Transcript 10_Proton_NMR

Proton Nuclear Magnetic Resonance
(1H-NMR) Spectroscopy - Part 1
Lecture Supplement page 133
Organic Structure Analysis
Crews, Rodriguez, Jaspers
1998 p.5-6
1H-NMR
Spectroscopy
Background and Theory
Fundamental principle
The energy required to cause nuclear spin flip is a function of the magnetic environment
of the nucleus.
•Protons, electrons, neutrons have “spin” (I)
•Motion of charged particle creates magnetic field
•In absence of external influence, magnetic poles (spin axis) randomly oriented
•Add external magnetic field (Bo)  spins align
Bo
add magnetic field
No external magnetic field
Spin alignment random
With external magnetic field
Spins aligned
Background and Theory
Nuclear Spin Flip
•I = +1/2 parallel to Bo (lower energy); I = -1/2 antiparallel to Bo (higher energy)
Increasing energy
•Addition of energy results in nuclear spin flip
I = -1/2
I = +1/2
Absorb energy
(excitation)
Release energy
(relaxation)
Ground state
Nuclear spin parallel to Bo
Lower energy
DE ~ 0.02 cal mol-1
= radio wave photons
Excited state
Nuclear spin antiparallel to Bo
Higher energy
Contrast this with absorption of infrared light (p. 114 of lecture supplement)
Background and Theory
Magnetic Field Controls DE
• DE influenced by magnetic field strength at nucleus
Spin state energy
I = -1/2
DE
DE
Small magnetic field  small DE
Large magnetic field  large DE
I = +1/2
Magnetic field strength at nucleus
Energy required for spin flip (DE)

Information about magnetic field strength at nucleus

Information about chemical structure
Background and Theory
The NMR Spectrum
•Spectrum = plot of photon energy versus photon quantity
Intensity of signal
(photon quantity)
NMR signal
Spin flip energy (photon energy)
Background and Theory
The NMR Spectrum
Nuclear: Manipulation of nuclear spin
Magnetic: Magnetic field strength influences DE
Resonance: Tendency of a system to oscillate at maximum
amplitude at a certain frequency
NMR
C
1H
O
X
C
O
nucleus = a proton  1H-NMR = proton NMR
Background and Theory
Spectrum  Structure
How do we deduce structure from NMR spectrum?
Information from NMR spectrum:
•Number of signals  number of nonequivalent proton groups in molecule
•Position of signals (chemical shift)  magnetic environment of protons
•Relative intensity of signals (integration)  ratio of equivalent proton types
•Splitting of signals (spin-spin coupling)  proton neighbors
Number of Signals
Proton Equivalency
•NMR signal due to photon absorption
•Photon energy controlled by magnetic environment of nucleus
•Nuclei in same magnetic environment = equivalent
•Multiple magnetic environments  multiple signals
•Number of signals = number of equivalent proton sets
H
O
H
H
H
Cl
Protons equivalent
One NMR signal
CN
Protons not equivalent
Two NMR signals
Number of Signals
Proton Equivalency
How to test for equivalency?
•Equivalent = proton magnetic environments identical in every way
•Nonequivalent = proton magnetic environments not identical in one or more ways
•Easier to test for nonequivalency than for equivalency
Useful vocabulary
CH3 = methyl
H
H
H
Cl
C
C
C
H
H
Cl
CH2 = methylene
H
H
H
H
Cl
C
C
C
H
H
Cl
H
CH = methine
H
H
H
Cl
C
C
C
H
H
Cl
H
Number of Signals
Proton Equivalency
Proton equivalency examples
Hb, Ha not equivalent
Ha, Hc not equivalent
H
H
C
Cl
H
One signal
H
H
H
C
C
H
H
rapid equilibrium
Cl
Two signals ?
Hb, Hc equivalent
Ha, Hc equivalent
•NMR = camera with slow shutter speed
•NMR detects only average when rotation is fast
•Thousands of 360o bond rotations per second
•Therefore Ha, Hb, Hc appear equivalent
Single bond rotation in acyclic molecules often allows equivalency
Number of Signals
Proton Equivalency
Proton equivalency examples
Hb, Ha not equivalent
Ha, Hc not equivalent
H
H
C
Cl
H
One signal
H
H
H
C
C
H
H
rapid equilibrium
Cl
Two signals ?
Hb, Hc equivalent
vs.
