Proton Nuclear Magnetic Resonance 1HNMR Spectroscopy Part 1

1
Proton Nuclear Magnetic Resonance (1H-NMR)
Spectroscopy Part 1

• Lecture Supplement
• Take one handout from the stage

2
Midterm 1
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1H-NMR SpectroscopyBackground 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

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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

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Background and TheoryMagnetic Field Controls DE

• DE influenced by magnetic field strength at
  nucleus

? Information about magnetic field strength at
nucleus
? Information about chemical structure
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Background and TheoryThe NMR Spectrum

• Spectrum plot of photon energy versus photon
  quantity

Shielded (upfield) High magnetic field strength
Deshielded (downfield) Low magnetic field strength
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Background and TheoryThe NMR Spectrum
Nuclear manipulation of nuclear spin
Magnetic magnetic field strength influences DE
X
1H nucleus a proton ? 1H-NMR proton NMR
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Background and TheorySpectrum ? 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

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Number of SignalsProton 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

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Number of SignalsProton 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
• Models

Build two copies label protons in
question Superpose protons in question If rest of
molecule superposable then protons in question
are equivalent
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Number of SignalsProton Equivalency
Proton Equivalency Examples
One signal
Two signals ?

• NMR slow camera
• NMR detects only average if rotation is fast
• Thousands of H3C-CH3 rotations per second
• Ha, Hb, Hc appear equivalent
• In general single bond rotation in acyclic
  molecules allows equivalency

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Number of SignalsProton Equivalency
More Proton Equivalency Examples
Three signals
Two signals
One signal
Four signals
One signal
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Number of SignalsProton Equivalency
Sample Spectra

• Verify what we have learned about equivalent
  protons
• How many signals in 1H-NMR spectra of these
  molecules?

Three proton sets ? three signals
Two proton sets ? two signals
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Proton Nuclear Magnetic Resonance (1H-NMR)
Spectroscopy Part 2

• Lecture Supplement
• Take one handout from the stage

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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 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)

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Position of SignalsThe Chemical Shift

• How does spin flip energy relate to molecular
  structure?
• Spin flip energy depends on magnetic field
  strength

• High magnetic field higher spectral resolution
  (more spectral detail)
• Magnetic field strength varies between NMR
  spectrometers
• Need a scale that is independent of magnetic
  field strength
• Chemical shift spin flip energy scale normalized
  to be independent of field strength

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Position of SignalsThe Chemical Shift

• How does molecular structure influence chemical
  shift?
• Chemical shift ? DE ? magnetic field at nucleus

• What contributes to magnetic field at nucleus?
• Earths magnetic field (weak 0.3-0.6 gauss)
• Spectrometers magnetic field (strong typically
  94 kilogauss)
• Other atoms in molecule
• Electron cloud of nucleus in question shield it
  from external magnetic fields

Shielded nucleus feels weaker magnetic
field Deshielded nucleus feels stronger magnetic
field
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Position of SignalsThe Chemical Shift
Intensity of signal (photon quantity)
Reference point?
Spin flip energy (photon energy)
15 ppm
0 ppm
Chemical shift scale (ppm)
Deshielded (downfield)
Shielded (upfield)
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Position of SignalsThe Chemical Shift

• How does molecular structure influence chemical
  shift?

Conclusion ? EN of atoms near H ? chemical shift
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Position of SignalsThe 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

Decreasing electron density around H Less
shielding Higher chemical shift
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Position of Signals
Do not memorize chemical shifts. Table given on
exams.
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Position of SignalsNotes On Characteristic
Chemical Shifts Table

• Characteristic shifts are typical proton
  averages. Actual shifts may lie outside given
  range.

• Useful chemical shift trends
• RCH3 lt RCH2R lt R3CH

EN of C (in R) gt EN of H

• EN effects decrease with distance

CH4 CH3OH CH3CH2OH
CH3CH2CH2OH
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Position of SignalsAvoid This Common
Misconception

• Unlike IR peaks, we cannot assign NMR peaks based
  only on chemical shift
• Example

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Relative Intensity of PeaksIntegration

• Beers Law amount of energy absorbed or
  transmitted proportional to moles of stuff present

• NMR amount of radio wave energy proportional to
  peak area
• Measurement of peak areas integration

• 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 21 might be 2H1H or 4H2H
  or...

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Sample Spectra

• Verify what we have learned about equivalent
  protons, chemical shifts, and integration
• Assign peaks to corresponding hydrogens

4.19 ppm integral 1.0 3.41 ppm integral 3.0
(1 H) (3 H)
CH3OH has 4 H
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Sample Spectra

• Assign peaks to corresponding hydrogens

3.19 ppm integral 1.0 1.33 ppm integral 1.0
(6 H) (6 H)
C5H12O2 has 12 H Two equal integrals Two groups
of equivalent H
Smallest integral often set 1 Integration gives
proton ratio
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Sample Spectra

• Assign peaks to corresponding hydrogens

3.55 ppm integral 1.0 3.39 ppm integral 1.5
(4 H) (6 H)
CH3OCH2CH2OCH3
Two groups of equivalent H Two unequal
integrals C4H10O2 has 10 H 10 H / (1.0 1.5) 4
H per unit
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Sample Spectra

