52 spectroscopy

8/13/2019 52 Spectroscopy

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A-level Spectroscopy

An image of a human brain from a live patient recorded using magnetic

resonance imaging - a 3D form of n.m.r. spectroscopy

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Introduction

Spectroscopy is a collective name for the various techniques that use theinteraction between molecules and electromagnetic radiation to elucidate the

structure of molecules. Spectroscopic methods are fundamental to the study ofChemistry, Molecular Biology, Medicine and Astrophysics.

This booklet covers the following techniues!-

A" Infrared Spectroscopy- from !olymer "evolution# and $hat#s in aMedicine%#

&se relevant given data to interpret 'and ma(e predictions of) infraredspectra for organic compounds containing a limited range of functional

groups 'hydro*yl, carbonyl and carbo*ylic acid groups). &nderstand that every compound has a distinctive fingerprint# in its

infrared spectrum. &se information given in the +ata Sheet to interpret and predict

infrared spectra for organic compounds, in terms of the functionalgroup's) present understand that specific frequencies of infraredradiation ma(e specific bonds vibrate more.

#" $ass Spectrometry- from $hat#s in a Medicine%#

nterpret and predict mass spectradentify the M/ pea( and e*plain that it indicates the Mr 'synoptic) e*plainhow the molecular formula can be wor(ed out from the high-resolution valueof the M/ pea( recall that other pea(s are due to positive ions fromfragments and the mass differences between pea(s indicate the loss ofgroups of atoms, suggest the origins of pea(s e.g. pea(s at masses of 01 and22 are usually due to the presence of the methyl and phenyl positive ionsloss of a methyl group would be indicated by a mass difference of 01.

%" &uclear $agnetic Spectroscopy - from Medicines by +esign#

+escribe and e*plain how proton nuclear magnetic resonancespectroscopy '3M") can be used for the elucidation of molecularstructure 'including splitting patterns up to quartets 4 using the n/0#rule further e*planation of splitting not required

5*plain how a combination of spectroscopic techniques 6MS, "'synoptic)and 3M"7 can be used to elucidate the structure of organic molecules.

This work builds on AS topics of!-

nteraction of radiation with matter '5lements of 8ife# and Atmosphere#) Mass spectrometry '5lements of 8ife#

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A) Infrared 'i.r." spectroscopy

&sed to identify bonds 9 functional groups Can only identify the e*act molecule by comparison with library

spectra

nfrared radiation is passed simultaneously through the sample and areference cell.

:he reference ensures that pea(s due to water or carbon dio*ide in theair can be cancelled out.

:he frequencies of i.r. radiation absorbed are determined by passingthrough a rotating prism to focus one frequency at a time onto thedetector.

:he spectrum shows the ;;;;;;;;;;;;;;;; 'cm-(

) on the * a*is'which is 09) and the ;;;;;;;;;;;;;;;;;;;; on the y-a*is.

%alculations

(" c ) *" +avenumber ) (,'cm)

e.g. $hat wavenumber would appear on an i.r. spectrum if the frequency of

radiation absorbed by a molecule was ?%

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c.f.springoscillations

weight

Theory

" radiation corresponds to the energy required to ma(e chemical bondsvibrate more 9 move to a higher vibrational energy level.

:herefore, energy of certain wavelengths is absorbed by molecules. :he actual energy depends on the mass of the atoms and the strength ofthe bond, so different bonds will absorb at different frequencies.

An A3A8@ between covalent bond and spring

Stronger bonds need more energy to ma(e them vibrate, so absorb ahigher frequency of i.r. radiation 'higher wavenumber)

e.g. hydrogen halides

Molecules with more than < atoms can vibrate in different wayse.g. sulphur dio*ide

So these spectra will contain more absorptions

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Most organic molecules contain a number of types of bond, socharacteristic absorptions will be seen for each bond.

e.g. ethanol

:he following types of bond need to be recognised-

#ond unctional group +ave number forAbsorbance 'cm-("

@ 4 > Alcohols Carbo*ylic acids groups

i.r. bands.ppt

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/0amples of infrared spectra

(" ethanol'C>C>)

displayed

formula

i.r. spectrum

#ond , 'unctional group" Absorption , cm-(

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*" ethanoic acid 'C>C@@>)

displayed

formula

i.r. spectrum


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3" /thyl /thanoate 'C>C@@C>)

i.r. spectrum


021=

0


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1" a" i.r. spectrum of an alcohol with molecular formula %324.

! :his Alcohol is o*idised to compound H)b) when heated under distillationwith acidified potassium dichromate and H)c)when heated to reflu* withacidified potassium dichromate.