Ha, Hc equivalent
Number of Signals
Proton Equivalency
More proton equivalency examples
H
H
H
C
C
H
H
OH
H
Three signals
H
C
F
Cl
H
H
C
C
C
Four signals
C
H
C
mirror plane
C
C
H
H
H
C
H
H
F
C
C
F
Cl
One signal
H
C
C
H
C
H
H
C
Cl
Two signals
H
C
F
H
C
H
One signal
H
Cl
Number of Signals
Proton Equivalency
Sample spectra
•Verify what we have learned about equivalent protons
•Which spectrum belongs to this molecule?
H3 C
H3 C
H
C
C
H3 C
H
OH
Three proton sets  three signals
O
H3C
H3C
C
H
C
H
Number of Signals
Proton Equivalency
Which spectrum belongs to this molecule?
H
H
HO
OH
H
Two proton sets  two signals
H
HO
OH
H3 C
CH3
Proton Nuclear Magnetic Resonance
(1H-NMR) Spectroscopy - Part 2
Lecture Supplement page 139
OCH3
H3C
C
CH3
OCH3
1H-NMR
Spectroscopy Part 1 Summary
•Atomic nucleus has spin, and therefore generates a magnetic field
•Nuclear spin axis can be parallel or antiparallel to external magnetic field (Bo)
•Spin parallel to Bo (I = +1/2) lower energy than spin antiparallel to Bo (I = -1/2)
•Energy difference between spin states (DE) controlled by magnetic field at nucleus
•Absorption of radio wave photon with energy = DE causes nuclear spin flip
•NMR spectrum = plot of photon energy (spin flip energy) versus photon quantity
Information from NMR spectrum
•Number of signals reveals number of sets of equivalent protons
Equivalency: Protons must be identical in all ways to be equivalent
Nonequivalency: Protons can be different in just one way
Example: 1H-NMR spectrum of CH3CH2OH has three signals
•Position of signal (chemical shift)
•Relative intensity of signals (integration)
•Splitting of signals (spin-spin coupling)
Vollhardt, 10-2
Position of Signals
The Chemical Shift
How does spin flip energy relate to molecular structure?
•Spin flip energy depends on magnetic field strength:
Spin state energy
I = -1/2
DE
DE
Small magnetic field  small DE
Large magnetic field  large DE
I = +1/2
Magnetic field strength at nucleus
•Magnetic field strength varies between NMR spectrometers
•High magnetic field = higher spectral resolution (more spectral detail)
•Need a spin flip energy scale that is independent of magnetic field strength
•Chemical shift: Spin flip energy scale normalized to be independent of field strength
Position of Signals
The Chemical Shift
How does molecular structure influence chemical shift?
•Chemical shift controlled by DE which is controlled by magnetic field at nucleus
What contributes to magnetic field at nucleus?
Earth’s magnetic field
Weak; 0.3-0.6 gauss
Spectrometer’s magnetic field
Strong; typically 94 kilogauss
Other electrons and
nuclei in the molecule
•Electron cloud shields atomic nucleus from external magnetic fields
Shielded: Nucleus feels weaker magnetic field
Deshielded: Nucleus feels stronger magnetic field
Position of Signals
Intensity of signal
(photon quantity)
The Chemical Shift
Reference point?
0.00 ppm (CH3)4Si
Tetramethylsilane (TMS)
15 ppm Spin flip
Chemical
energy shift
(photon
scale
energy) 0 ppm
(ppm)
Deshielded
Shielded
Low Magnetic Field Strength
High Magnetic Field Strength
Position of Signals
The Chemical Shift
•How does molecular structure influence chemical shift?
5.0
4.0
2.0
1.0
2.0
3.0
4.0
O
3.5
F
4.0
Cl
3.0
Br
2.8
I
2.5
C
2.5
0.0 ppm
(CH3)4Si 0.00 ppm
CH3CH3 0.86 ppm
CH3Br 2.68 ppm
CH3Cl 3.05 ppm
CH3OH 3.42 ppm
1.0
EN of X in CH3-X
3.0
Chemical shifts for H3C-X:
Si
H 1.8
2.1
Conclusion:  EN of atoms near H  chemical shift
•Electron cloud shields atomic nucleus from external magnetic fields
Shielded: Nucleus feels weaker magnetic field
Deshielded: Nucleus feels stronger magnetic field
deshielded
Vollhardt, Fig 10-9
shielded
Position of Signals
The Chemical Shift
How does electronegativity influence chemical shift?
