mc 13.1 spectroscopy, pt i 1 spectrocopy nuclear magnetic resonance (nmr)spectroscopy infrared...
TRANSCRIPT
MC 13.1 Spectroscopy, Pt I1
Spectrocopy
Nuclear Magnetic Resonance (NMR)spectroscopy
Infrared (IR) Spectroscopy
Ultraviolet-Visible (UV-VIS) Spectroscopy
Mass Spectrometry (MS)
continue…..
MC 13.1 Spectroscopy, Pt I2
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
continue…..
MC 13.1 Spectroscopy, Pt I3
The Visible Electromagnetic Spectrum
400 nm 750 nm
Visible Light
Longer Wavelength ()Shorter Wavelength ()
Higher Frequency () Lower Frequency ()
Higher Energy (E) Lower Energy (E)
continue….
MC 13.1 Spectroscopy, Pt I4
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 I5
Cosmic rays
Rays
X-rays
Ultraviolet light
Visible light
Infrared radiation
Microwaves
Radio waves
Energy
The Electromagnetic Spectrum (cont)
continue…..
MC 13.1 Spectroscopy, Pt I6
Quantized Energy States
Electromagnetic radiation is absorbed when theenergy of the photon corresponds to difference in energy between two states
E = h
State #1
State #2
continue…..
MC 13.1 Spectroscopy, Pt I7
Quantized Energy States (cont)
What Kind of States are of Interest?
Electronic
Nuclear spin
Vibrational
Rotational
UV-Vis
Radiofrequency
Infrared
Microwave
State Spectral Region Type of Spectroscopy
Ultraviolet-Visible
Nuclear Magnetic Resonance
Infrared
Raman
continue…..
MC 13.1 Spectroscopy, Pt I8
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
continue…..
MC 13.1 Spectroscopy, Pt I9continue…..
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 +1/2 is opposite in direction from that generated by a nucleus of spin –1/2 + +
10
The distribution of nuclear spins is random in the absence of an external magnetic field
+
++
+
+
continue…..
11
An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field
++
+
+
+
H0
continue…..
MC 13.1 Spectroscopy, Pt I12
There is a slight excess of nuclear magnetic moments aligned parallel to the 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 the external magnetic field
+
+
E E '
Increasing Field Strength
No Magnetic
Field
continue…..
MC 13.1 Spectroscopy, Pt I14
Introduction to 1H NMR Spectroscopy (cont)
Some important relationships in NMR:
The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1H) depends on its molecular environment
This is why NMR is such a useful tool for structure determination
continue…..
MC 13.1 Spectroscopy, Pt I15
Introduction to 1H NMR Spectroscopy (cont)
Nuclear Shielding and 1H Chemical Shifts:
1) What do we mean by "shielding?“
2) What do we mean by "chemical shift?"
The induced field shields the nuclei (in this case, C and H) from the applied field
A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation
C H
H 0
continue…..
MC 13.1 Spectroscopy, Pt I16
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 of the degree to which a nucleus in a molecule is shielded
Protons in different environments are shielded to greater or lesser degrees;
They have different chemical shifts
C H
H 0
continue…..
MC 13.1 Spectroscopy, Pt I17
Introduction to 1H NMR Spectroscopy (cont)
Chemical Shift (cont):
Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard
Si CH3
CH3
CH3
H3C
=Position of Signal - Position of TMS Signal
Spectrometer Frequencyx 106
Calculating Chemical Shift Numbers:
continue…..
MC 13.1 Spectroscopy, Pt I18
Introduction to 1H NMR Spectroscopy (cont)
The Proton NMR Chemical Shift Scale:
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (, ppm) - measured relative to TMS
DownfieldDecreased shielding
UpfieldIncreased shielding
(CH3)4Si (TMS)
continue…..
19
01.02.03.04.05.06.07.08.09.010.0
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 different degrees of shielding and have different chemical shifts
7.28 ppm
H C
Cl
Cl
Cl
continue…..
MC 13.1 Spectroscopy, Pt I20
Introduction to 1H NMR Spectroscopy (cont)
Effects of Molecular Structure on 1H Chemical Shifts (cont):
Protons in different environments experience different degrees of shielding and have different chemical shifts
Electronegative substituents decrease the shielding of hydrogen 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 I21
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 I22
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 I23
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 I24
01.02.03.04.05.06.07.08.09.010.0
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)
01.02.03.04.05.06.07.08.09.010.0Chemical shift (, ppm)
(TMS)
3 lines;tripletCH3
BrCH2 CH3
Br CH2 CH3
4 lines;quartetCH2
Splitting Patterns: The Ethyl Group BrCH2CH3
continue…..