Clue%


Displayed ormula of 1a

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1" b" i.r. spectrum of the compound with molecular formula %3254

obtained by distilling compound 1"a" with acidified potassium

dichromate


Displayed ormula of compound 1b

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1"c" i.r. spectrum of the compound with molecular formula %3254*formed

when compound 1a is heated to reflu0 with acidified potassium

dichromate


Displayed ormula of %ompound 1c

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6"a" i.r. spectrum of an isomer of 1a which forms the same product 6"b"

whether it is heated to distil or reflu0 with acidified potassium

dichromate


Displayed ormula of %ompound 6a

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6"b" i.r. spectrum of the product of the reaction of 6a with acidified

potassium dichromate when heated to reflu0 or distillation.


Displayed ormula of %ompound 6b

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5" Salicylic Acid '*-hydro0yben7oic acid - '>@CD>HC@@>)"

displayed

formula

i.r. spectrum


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B) $ass Spectrometry

&se M9'molecular ion) to measure Mr &se M/< isotope pea(s to identify Cl or Br

&se fragmentation pattern to confirm structure of molecule

/0periment *: ;S% video

AS-level


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ragmentation

:he atoms or molecules are ionised by bombarding with high energyelectrons-

e.g. C>C@C>/ e-

6C>C@C>7/ / < e

-

M/

&sually, the resulting molecular ionhas such high energy that it splits upinto a smaller ion and an uncharged molecule 'fragmentation)

e.g. 6C>C@C>7/ 6C>C@7

/ / C>

M/m9e E 1F HC

or 6C>C@C>7/ C>C@ / 6C>7

/

1F 01

:he first fragmentation route is more li(ely because fragments containing

the 6"-CE@79

group 'acylium cations) are particularly stable.

:he following pea(s are often seen in the fragmentation patterns of massspectra 4 the highlighted pea(s usually provide very useful clues indetermining the structure of a molecule

fragment m,e

%23 (6

%23%2*or %24 *>

C>< =

C> 0

%23%4 or %328 13

C@3>< HH

C@@> H1

%526 88

CD>1C>< I0

%526%4 (?6

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/0amples of fragmentation and the interpretation of mass spectra

(" @ropanone 'C>C@C>)

displayedformula

mass spectrum

m,7 ormula m,7 lost roup lost

1F

H

01

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*" @ropanal 'C>C>@)

displayed

formula

mass spectrum


1F

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3" $ethyl #en7oate 'CD>1C@@C>)

mass spectrum


0D

0=1

22

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C C H

H

O

O

H


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1" /thyl /thanoate 'C>C@@C>)

mass spectrum


FF

2

H


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6" Salicylic Acid '*-hydro0yben7oic acid - '>@CD>HC@@>)"

displayed

formula

mass spectrum


0F

0


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5" Aspirin '%23%44%521%442"

mass spectrum


0F=

0F

0


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8" ethanamide '%23%4&2*"

displayed

formula

mass spectrum


1I

HH

H

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" paracetamol '1-hydro0yphenylethanamide" '>@CD>H3>C@C>)

displayed

formula

mass spectrum


010

0=I

0=F

H

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C) &uclear $agnetic ;esonance 'n.m.r." spectroscopy

:he number of pea(s 4 number of proton types :he chemical shift ') 4 what are the proton types

:he integration 4 how many protons of each type

5*periment 4 "SC video

Sample is placed in a very strong magnetic field A pulse of radiofrequency radiation is applied "adiofrequency signal emitted from sample is detected

Theory

3uclei have a property called nuclear spin which generates a tinymagnetic field. :he nuclei therefore behave li(e tiny bar magnets.

$hen such nuclei are placed in a large magnetic field they will becomealigned with or against the direction of the e*ternal field.

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:he nuclei lined up with the field are slightly more stable 'lower energy)than those that oppose the e*ternal field.

:he energy gap between these two states corresponds to radiofrequencyradiation.

f the sample is irradiated with a pulse of radio waves, the nuclei in thelower energy state may be promoted to the higher energy state 'the tinybar magnets flip# from being aligned with to against the e*ternal field).

:he e*cited nuclei will then return to the ground state releasing fi*edquanta of energy which will be detected.

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:he energy gap depends on the chemical environment of the nuclei and can beused to deduce the e*act structure of the molecule.

5thanal has two proton types, so produces < signals in the n.m.r. spectrum.