•Chemical shift related to magnetic field strength at nucleus
•Electron cloud shields nucleus from effects of Bo
H
H
I
Iodine has low EN
Br
H
Cl
H
F
Fluorine has high EN
Electron density at H is high
Electron density at H is low
H more shielded
H is less shielded
H has lower chemical shift
H has higher chemical shift
Metaphor: Ozone layer shields Earth
Position of Signals
Do not memorize chemical shifts. Table given on exams.
Position of Signals
Notes On Characteristic Chemical Shifts Table
•Characteristic shifts are typical proton averages. Actual shifts may lie outside given range.
O
O
CH3 Typically 2.0-2.6 ppm
Useful chemical shift trends
•RCH3 < RCH2R < R3CH
0.23 ppm
CH3OH
3.39 ppm
CH3 2.01 ppm
CH3O
CH3 2.59 ppm
EN of C (in R) > EN of H
•EN effects decrease with distance:
CH4
O
3.59 ppm
CH3CH2OH
1.18 ppm
1.53 ppm
CH3CH2CH2OH
0.93 ppm 3.49 ppm
Position of Signals
•Example:
Avoid this common misconception:
“NMR peaks can be assigned based on chemical shift alone”
3.8 ppm not always ROCH3
2.3 ppm not always ArCH3
Common exception
6.5-8.0 ppm usually
benzene ring protons
C=O stretch
Relative Intensity of Peaks
Integration
Information from NMR spectrum
•Number of signals  number of nonequivalent proton groups in molecule
•Position of signals (chemical shift)  magnetic environment of protons
•Relative intensity of signals (integration)  ratio of equivalent proton types
•Splitting of signals (spin-spin coupling)  proton neighbors
Relative Intensity of Peaks
Integration
•Beer’s Law: Amount of energy absorbed or released proportional to moles of stuff present
•NMR: Amount of radio wave energy proportional to peak area
•Measurement of peak areas = integration
Peak
height
Peak
area
∫ir I∫aac Newton Gottfried Leibniz
Inventor∫ of calculu∫
•Relative intensities of NMR signals proportional to relative number of equivalent protons
•Integrals do not always correspond to exact number of protons
Example: Integrals of 2:1 might be 2H:1H or 4H:2H or...
Sample Spectra
•Verify what we have learned about equivalent protons, chemical shifts, and integration
•Assign peaks to corresponding hydrogens:
H
H
C
H
OH
4.19 ppm: integral = 1.0 (1 H)
3.41 ppm: integral = 3.0 (3 H)
CH3OH has 4 H
Sample Spectra
Assign peaks to corresponding hydrogens:
OCH3
H3C
C
CH3
OCH3
3.19 ppm: integral = 1.0 (6 H)
1.33 ppm: integral = 1.0 (6 H)
C5H12O2 has 12 H
Two equal integrals
Two groups of equivalent H
Smallest integral often set = 1
Integration gives proton ratio
Sample Spectra
Assign peaks to corresponding hydrogens:
CH3OCH2CH2OCH3
3.55 ppm: integral = 1.0 (4 H)
3.39 ppm: integral = 1.5 (6 H)
Two groups of equivalent H
Two unequal integrals
C4H10O2 has 10 H
10 H / (1.0 + 1.5) = 4 H per unit
OCH3
H3C
OCH3
CH3
1
1
2
CH3OCH2CH2OCH3
O
H3C
C
H
C
H
H3C
HO
2
H3 C
3
C
OH
2
2
H3 C
H
C
C
H3 C
H
H3 C
CH3
CH3CH2Br
OH
2
2
Lecture Supplement p. 138, 139, 146
Sample Spectra
•Assign peaks to corresponding hydrogens:
CH3CH2Br
Three peaks!
Four peaks!
•Why the extra peaks? Hint: Think about spin and magnetic fields
Proton Nuclear Magnetic Resonance
(1H-NMR) Spectroscopy - Part 3
Lecture Supplement page 147
CH3CH2Br
1H-NMR
Spectroscopy Part 2 Summary
Information from NMR Spectrum
1. Number of signals  how many sets of equivalent protons
2. Position of signals (chemical shift)  magnetic environment of nucleus
Deshielding by electronegative atoms  higher chemical shift
3. Relative intensity of signals (integration)  how many hydrogens per signal
Integrals give proton ratio; not always equal to absolute proton count (i.e., 1.5:1)
4. Splitting of signals (spin-spin coupling)
Example:
3.55 ppm: integral = 1.0 4 H
CH3OCH2CH2OCH3
3.39 ppm: integral = 1.5 6 H
Two groups of equivalent H
Two unequal integrals
C4H10O2 has 10 H
10 H / (1.0 + 1.5) = 4 H per unit
Signal Splitting
1H-NMR
spectrum of CH3CH2Br has more details...
