proton nuclear magnetic resonance ( 1 h-nmr) spectroscopy - part 1 lecture supplement page 133
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Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy - Part 1
Lecture Supplement page 133
Organic Structure AnalysisCrews, Rodriguez, Jaspers1998 p.5-6
1H-NMR SpectroscopyBackground and Theory
Fundamental principleThe 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
No external magnetic fieldSpin alignment random
With external magnetic fieldSpins aligned
Bo
•Add external magnetic field (Bo) spins align
add magnetic field
Background and TheoryNuclear Spin Flip
•I = +1/2 parallel to Bo (lower energy); I = -1/2 antiparallel to Bo (higher energy)
•Addition of energy results in nuclear spin flip
Excited stateNuclear spin antiparallel to Bo
Higher energy
I = +1/2
I = -1/2
Ground stateNuclear spin parallel to Bo
Lower energy
Incr
easi
ng e
nerg
y
Absorb energy(excitation)
Release energy(relaxation)
E ~ 0.02 cal mol-1 = radio wave photons
Contrast this with absorption of infrared light (p. 114 of lecture supplement)
Background and TheoryMagnetic Field Controls E
• E influenced by magnetic field strength at nucleus
Information about magnetic field strength at nucleus
Energy required for spin flip (E)
Information about chemical structure
I = -1/2
I = +1/2
Spin
sta
te e
nerg
y
Magnetic field strength at nucleus
Large magnetic field large E
Small magnetic field small EE E
Background and TheoryThe NMR Spectrum
•Spectrum = plot of photon energy versus photon quantityIn
tens
ity o
f sig
nal
(pho
ton
quan
tity)
Spin flip energy (photon energy)
NMR signal
Background and TheoryThe NMR Spectrum
Nuclear: Manipulation of nuclear spin
Magnetic: Magnetic field strength influences E
X
Resonance: Tendency of a system to oscillate at maximumamplitude at a certain frequency
1H nucleus = a proton 1H-NMR = proton NMR
C O C O
Background and TheorySpectrum Structure
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
How do we deduce structure from NMR spectrum?
Number of SignalsProton Equivalency
•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
Protons equivalentOne NMR signal
OH H
Protons not equivalentTwo NMR signals
Cl CN
HH
•NMR signal due to photon absorption
Number of SignalsProton 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
How to test for equivalency?
Useful vocabulary
CH3 = methyl CH2 = methylene CH = methine
H C
H
H
C
H
H
C H
Cl
Cl
H C
H
H
C
H
H
C H
Cl
Cl
H C
H
H
C
H
H
C H
Cl
Cl
Number of SignalsProton Equivalency
Proton equivalency examples
One signal
CH
H
Cl
H
CC
H
Cl
H
H
H
H
Two signals ?
•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
Hb, Hc equivalent
Ha, Hc not equivalent
rapid equilibrium
Ha, Hc equivalent
Hb, Ha not equivalent
Single bond rotation in acyclic molecules often allows equivalency
Number of SignalsProton Equivalency
Proton equivalency examples
One signal
CH
H
Cl
H
CC
H
Cl
H
H
H
H
Two signals ? Hb, Hc equivalent
Ha, Hc not equivalent
rapid equilibrium
Ha, Hc equivalent
Hb, Ha not equivalent
vs.
C
C C
C
CC
H
HH
H
H H
Number of SignalsProton Equivalency
More proton equivalency examples
Three signals
CC
H
OH
H
H
H
H
CC
Cl
H
Cl
F
H
F
Two signals
CC
F
Cl
Cl
H
H
F
One signal
C
C C
C
CC
C
HH
H
H H
H
H
H
Four signals
mirror plane
One signal
Number of SignalsProton Equivalency
•Verify what we have learned about equivalent protons•Which spectrum belongs to this molecule?
Sample spectra
Three proton sets three signals
C C
O
H3C
H3C H
H
H3C C
H3C
H3C
C OH
H
H
Number of SignalsProton Equivalency
Which spectrum belongs to this molecule?
Two proton sets two signals
HH
H H
OHHO
HO OH H3C CH3
Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy - Part 2
Lecture Supplement page 139
CH3C
OCH3
OCH3
CH3
1H-NMR Spectroscopy Part 1 Summary
•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 (E) controlled by magnetic field at nucleus•Absorption of radio wave photon with energy = E 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 equivalentNonequivalency: Protons can be different in just one wayExample: 1H-NMR spectrum of CH3CH2OH has three signals
•Position of signal (chemical shift)•Relative intensity of signals (integration)•Splitting of signals (spin-spin coupling)
•Atomic nucleus has spin, and therefore generates a magnetic field
Vollhardt, 10-2
Position of SignalsThe Chemical Shift
How does spin flip energy relate to molecular structure?
