MC 13.1 Spectroscopy, Pt I
1
Spectrocopy
Nuclear Magnetic Resonance (NMR)spectroscopy
Infrared (IR) Spectroscopy
Ultraviolet-Visible (UV-VIS) Spectroscopy
Mass Spectrometry (MS)
continue…..
MC 13.1 Spectroscopy, Pt I
2
Principles of Molecular Spectroscopy:Electromagnetic Radiation
Electromagnetic Radiation travels at the speed of light
Has both particle and wave properties
A single packet of energy is a photon
The energy of a photon is proportional to its frequency
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MC 13.1 Spectroscopy, Pt I
3
The Visible Electromagnetic Spectrum
400 nm
750 nm
Visible Light
Longer Wavelength ()
Shorter Wavelength ()
Higher Frequency ()
Lower Frequency ()
Higher Energy (E)
Lower Energy (E)
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MC 13.1 Spectroscopy, Pt I
4
The Electromagnetic Spectrum (cont)
Ultraviolet
Infrared
Longer Wavelength ()
Shorter Wavelength ()
Higher Frequency ()
Lower Frequency ()
Higher Energy (E)
Lower Energy (E)
continue…..
MC 13.1 Spectroscopy, Pt I
5
Cosmic rays
Rays
X-rays
Ultraviolet light
Visible light
Infrared radiation
Microwaves
Radio waves
Energy
The Electromagnetic Spectrum (cont)
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MC 13.1 Spectroscopy, Pt I
6
Quantized Energy States
Electromagnetic radiation is absorbed when theenergy of the photon corresponds to difference inenergy between two states
E = h
State #1
State #2
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MC 13.1 Spectroscopy, Pt I
7
Quantized Energy States (cont)
What Kind of States are of Interest?
Electronic
Nuclear spin
Vibrational
Rotational
UV-Vis
Radiofrequency
Infrared
Microwave
StateSpectral RegionType of Spectroscopy
Ultraviolet-Visible
Nuclear Magnetic Resonance
Infrared
Raman
continue…..
MC 13.1 Spectroscopy, Pt I
8
Introduction to 1H NMR Spectroscopy
The nuclei that are most useful to organic chemists are:
1H and 13C
Both have spin = + or - 1/2
1H is 99% natural abundance
13C is 1.1% natural abundance
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MC 13.1 Spectroscopy, Pt I
9
continue…..
Introduction to 1H NMR Spectroscopy (cont)
Nuclear Spin:
A spinning charge, such as the nucleus of 1H or 13C,generates a magnetic field
The magnetic field generated by a nucleus of spin +1/2is opposite in direction from that generated by anucleus of spin –1/2
+
+
10
The distribution ofnuclear spins israndom in theabsence of anexternal magneticfield
+
+
+
+
+
continue…..
11
An externalmagnetic fieldcauses nuclearmagnetic momentsto align paralleland antiparallel toapplied field
+
+
+
+
+
H0
continue…..
MC 13.1 Spectroscopy, Pt I
12
There is a slightexcess of nuclearmagnetic momentsaligned parallel tothe applied field
+
+
+
+
+
H0
continue…..
13
Introduction to 1H NMR Spectroscopy (cont)
Energy Differences Between Nuclear Spin States:
There is no difference in the absence of a magnetic field
The difference is proportional to the strength of theexternal magnetic field
+
+
E
E '
Increasing Field Strength
NoMagneticField
continue…..
MC 13.1 Spectroscopy, Pt I
14
Introduction to 1H NMR Spectroscopy (cont)
Some important relationships in NMR:
The frequency of absorbed electromagneticradiation for a particular nucleus (such as 1H)depends on its molecular environment
This is why NMR is such a useful toolfor structure determination
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MC 13.1 Spectroscopy, Pt I
15
Introduction to 1H NMR Spectroscopy (cont)
Nuclear Shielding and 1H Chemical Shifts:
What do we mean by "shielding?“
What do we mean by "chemical shift?"