• Assign peaks to corresponding hydrogens

CH3CH2Br

• Homework
• Why the extra peaks? Hint think about spin and
  magnetic fields

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Proton Nuclear Magnetic Resonance (1H-NMR)
Spectroscopy Part 3

• Lecture Supplement
• Take one handout from the stage

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1H-NMR Spectroscopy Part 2 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., 1.51)
• Splitting of signals (spin-spin coupling)
• Example

3.55 ppm integral 1.0 3.39 ppm integral 1.5
4 H 6 H
CH3OCH2CH2OCH3
Two groups of equivalent H Two unequal
integrals C4H10O2 has 10 H 10 H / (1.0 1.5) 4
H per unit
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Signal Splitting
three lines triplet
1H-NMR spectrum of CH3CH2Br has more details...
Signals are split
3.43 ppm integral 1.0 1.68 ppm integral 1.5
2 H 3 H
CH3CH2Br
Two unequal integrals 5 H / (1.0 1.5) 2 H per
unit
four lines quartet
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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
This nucleus has only one magnetic environment A
singlet
This nucleus has two magnetic environments A
doublet
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Signal Splitting
How can one nucleus have different magnetic
environments?

• Spin direction of adjacent nuclei

Ha feels Bo Hb
Ha feels Bo - Hb

• 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

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Signal SplittingSome Useful Terms
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
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Signal SplittingMore Than One Neighbor
What is splitting when there is more than one
neighbor?
Ha feels Bo Hb Hc
Ha feels Bo - Hb Hc
Ha feels Bo Hb - Hc
Ha feels Bo - Hb - Hc
121 because of energy state population
probabilities

• Ha has three different (but very similar)
  chemical shifts
• Ha signal is split into a triplet

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Signal SplittingRules and Restrictions
Rules and Restrictions for Proton-Proton
Spin-Spin Coupling 1. Only nonequivalent protons
couple

• Hb couples with Hc

• Hb and Ha do not couple because they are
  equivalent

• Hc and Hd do not couple because they are
  equivalent

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Signal SplittingRules and Restrictions
2. Protons separated by more than three single
bonds usually do not couple

• Ha couples with Hb

• Ha couples with Hc

• Ha does not couple with Hd

Pi bonds do not count toward this bond limit, but
J may be too small to observe

• Ha couples with Hb

• Ha couples with Hc

• Ha couples with Hd but J may be very small

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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

Benzene ring is a gated community it blocks
some coupling that we expect to observe
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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

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Sample Spectra

• Verify what we have learned about equivalency,
  chemical shifts, integration, and splitting
  patterns
• Assign peaks to corresponding hydrogens in
  structure

3.39 ppm (triplet integral 1.0) 1.87 ppm
(sextet integral 1.0) 1.03 ppm (triplet
integral 1.5)
2 H 2 H 3 H
BrCH2CH2CH3
7 H / (1.0 1.0 1.5) 2 H per unit
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Sample Spectra

• Assign peaks to corresponding hydrogens in
  structure

3.79 ppm (septet integral 1.0) 1.31 ppm
(doublet integral 6.0)
1 H 6 H
7 H / (1.0 6.0) 1 H per unit
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Sample Spectra

• Assign peaks to corresponding hydrogens in
  structure

2 H 4 H 4 H
5.66 ppm (multiplet integral 1.0)
1.98 ppm (multiplet integral 2.0) 1.16 ppm
(multiplet integral 2.0)
10 H / (1.0 2.0 2.0) 2 H per unit
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Non-First Order SplittingWhy Is Cyclohexene
Splitting Not Simple?

• First order splitting all J values in a
  splitting pattern are equal
• n1 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.
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Proton Nuclear Magnetic Resonance (1H-NMR)
Spectroscopy Part 4

• Lecture Supplement
• Take one handout from the stage

45
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., 1.51)
• Splitting of signals (spin-spin coupling)
• The signal of a proton with n neighbors is split
  into n1 lines (first order coupling)
• Example CH3CH2Br CH3 is a triplet, CH2 is a
  quartet
• More complex patterns (non first-order coupling)
  are common
• Splitting rules
• 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 not split

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Sample SpectraEffects of Pi Electron Clouds
Vinyl Protons (CC-H)
5.83 ppm (doublet of doublets 1H) 4.88 ppm
(multiplet 2 H) 1.00 ppm (singlet 9 H)

• Vinyl H splitting often not first order
• Pi electron cloud causes deshielding, downfield
  shift
• Shift effect by pi electrons magnetic induction

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Sample SpectraBenzene Ring Protons
10.00 ppm (singlet) 7.87-7.56 ppm (multiplet)

• Magnetic induction causes benzene ring proton
  chemical shifts 6.5-8.0 ppm

• Magnetic induction by CO causes aldehyde proton
  chemical shift 9.5-11 ppm

• Due to long range coupling, benzene ring proton
  signals often multiplets

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Sample SpectraBenzene Ring Protons
Benzene ring proton signals can be deceptively
simple...

• 7.2 ppm singlet?
• Benzene ring protons not equivalent