3.43 ppm: integral = 1.0 2 H
1.68 ppm: integral = 1.5 3 H
Two unequal integrals
5 H / (1.0 + 1.5) = 2 H per unit
CH3CH2Br
Three lines
A triplet
Four lines
A quartet
Signals are split
Signal Splitting
What is the origin of signal splitting?
Each line in signal...
...has slightly different chemical shift
...represents slightly different spin flip energy
...represents nucleus with slightly different magnetic environment
A nucleus with only one magnetic environment causes a singlet
A nucleus with two magnetic environments causes a doublet
Signal Splitting
How can one nucleus have different magnetic environments?
•Caused by spin direction of adjacent nuclei
Ha
Bo
C
C
Hb
Ha feels Bo + Hb
Larger DE
Ha feels Bo - Hb
Smaller DE
NMR signal for Ha
•Ha feels two different magnetic environments
•Ha has two different spin flip DE
•Ha has two different (but very similar) chemical shifts
•Ha signal is split into a doublet
Signal Splitting
Some Useful Terms
Spin-spin coupling: One nuclear spin influences spin of another nucleus
Splitting: Effect on NMR signal caused by spin-spin coupling
J
Coupling constant (J): Spacing between lines in a splitting pattern
Signal Splitting
More Than One Neighbor
What is splitting when there is more than one neighbor?
Ha
C
C
Hc
Hb
Ha feels Bo + Hb + Hc
Ha feels Bo - Hb + Hc = Bo
Bo
equal energy
}
H feels B + H - H = B
a
o
b
c
o
Ha feels Bo - Hb - Hc
•Ha has three different (but very similar) chemical shifts
•Ha signal is split into a triplet
NMR signal for Ha
1:2:1 because of energy state
population probabilities
Signal Splitting
Rules and Restrictions
General rule: The signal for a proton with n neighbors is split into n+1 lines
Rules and Restrictions for Proton-Proton Spin-Spin Coupling
1. Only nonequivalent protons couple.
X
X
Ha
Hb
Hc
Hd
H
C
C
C
C
H
H
H
H
•Hb couples with Hc
H
•Hb and Ha do not couple because they are equivalent
•Hc and Hd do not couple because they are equivalent
Signal Splitting
Rules and Restrictions
2. Protons separated by more than three single bonds usually do not couple.
Hb
Hc •H and H _____
2 bonds apart Can couple * Cannot couple
a
b
C
Ha
X
*Assuming Ha and Hb are not equivalent
C
C
•Ha and Hc _____
3 bonds apart Can couple Cannot couple
•Ha and Hd _____
4 bonds apart Can couple Cannot couple
Hd
Pi bonds do not count toward this bond limit, but J may be too small to observe.
Free spacer
Hb
Hc
C
2 (+1) bonds apart Can couple Cannot couple
•Ha and Hc _____
C
Ha
C
Hd
•Ha and Hb _____
2 bonds apart Can couple Cannot couple
3 (+1) bonds apart Can couple* Cannot couple
•Ha and Hd _____
*But Jad may be very small
Signal Splitting
Rules and Restrictions
2. Protons separated by more than three single bonds usually do not couple.
Hd
•Benzene ring = one big free spacer
Hc
F
Hb
Cl
Ha
•All benzene ring protons may couple with each other but J may be small
•Ha, Hb, Hc, and Hd all couple with each other
•Jad may be too small to observe
Benzene ring is a “gated community”; it blocks some coupling that we expect to observe.