The induced field shields thenuclei (in this case, C and H)from the applied field
A stronger external field isneeded in order for energydifference between spin statesto match energy of rf radiation
C
H
H 0
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MC 13.1 Spectroscopy, Pt I
16
Introduction to 1H NMR Spectroscopy (cont)
Chemical Shift:
What do we mean by "shielding?“
What do we mean by "chemical shift?"
Chemical shift is a measure ofthe degree to which a nucleusin a molecule is shielded
Protons in differentenvironments are shielded togreater or lesser degrees;
They have different chemicalshifts
C
H
H 0
continue…..
MC 13.1 Spectroscopy, Pt I
17
Introduction to 1H NMR Spectroscopy (cont)
Chemical Shift (cont):
Chemical shifts (d) aremeasured relative to theprotons in tetramethylsilane(TMS) as a standard
Si
CH3
CH3
CH3
H3C
=
Position of Signal - Position of TMS Signal
Spectrometer Frequency
x 106
Calculating Chemical Shift Numbers:
continue…..
MC 13.1 Spectroscopy, Pt I
18
Introduction to 1H NMR Spectroscopy (cont)
The Proton NMR Chemical Shift Scale:
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm) - measured relative to TMS
DownfieldDecreased shielding
UpfieldIncreased shielding
(CH3)4Si (TMS)
continue…..
19
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
(CH3)4Si (TMS)
Introduction to 1H NMR Spectroscopy (cont)
Effects of Molecular Structure on 1H Chemical Shifts:
Protons in different environments experience differentdegrees of shielding and have different chemical shifts
7.28 ppm
H
C
Cl
Cl
Cl
continue…..
MC 13.1 Spectroscopy, Pt I
20
Introduction to 1H NMR Spectroscopy (cont)
Effects of Molecular Structure on 1H Chemical Shifts (cont):
Protons in different environments experience differentdegrees of shielding and have different chemical shifts
Electronegative substituents decrease the shielding ofhydrogen atoms on methyl groups
Decreased Shielding
Increased shielding
CH3F
CH3OCH3
(CH3)3N
CH3CH3
(CH3)4Si
4.3
3.2
2.2
0.9
0.0
continue…..
MC 13.1 Spectroscopy, Pt I
21
Introduction to 1H NMR Spectroscopy (cont)
Effects of Molecular Structure on 1H Chemical Shifts (cont):
Electronegative substituents decrease shielding
H3C — CH2 — CH3
0.9
0.9
1.3
O2N — CH2 — CH2 — CH3
1.0
4.3
2.0
continue…..
MC 13.1 Spectroscopy, Pt I
22
Introduction to 1H NMR Spectroscopy (cont)
Effects of Molecular Structure on 1H Chemical Shifts (cont):
Electronegative substituents decrease shielding
The electronegativity effect is cumulative
continue…..
CHCl3 7.3
CH2Cl2 5.3
CH3Cl 3.1
MC 13.1 Spectroscopy, Pt I
23
Methyl ( CH3 ), Methylene ( CH2 ), and Methine ( CH ), :
Introduction to 1H NMR Spectroscopy (cont)
Effects of Molecular Structure on 1H Chemical Shifts (cont):
CH3 more shielded than CH2
CH2 more shielded than CH
H3C
C
CH3
CH3
H
0.9
1.6
0.8
0.9
H3C
C
CH3
CH3
CH2
CH3
1.2
continue…..
MC 13.1 Spectroscopy, Pt I
24
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
(TMS)
Example:
continue…..
Interpreting Proton NMR Spectra (cont)
4 lines;
quartet
2 lines;
doublet
Cl2CH CH3
Cl2CH CH3
Cl2CH CH3
25
Interpreting Proton NMR Spectra (cont)
CH3CH2X is characterized by a triplet-quartet pattern(quartet at lower field than the triplet)
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
(TMS)
3 lines;
triplet
CH3
BrCH2 CH3
Br CH2 CH3
4 lines;
quartet
CH2
Splitting Patterns: The Ethyl Group
BrCH2CH3
continue…..