701.30 2.60 4.605.00

1.201.35 3.103.30 5.205.70

1.401.65 3.40 5.805.90

1.601.90 3.50 6.608.00

2.102.60 3.503.80 9.509.70

2.103.00 3.503.80 10.0013.00

2.202.50 4.008.00 0.505.50

2.402.70 4.304.40

correlation table

correlation table


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C CH3C

H2C Ph

C CH2

C

H2C C C

H2C I

C CH

C

C CH

C

C C H2

C BrCHCl2

H3C C CC

H2C Cl

Ph H

H3C C O H2C OC C

O

H

H3C N H3C O

C C

O

OH

Ph CH3 Ar OH R OH

C C HC

H2C F

Type of

hydrogen (ppm)

Type of

hydrogen (ppm)

Type of

hydrogen (ppm)

0.701.30 2.60 4.605.00

1.201.35 3.103.30 5.205.70

1.401.65 3.40 5.805.90

1.601.90 3.50 6.608.00

2.102.60 3.503.80 9.509.70

2.103.00 3.503.80 10.0013.00

2.202.50 4.008.00 0.505.50

2.402.70 4.304.40

correlation table

shows where we wouldexpect to find peaks in our

NMR data...you will always

be given a table like this

for your exams...

chemically equivalent hydrogen


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SCH3

O

H

HH


this is DMSO

(dimethylsulfoxide)...it has

six hydrogen atoms...dowe see six peaks?



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H3CS

CH3

O

nope, just onepeak...why?


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CH3

NCH3H3C


trimethylamine has 9

hydrogen but allchemically equivalentso one peak



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CH3

NCH3H3C


the peak is slightly

shifted compared to

DMSO as N does not

pull the electrons

towards it as much as

the S=O group

two chemical environments


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CH3

H

H3C

H

CH3

H

two chemical environments

mesityl has two peaks asit has one set of H

attached to the aromatic

ring and one set attachedto methyl groups

three chemical environments


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OCH3

O

H3C

H H

3.42 ppm

4.03 ppm

2.15 ppm

three chemicalenvironments (methyl ether,methyl ketone and the one

in the middle) so three

peaks



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OCH3

O

H3C

H H

3.42 ppm

4.03 ppm

2.15 ppm

this peak shifted down field(deshielded) as the ketone is electron

withdrawing and the electronegative

O is electron withdrawing so lowelectron density near the two H

integration (hydrogen counting)


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OCH3

O

H3C

H H


when the NMR prints a spectrum

in normally has a second bit of

information attached...this blue

line (although the colour is

unimportant)



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OCH3

O

H3C

H H


this is called the

integral and its height

is proportional to the

area of each peak



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OCH3

O

H3C

H H


3x3x2x

it is proportional to

the number of H

responsible for the

peak



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OCH3

O

H3C

H H


3x3x2x

so without any knowledge of polarity(electron withdrawing groups) we know that

this peak must be due to the central CH2 as

it is smaller than the other two peaks (which

are caused by 3 H each)



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CH3

H

H3C

H

CH3

H


9x

3x

this peak must belong to

the nine H of the threeCH3 groups



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H3C O CH3


6x4x

we have 6 x H on the

two CH3 groups so theymust be the peak near

1 ppm whilst we have 4

x H near the O so theyare the deshielded H at

3.5



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H3C O CH3


6x4x

but note...it is only a

ratio...ethanol

(CH3CH2OH) would give

a very similar spectrum

as the ratio 3:2 is the

same as 6:4



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H3C O CH3


6x4x

also note how broad thepeaks are...this is

because nmr is a lot

more complicated than

Ive shown...

this is chemistry...


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y

it gets more complicated signal splitting


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H3C O CH3

signal splitting

6x

4x

if we look closer the

spectrum actually

shows seven lines (a

quartet and a triplet)

signal splitting


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H3C O CH3

s g a sp g

6x

4x

this is due to peak splitting

and it tells me a lot about

the structure of the

molecule...luckily you dont

need to know about it...yet...

a real 1H NMR spectrum


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a real H NMR spectrum

S

O

Tol

H

H

looks nasty but

is very useful!

what have


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all about

....welearnt?

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Nuclear Magnetic Resonance Spectroscopy (NMR)