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Chapter 12
Infrared Spectroscopy andMass Spectrometry
Jo BlackburnRichland College, allas, !"
allas County Community College istrict 2##$, %rentice &all
Organic Chemistry , 'th (dition
)* +* ade, Jr*
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Chapter 12 3
!ypes of Spectroscopy
- Infrared IR spectroscopy measures the bond
0ibration fre.uencies in a molecule and is used
to determine the functional group*
- Mass spectrometry MS fragments the moleculeand measures the masses*
- 3uclear magnetic resonance 3MR
spectroscopy detects signals from hydrogen
atoms and can be used to distinguish isomers*
- 4ltra0iolet 45 spectroscopy uses electron
transitions to determine bonding patterns* =>
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Chapter 12 4
(lectromagnetic
Spectrum- (6amples7 " rays, micro/a0es, radio
/a0es, 0isible light, IR, and 45*
- 8re.uency and /a0elength are
in0ersely proportional*
- c 9 λν , /here c is the speed of light*
- (nergy per photon 9 hν , /here h is
%lanck:s constant* =>
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Chapter 12 5
!he Spectrum and
Molecular (ffects
=>
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Chapter 12 6
!he IR Region
- Just belo/ red in the 0isible region*
- a0elengths usually 2*'
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Chapter 12 7
Molecular 5ibrations
Co0alent bonds 0ibrate at only certain
allo/able fre.uencies*
=>
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Chapter 12 8
Stretching 8re.uencies
- 8re.uency decreases /ith increasingatomic /eight*
- 8re.uency increases /ith increasing
bond energy* =>
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Chapter 12 9
5ibrational Modes
3onlinear molecule /ith n atoms usually has
$n < = fundamental 0ibrational modes*
=>
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Chapter 12 10
8ingerprint of Molecule
- holeed*
- 3o t/o molecules /ill gi0e e6actly the
same IR spectrum e6cept enantiomers*
- Simple stretching7 1=##
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Chapter 12 11
IR
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Chapter 12 12
n Infrared
Spectrometer
=>
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Chapter 12 13
8!
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Chapter 12 14
Carbon
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Chapter 12 15
Carbon
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Chapter 12 16
n lkane IR Spectrum
=>
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Chapter 12 17
n lkene IR Spectrum
=>
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Chapter 12 18
n lkyne IR Spectrum
=>
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Chapter 12 19
E
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Chapter 12 20
n lcohol IR Spectrum
=>
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Chapter 12 21
n mine
IR Spectrum
=>
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Chapter 12 22
Carbonyl Stretching
- !he C9E bond of simple ketones,
aldehydes, and carbo6ylic acids absorb
around 1F1# cm
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Chapter 12 23
Getone
IR Spectrum
=>
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Chapter 12 24
n ldehyde
IR Spectrum
=>
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Chapter 12 25
E
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Chapter 12 26
5ariations in
C9E bsorption- Conugation of C9E /ith C9C lo/ers thestretching fre.uency to H1=D# cm
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Chapter 12 27
n mide
IR Spectrum
=>
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Chapter 12 28
Carbon < 3itrogen
Stretching- C < 3 absorbs around 12## cm
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Chapter 12 29
3itrile
IR Spectrum
=>
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Chapter 12 30
Summary of IR
bsorptions
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Chapter 12 31
Strengths and )imitations
- IR alone cannot determine a structure*
- Some signals may be ambiguous*
- !he functional group is usually indicated*- !he absence of a signal is definite proof
that the functional group is absent*
- Correspondence /ith a kno/n sample:sIR spectrum confirms the identity of the
compound* =>
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Chapter 12 32
Mass Spectrometry
- Molecular /eight can be obtained from a0ery small sample*
- It does not in0ol0e the absorption or
emission of light*- beam of high
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Chapter 12 34
Separation of Ions
- Enly the cations are deflected by the
magnetic field*
- mount of deflection depends on m/z *
- !he detector signal is proportional to
the number of ions hitting it*
- By 0arying the magnetic field, ions of all
masses are collected and counted* =>
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Chapter 12 35
Mass Spectrometer
=>
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Chapter 12 36
!he Mass Spectrum
Masses are graphed or tabulated according to
their relati0e abundance*
=>
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Chapter 12 37
!he +C
mi6ture of compounds is separatedby gas chromatography, then identified
by mass spectrometry*
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Chapter 12 38
&igh Resolution MS
- Masses measured to 1 part in 2#,###*
- molecule /ith mass of ?? could be
C$&D, C2&?E, CE2, or C32&?*- If a more e6act mass is ??*#2, pick the
correct structure from the table7
C$&D C2&?E CE2 C32&?
??*#=2=# ??*#2=2# ?$*DD$ ??*#$F?#
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Chapter 12 40
Isotopic bundance
=>
81Br
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Chapter 12 41
Mass Spectrum
/ith Sulfur
=>
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Chapter 12 42
Mass Spectrum
/ith Chlorine
=>
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Chapter 12 43
Mass Spectrum
/ith Bromine
=>
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Chapter 12 44
Mass Spectra
of lkanesMore stable carbocations /ill be moreabundant*
=>
Mass Spectra
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Chapter 12 45
Mass Spectra
of lkenes
Resonanceed cations fa0ored*
=>
M S t
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Chapter 12 46
Mass Spectra
of lcohols- lcohols usually lose a /ater molecule*
- MI may not be 0isible*
=>
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(nd of Chapter 12