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

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Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy Part 1 Lecture Supplement: Take one handout from the table 1H-NMR Spectroscopy Background and Theory ... – PowerPoint PPT presentation

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


1
Proton Nuclear Magnetic Resonance (1H-NMR)
Spectroscopy Part 1
  • Lecture Supplement
  • Take one handout from the table

2
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

3
Background and Theory Nuclear 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

4
Background and Theory Magnetic Field Controls DE
  • DE influenced by magnetic field strength at
    nucleus

? Information about magnetic field strength at
nucleus
? Information about chemical structure
5
Background and Theory The NMR Spectrum
  • Spectrum plot of photon energy versus photon
    quantity

Shielded (upfield) High magnetic field strength
Deshielded (downfield) Low magnetic field strength
6
Background and Theory The NMR Spectrum
Nuclear manipulation of nuclear spin
Magnetic magnetic field strength influences DE
X
1H nucleus a proton ? 1H-NMR proton NMR
7
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

8
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

9
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
  • Models

Build two copies label protons in question
(replace H with D) Superimpose protons in
question If rest of molecule superimposable then
protons in question are equivalent
Not superimposable
10
Number of Signals Proton 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

11
Number of Signals Proton Equivalency
More Proton Equivalency Examples a plane of
symmetry or axis of symmetry renders nuclei
equivalent.
Three signals
Two signals
One signal
mirror plane
Four signals
One signal
12
Number of Signals Proton 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
13
1H-NMR Spectroscopy 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)

14
Position of Signals The 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

15
Position of Signals The 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) (B0)
  • Other atoms in molecule
  • Electron cloud of nucleus in question shields it
    from external magnetic fields

B0 causes these electrons to circulate this
generates a small magnetic field that opposes the
spectrometer magnetic field the nucleus is
shielded from seeing the total B0 DE is
different
Bat nucleus B0 - Bshielding
Shielded nucleus feels weaker magnetic
field Deshielded nucleus feels stronger magnetic
field
16
Position of Signals The Chemical Shift
Intensity of signal (photon quantity)
Reference point?
15 ppm
0 ppm
Chemical shift scale (ppm)
Deshielded (downfield)
Shielded (upfield)
Spin flip energy (photon energy)
17
Position of Signals The Chemical Shift
  • How does molecular structure influence chemical
    shift?

Conclusion ? EN of atoms near H ? chemical shift
18
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

Decreasing electron density around H Less
shielding Higher chemical shift
19
Position of Signals
Do not memorize chemical shifts. Table given on
exams/quizzes.
20
Position of Signals Notes 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
ENC 2.5 ENH 2.3
  • EN effects decrease with distance

CH4 CH3OH CH3CH2OH
CH3CH2CH2OH
21
Position of Signals Avoid This Common
Misconception
  • Unlike IR peaks, we cannot assign NMR peaks based
    only on chemical shift
  • Example

22
Relative Intensity of Peaks Integration
  • 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...

23
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
24
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
25
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
26
Sample Spectra
  • Assign peaks to corresponding hydrogens

CH3CH2Br
  • Homework
  • Why the extra peaks? Hint think about spin and
    magnetic fields
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