•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
I = -1/2
I = +1/2
Spin
sta
te e
nerg
y
Magnetic field strength at nucleus
Large magnetic field large E
Small magnetic field small E
•Spin flip energy depends on magnetic field strength:
E E
Position of SignalsThe Chemical Shift
How does molecular structure influence chemical shift?
•Chemical shift controlled by E which is controlled by magnetic field at nucleus
What contributes to magnetic field at nucleus?
•Electron cloud shields atomic nucleus from external magnetic fields
Shielded: Nucleus feels weaker magnetic field
Deshielded: Nucleus feels stronger magnetic field
Spectrometer’s magnetic fieldStrong; typically 94 kilogauss
Earth’s magnetic fieldWeak; 0.3-0.6 gauss
Other electrons andnuclei in the molecule
HO CH2 CH2 CH3
Position of SignalsThe Chemical Shift
Inte
nsity
of s
igna
l(p
hoto
n qu
antit
y)
Spin flip energy (photon energy)
Deshielded Shielded
15 ppm 0 ppmChemical shift scale(ppm)
Reference point?
0.00 ppm (CH3)4SiTetramethylsilane (TMS)
High Magnetic Field StrengthLow Magnetic Field Strength
Position of SignalsThe Chemical Shift
•How does molecular structure influence chemical shift? Chemical shifts for H3C-X:0.01.02.03.04.05.0 ppm
(CH3 )4 Si 0.00 ppm
CH4 0.23 ppm
CH3 I 2. 16 ppm
CH3 Br 2.68 ppm
CH3 Cl 3.05 ppm
CH3 O
H 3.42 ppm
CH3 F 4.26 ppm
1.0
2.0
3.0
4.0
EN o
f X in
CH
3-X
Conclusion: EN of atoms near H chemical shift
Si1.8H
2.1I
2.5Br2.8Cl
3.0O
3.5F
4.0
CH3 CH
3 0.86 ppm
C2.5
Vollhardt, Fig 10-9
HO CH2 CH2 CH3
•Electron cloud shields atomic nucleus from external magnetic fields
Shielded: Nucleus feels weaker magnetic field
Deshielded: Nucleus feels stronger magnetic field
deshielded shielded
Position of SignalsThe Chemical Shift
How does electronegativity influence chemical shift?
H I H Br H Cl H F
Iodine has low EN 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 higher chemical shiftH has lower chemical shift
Metaphor: Ozone layer shields Earth
•Chemical shift related to magnetic field strength at nucleus
•Electron cloud shields nucleus from effects of Bo
Position of Signals
Do not memorize chemical shifts. Table given on exams.
Position of SignalsNotes On Characteristic Chemical Shifts Table
•Characteristic shifts are typical proton averages. Actual shifts may lie outside given range.
CH3
O
Typically 2.0-2.6 ppm CH3O CH3
O
2.01 ppm CH3
O
2.59 ppm
Useful chemical shift trends•RCH3 < RCH2R < R3CH
•EN effects decrease with distance:
EN of C (in R) > EN of H
CH4 CH3OH CH3CH2OH CH3CH2CH2OH
0.23 ppm 3.39 ppm 1.18 ppm 0.93 ppm 3.49 ppm
3.59 ppm 1.53 ppm
Position of Signals
•Example:
3.8 ppm not always ROCH3
2.3 ppm not always ArCH3
Common exception
benzene ring protons6.5-8.0 ppm usually
C=O stretch
Avoid this common misconception:“NMR peaks can be assigned based on chemical shift alone”
Relative Intensity of PeaksIntegration
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 PeaksIntegration
•Beer’s Law: Amount of energy absorbed or released proportional to moles of stuff present
∫ir I∫aac Newton Gottfried LeibnizInventor∫ 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...