H
X
CH3
X
CH2CH3
Signal Splitting
Rules and Restrictions
3. Signals for O-H and N-H are usually singlets
singlet
H2N
triplet
H
H
C
C
H
H
OH
singlet
triplet
•Splitting of O-H or N-H protons may be observed in rare circumstances
Sample Spectra
•Verify what we have learned about equivalency, chemical shifts, integration, and splitting
•Assign peaks to corresponding hydrogens in structure
BrCH2CH2CH3
3.39 ppm (triplet; integral = 1.0) 2 H
1.87 ppm (sextet; integral = 1.0) 2 H
1.03 ppm (triplet; integral = 1.5) 3 H
7 H / (1.0 + 1.0 + 1.5) = 2 H per unit
Sample Spectra
•Assign peaks to corresponding hydrogens in structure
Br
H3C
C
CH3
H
3.78 ppm (septet; integral = 1.0) 1 H
1.31 ppm (doublet; integral = 6.0) 6 H
7 H / (1.0 + 6.0) = 1 H per unit
OCH3
H3C
C
OCH3
CH3
CH3OCH2CH2OCH3
O
H3C
C
H
H3C
H3 C
H
C
H3 C
H
C
C
H3 C
H
HO
H3 C
OH
CH3
CH3CH2Br
OH
Lecture Supplement p. 138, 139, 146
Sample Spectra
For next lecture:
•Assign peaks to corresponding hydrogens in structure
•Explain splitting patterns
H
5.66 ppm (multiplet; integral = 1.0)
H
C
H
H2C
C
H2C
1.61 ppm (multiplet; integral = 2.0)
C
C
H
H
H
1.98 ppm (multiplet; integral = 2.0)
Proton Nuclear Magnetic Resonance
(1H-NMR) Spectroscopy - Part 4
Lecture Supplement page 154
CH3
1H-NMR
Spectroscopy Part 3 Summary
Information from NMR Spectrum
•Number of signals  how many sets of equivalent protons
•Position of signals (chemical shift)  magnetic environment of nucleus
Deshielding by electronegative atoms  higher chemical shift
•Relative intensity of signals (integration)  how many hydrogens per signal
Integrals give proton ratio; not always equal to absolute proton count (i.e., 1.5:1)
•Splitting of signals (spin-spin coupling)
The signal of a proton with n neighbors is split into n+1 lines (first order coupling)
Example: CH3CH2Br
CH3 is a triplet, CH2 is a quartet
More complex patterns (non first-order coupling) are common
1H-NMR
Spectroscopy Part 3 Summary
Splitting rules
•Only nonequivalent hydrogens couple with each other
OH
•Hydrogens can be at most three single bonds distant
•Pi bonds and benzene rings are “free spacers”
H
C
H
H
C
H
H
C
H
H
H
H
H
•Benzene ring “gated community”
•OH, NH usually do not couple, and usually are not split
H
Sample Spectra
•Assign peaks to corresponding hydrogens in structure
Multiplet: A splitting pattern that is too complex to decipher
H
5.66 ppm (multiplet; integral = 1.0) 2 H
H
C
H
H2C
C
H2C
1.98 ppm (multiplet; integral = 2.0) 4 H
1.61 ppm (multiplet; integral = 2.0) 4 H
C
C
H
H
H
10 H / (1.0 + 2.0 + 2.0) = 2 H per unit
Non-First Order Splitting
Why Is Cyclohexene Splitting Not Simple?
First order splitting: All J values in a splitting pattern are equal
•n+1 rule obeyed; “normal” doublets, triplets, etc. result
Non-first order splitting: J values in a splitting pattern are unequal
•More complex splitting patterns result
Example:
Hb
Ha
C
C
Hc
Jab = Jac
“normal” triplet
Jab ≠ Jac
doublet of doublets
For Chem 14C we predict J values equal and “normal” coupling results. Exceptions are plentiful.
Sample Spectra
Effects of Pi Electron Clouds: Magnetic Induction
•Vinyl protons deshielded by magnetic induction
H
H
H
H
C
H
H2C
C
H2C
C
C
H
C
Bo
H
Deshielded
H
•May also cause shielding
Sample Spectra
Benzene Ring Protons
H
10.00 ppm (singlet)
H
7.87-7.56 ppm (multiplet)
H
C
H
O
H
9.5-11 ppm
H
6.5-8.0 ppm
•Magnetic induction causes benzene ring proton chemical shifts ~6.5-8.0 ppm
•Magnetic induction by C=O causes aldehyde proton chemical shift ~9.5-11 ppm
•Due to long range coupling, benzene ring proton signals often multiplets
Sample Spectra
Benzene Ring Protons
Benzene ring proton signals can be deceptively simple...
CH3
•7.2 ppm = singlet?
•Benzene ring protons not equivalent
When chemical shifts very similar
J  0; splitting disappears