:he important features of the spectrum are- :he number of pea(s 4 number of proton types :he integration 4 how many protons of each type :he chemical shift ') 4 what are the proton types

:he following table can be used to lin( the chemical shift to the proton type 'chemicalenvironment of > atom)-

type of proton chemical shift B , ppm

"C> 9 "C> 'estersC ethers" .< - .1

"@> 'alcohol" 0.= - D.=

"CD>H> 'arenes" D.= - I.="CD>HC> 'methylarene" 'carbo0ylic acids" I.= - 0


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(" propanone 'C>C@C>)

displayed

formula

nmr spectrum

@roton integration inference B , ppm inference

>a

*" ethanoic acid 'C>C@@>)

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nmr spectrum


>a 00.H

>b


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displayed

formula

nmr spectrum


>a I.2

>b c 0.0

2igh ;esolution nmr Spectra

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Most nmr spectra loo( more complicated than the first three e*amples. :he signal for each hydrogen atom may be split into a number of pea(s. :he pattern of the splitting tells us how many hydrogen atoms are

bonded to the adNacent carbon atom.

e.g. ethanal

:he >a protons have one adNacent > atom '>b) :he signal will be split into ;;;; pea(s 4 a doublet.

:he >b proton has three adNacent > atoms '>a) :he signal will be split into ;;;; pea(s 4 a quartet.

2igh resolution spectra may be analysed as follows!-

:he number of pea(s 4 number of proton types :he integration 4 how many protons of each type :he chemical shift ') 4 what are the proton types :he splitting pattern 4 the number of > atoms on the adNacent C atom

:his level of analysis is now required for the A-level e*aminations, as theinformation from splitting patterns is e*tremely useful in wor(ing out thestructure of comple* molecules.

+hy are the signals split by adacent protonsE

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>aC C

@ >a

>b >a

The n9( rule:he no. pea(s E the no. > atoms on the adNacent carbon / 0


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5ach > nucleus generates its own tiny magnetic field, which may bealigned with or against the e*ternal magnetic field. :his will affect themagnetic environment e*perienced by > nuclei bonded to adNacent Catoms.

e.g. ethanal

:he >a protons have one adNacent > atom '>b) >b may be aligned with or against the field

:his means that there are two possible environments for the >a protons,of equal probability.

:he signal for >a will be split into < pea(s 4 a 00 doublet.

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( ! (

>aC C

@ >a

>b >a

>aC C

@ >a

>b >a


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:he situation is slightly more complicated for the >b proton, which hasthree adNacent > atoms '>a).

5ach of the three >a protons may be aligned with or against the field. :his means that there are four possible orientations of the >a nuclei-

a. All nuclei are aligned with the field '0)b. :wo nuclei with and one against the field ')c. @ne nucleus with and two nuclei against the field ')d. All nuclei aligned against the e*ternal magnetic field '0).

:his means that there are four possible environments for the >b proton,with a relative probability of 00

:he signal will be split into H pea(s 4 a 00 quartet.

1" /thanal 'C>C>@)

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( ! 3 ! 3 ! (


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>igh "esolution nmr Spectrum of ethanal

@roton integration inference B , ppm inference splitting inference

>a b I.2

6" ethyl ethanoate 'C>C@@C>)

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9 ppm0= F D H < =

>a

>b1

3

>aC C

@ >a

>b >a


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nmr spectrum


>a b H.0

>c 0.)C>)

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Ha C C CC

Hb

O

O Hc

Hc

Ha

HbHa

Hc

>c C CC

>c

>c

>b>c

>c

>c@

>a


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nmr spectrum


>a b .I septet

>c 0.< doublet

8" Salicylic Acid '*-hydro0yben7oic acid - '24%521%442""

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displayedformula

nmr spectrum


>a F.=

>b 2.D

>c 2.=

$here are the @-> groups%

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" Aspirin 'C>C@@CD>HC@@>)

nmr spectrum


>a 0 00.

>b H * 0 2 - F

>c


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>" $ystery compound : O$hy are there no aspirin in the Nungle%P

n.m.r. spectrum


>a 0 I.2

>b 0 I.0

>c < 2.H

>d < D.2

>e


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%ombined Spectral Techniues

(" @redict the irC nmr and mass spectra of propanoic acid

a" I; spectroscopy


b" &mr spectroscopy


>a

>b

>c

c" $ass Spectrometry


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*" Deduce the structure of the molecule from these spectra

a. ir spectrum


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b" nmr spectrum


>a b .D triplet

>c 0.1

overlapping

quartet oftriplets

>d =.I triplet

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c" mass spectrum


D=

0

D=

0

Structure of &n(nown Molecule

52 spectroscopy

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