•NMR: Amount of radio wave energy proportional to peak area
•Measurement of peak areas = integration
Peakheight
Peakarea
Sample Spectra
•Verify what we have learned about equivalent protons, chemical shifts, and integration
4.19 ppm: integral = 1.03.41 ppm: integral = 3.0
(1 H)(3 H)
CH3OH has 4 H
H C
H
OH
H
•Assign peaks to corresponding hydrogens:
Sample Spectra
Assign peaks to corresponding hydrogens:
3.19 ppm: integral = 1.01.33 ppm: integral = 1.0
(6 H)(6 H)
C5H12O2 has 12 HTwo equal integrals
Two groups of equivalent H
CH3C
OCH3
OCH3
CH3
Smallest integral often set = 1Integration gives proton ratio
Sample Spectra
Assign peaks to corresponding hydrogens:
3.55 ppm: integral = 1.03.39 ppm: integral = 1.5
(4 H)(6 H)
Two groups of equivalent HTwo unequal integrals
C4H10O2 has 10 H10 H / (1.0 + 1.5) = 4 H per unit
CH3OCH2CH2OCH3
C C
O
H3C
H3C H
H
H3C C
H3C
H3C
C OH
H
H
Lecture Supplement p. 138, 139, 146
HO OH
H3C CH3
CH3C
OCH3
OCH3
CH3
CH3OCH2CH2OCH3
CH3CH2Br
1
2
3
1
2
2
2
2
2
Sample Spectra•Assign peaks to corresponding hydrogens:
CH3CH2Br
•Why the extra peaks? Hint: Think about spin and magnetic fields
Four peaks!
Three peaks!
Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy - Part 3
Lecture Supplement page 147
CH3CH2Br
1H-NMR Spectroscopy Part 2 Summary
1. Number of signals how many sets of equivalent protons2. Position of signals (chemical shift) magnetic environment of nucleus
Deshielding by electronegative atoms higher chemical shift3. 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
3.39 ppm: integral = 1.54 H6 H
Two groups of equivalent HTwo unequal integrals
C4H10O2 has 10 H10 H / (1.0 + 1.5) = 4 H per unit
CH3OCH2CH2OCH3
Information from NMR Spectrum
Signal Splitting
3.43 ppm: integral = 1.01.68 ppm: integral = 1.5
Two unequal integrals5 H / (1.0 + 1.5) = 2 H per unit
2 H3 H
Four linesA quartet
Three linesA triplet
Signals are split
1H-NMR spectrum of CH3CH2Br has more details...
CH3CH2Br
Signal Splitting
What is the origin of signal splitting?
A nucleus with only one magnetic environment causes a singlet
A nucleus with two magnetic environments causes a doublet
Each line in signal... ...has slightly different chemical shift ...represents slightly different spin flip energy ...represents nucleus with slightly different magnetic environment
Signal Splitting
How can one nucleus have different magnetic environments?
Ha feels Bo + Hb
•Ha feels two different magnetic environments
•Ha has two different spin flip E
Ha feels Bo - Hb
Bo
Ha C C Hb
•Caused by spin direction of adjacent nuclei
Larger E
Smaller E
•Ha has two different (but very similar) chemical shifts
•Ha signal is split into a doublet
NMR signal for Ha
Signal SplittingSome Useful Terms
J
Spin-spin coupling: One nuclear spin influences spin of another nucleus
Splitting: Effect on NMR signal caused by spin-spin coupling
Coupling constant (J): Spacing between lines in a splitting pattern
Signal SplittingMore Than One Neighbor
What is splitting when there is more than one neighbor?
Ha feels Bo + Hb + Hc
•Ha has three different (but very similar) chemical shifts
•Ha signal is split into a triplet
Ha C C Hc
Hb
Bo
Ha feels Bo - Hb + Hc = Bo
Ha feels Bo + Hb - Hc = Bo
Ha feels Bo - Hb - Hc
} equal energy
1:2:1 because of energy state population probabilities
NMR signal for Ha
Signal SplittingRules 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 Coupling1. Only nonequivalent protons couple.
•Hc and Hd do not couple because they are equivalent
•Hb couples with Hc
•Hb and Ha do not couple because they are equivalentHa C
Hb
H
C
Hc
H
C
Hd
H
C
H
H
H
XX
Signal SplittingRules and Restrictions
2. Protons separated by more than three single bonds usually do not couple.
•Ha and Hb _____ bonds apart Can couple Cannot couple
•Ha and Hc _____ bonds apart Can couple Cannot couple
•Ha and Hd _____ bonds apart Can couple Cannot couple
Pi bonds do not count toward this bond limit, but J may be too small to observe.
CC
Ha
Hd
C
HcHb •Ha and Hb _____ bonds apart Can couple Cannot couple
•Ha and Hc _____ bonds apart Can couple Cannot couple
•Ha and Hd _____ bonds apart Can couple Cannot couple
Free spacer
CC
Ha
Hd
C
HcHb
X
*But Jad may be very small
*
*Assuming Ha and Hb are not equivalent
*
2
3
4
2
2 (+1)
3 (+1)
Signal SplittingRules and Restrictions
2. Protons separated by more than three single bonds usually do not couple.
•Benzene ring = one big free spacer
•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
F
Hd
Ha
Cl
Hc
Hb
Benzene ring is a “gated community”; it blocks some coupling that we expect to observe.
CH3
H
CH2CH3
X
X
Signal SplittingRules and Restrictions
3. Signals for O-H and N-H are usually singlets
•Splitting of O-H or N-H protons may be observed in rare circumstances
singletsinglet
triplettriplet
H2N C
H
H
C
H
H
OH
Sample Spectra•Verify what we have learned about equivalency, chemical shifts, integration, and splitting
3.39 ppm (triplet; integral = 1.0)
1.87 ppm (sextet; integral = 1.0)
1.03 ppm (triplet; integral = 1.5)
7 H / (1.0 + 1.0 + 1.5) = 2 H per unit
2 H
2 H
3 H
•Assign peaks to corresponding hydrogens in structure
BrCH2CH2CH3
Sample Spectra•Assign peaks to corresponding hydrogens in structure
3.78 ppm (septet; integral = 1.0)
1.31 ppm (doublet; integral = 6.0)7 H / (1.0 + 6.0) = 1 H per unit
1 H
6 H
H3C C
Br
CH3
H
C C
O
H3C
H3C H
H
H3C C
H3C
H3C
C OH
H
H
Lecture Supplement p. 138, 139, 146
HO OH
H3C CH3
CH3C
OCH3
OCH3
CH3
CH3OCH2CH2OCH3
CH3CH2Br
Sample SpectraFor next lecture:
5.66 ppm (multiplet; integral = 1.0)
1.98 ppm (multiplet; integral = 2.0)
1.61 ppm (multiplet; integral = 2.0)
CH2C
H2CC
C
C
H
H
H H
HH
•Assign peaks to corresponding hydrogens in structure
•Explain splitting patterns
Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy - Part 4
Lecture Supplement page 154
CH3
1H-NMR Spectroscopy Part 3 Summary
•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
Information from NMR Spectrum
1H-NMR Spectroscopy Part 3 Summary
•Only nonequivalent hydrogens couple with each other
•Hydrogens can be at most three single bonds distant
•Pi bonds and benzene rings are “free spacers”
•Benzene ring “gated community”
•OH, NH usually do not couple, and usually are not split
Splitting rules
C HH
C HH
C HH
OH
H
H
H
H
H
Sample Spectra•Assign peaks to corresponding hydrogens in structure
5.66 ppm (multiplet; integral = 1.0)
1.98 ppm (multiplet; integral = 2.0)
1.61 ppm (multiplet; integral = 2.0)
10 H / (1.0 + 2.0 + 2.0) = 2 H per unit
2 H
4 H
4 H
CH2C
H2CC
C
C
H
H
H H
HH
Multiplet: A splitting pattern that is too complex to decipher
Non-First Order SplittingWhy Is Cyclohexene Splitting Not Simple?
•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:
For Chem 14C we predict J values equal and “normal” coupling results. Exceptions are plentiful.
Ha C C
Hb
Hc Jab = Jac
“normal” tripletJab ≠ Jac
doublet of doublets
First order splitting: All J values in a splitting pattern are equal
Sample SpectraEffects of Pi Electron Clouds: Magnetic Induction
CH2C
H2CC
C
C
H
H
H H
HH
•Vinyl protons deshielded by magnetic induction
BoDeshielded
H C H
•May also cause shielding
Sample SpectraBenzene Ring Protons
•Magnetic induction causes benzene ring proton chemical shifts ~6.5-8.0 ppm
10.00 ppm (singlet)
7.87-7.56 ppm (multiplet)
•Magnetic induction by C=O causes aldehyde proton chemical shift ~9.5-11 ppm
6.5-8.0 ppm
C
O
H
HH
H H
H
•Due to long range coupling, benzene ring proton signals often multiplets
9.5-11 ppm
Sample SpectraBenzene Ring Protons
Benzene ring proton signals can be deceptively simple...
•7.2 ppm = singlet?•Benzene ring protons not equivalent
When chemical shifts very similarJ 0; splitting disappears
CH3
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