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Nuclear Magnetic Resonance (NMR)

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Title: Nuclear Magnetic Resonance (NMR)


1
Nuclear Magnetic Resonance (NMR)
  • for beginners

2
Overview
  • NMR is a sensitive, non-destructive method for
    elucidating the structure of organic molecules
  • Information can be gained from the hydrogens
    (proton NMR, the most common), carbons (13C NMR)
    or (rarely) other elements

3
Spin States
  • All nuclei have a spin state (I )
  • Hydrogen nuclei have a spin of I ½ (like
    electrons)
  • Spin number gives number of ways a particle can
    be oriented in a magnetic field 2I 1

4
Spin States
  • In the absence of a magnetic field the spin
    states are degenerate
  • The spinning nucleus generates its own magnetic
    field

5
Spin States
  • In a magnetic field the states have different
    energies

B?
B?
Bo
6
Spin states in a magnetic field
  • Energy difference linearly depends on field
    strength

? magnetic moment of H (2.7927?N or
14.106067x10-27J/T)
7
Spin states in a magnetic field
  • Even in a very large field (1-20T) the energy
    difference is small (0.1cal/mol)

8
Spin states in a magnetic field
  • A small excess of protons will be in the lower
    energy state
  • These can be promoted to the higher state by
    zapping them with EM radiation of the proper
    wavelength
  • Wavelength falls in the radio/TV band (frequency
    of 60-500MHz)

9
Spin states in a magnetic field
  • Stronger magnetic field necessitates shorter
    wavelength (higher frequency)
  • After low energy protons are promoted to the
    higher energy state they relax back to the lower
    state

10
Making NMR work
  • Not all protons absorb at the same field values
  • Either magnetic field strength or radio frequency
    must be varied
  • Frequency/field strength at which the proton
    absorbs tells something about the protons
    surroundings

11
Making NMR work
12
  • Sample must be spun to average out magnetic field
    inhomogeneity

13
NMR data collection
  • Continuous wave data collection (CW)
  • Magnetic field value is varied
  • Intensity of emitted RF compared to RF at
    detector
  • Absorption is plotted on graph

14
NMR data collection
CW NMR of isopropanol
15
NMR data collection
  • Pulsed Fourier transform data collection
  • Short bursts of RF energy are shot at sample
  • Produces a decay pattern
  • FT done by computer produces spectrum

16
Simple FT FID and spectrum
17
More complex FT FID and spectrum
18
Even more complex FT FID
19
FT NMR Spectrum
20
Pulsed FT NMR of isopropanol
21
Chemical shift
  • Protons in different environments absorb at
    different field strengths (for the same
    frequency)
  • Different environment different electron
    density around the H

22
Chemical shift positions
High field, shielded
Low field, deshielded
PPM of applied field (?) from reference
Reference (tetramethylsilane)
23
Chemical shift positions
24
NMR reference
  • Tetramethylsilane ((CH3)4Si)
  • Advantages
  • Makes one peak
  • 12 equivalent H, so little is needed
  • Volatile, inert, soluble in organic solvents
  • Absorbs upfield of hydrogens in most organic
    compounds

25
Shielding/deshielding
  • Electron density affects chemical shift
  • Electrons generate a magnetic field opposed to
    the applied field
  • H in high electron density absorbs upfield
    (toward TMS, 0ppm) from H in low electron density

26
Shielding/deshielding
  • Effect of electronegativity electronegative
    atom nearby removes electron density and causes
    deshielding
  • TMS protons are extremely shielded because Si is
    electropositive compared to C

27
Shielding/deshielding
  • Few protons absorb upfield of TMS
  • Alkyl groups are electron donating, so alkanes
    absorb around 0-2ppm (?)
  • Hydrogens near electronegative atoms are
    deshielded
  • Absorption is around 3-4?

28
Anisotropy
  • Anisotropy any characteristic that varies
    with direction (asymmetric)
  • Applied to the shielding/deshielding
    characteristics of electrons in some systems

29
Anisotropy
  • Aromatic hydrogens are in the deshielding region
    of the magnetic field generated by circulating
    electrons

30
Typical chemical shifts
31
Spin-spin coupling
  • Magnetic field felt by a proton is affected by
    the spin states of nearby protons either
    shielding or deshielding
  • Case 1 neighboring single protons
  • These H can either be the same or opposite spins
    equal probability
  • Makes doublets of two equal peaks at both
    absorptions

32
NMR spectrum of dichloroacetaldehyde
33
Coupling constants
  • Separation between peaks is the coupling
    constant
  • Symbol J
  • Measured in Hz
  • It is the same for both coupled protons

34
Spin-spin coupling
  • Case 2 Single proton next to a pair
  • Single proton splits the pair into a doublet
  • Spin state possibilities for pair

? ?
Integration ratio 121
? ?
? ?
Bo
? ? Equal energy
35
Spin-spin coupling
  • Single proton is split into a triplet
  • Any group of n protons will split its neighbors
    into n 1 peaks
  • Intensity follows Pascals triangle (Fibonacci
    series)

36
Spin coupling example
  • Chloroethane CH3CH2Cl

37
Protons on Heteroatoms
  • Protons on N or O often give broad uncoupled
    peaks of uncertain chemical shift
  • Protons on nitrogen are broad due to coupling
    with nitrogen nucleus (spin 1)
  • Chemical shift can depend on concentration
  • Peaks will be sharp and coupled if there is no
    acid or water present

38
Protons on heteroatoms
Split into doublet by NH reciprocal splitting
is not seen
Proton on nitrogen broad due to interaction
with nitrogen (spin number 1)
39
Phenolic Protons and Concentration
40
Alcoholic protons and coupling
1H NMR spectrum of methanol at various
temperatures
41
Chemical Shift Differences and Coupling
  • Equivalent protons do not split each other
  • Adjacent protons (vicinal) exhibit simple
    coupling if their chemical shifts are very
    different (??/J gt10)
  • Designated an AaXx system (AaMmXx for three
    widely separated sets)
  • Subscripts designate the number of protons
    involved

42
Chemical Shift Differences and Coupling
  • Sets of protons close to each other are AaBb or
    AaBbCc
  • The closer two sets are the more the peaks are
    distorted

AX system becoming an AB system
43
Chemical Shift Differences and Coupling
44
AX system with some distortion
45
Ternary systems
  • AaMmXx systems exhibit simple splitting with two
    coupling constants

46
Ternary Systems
47
Ternary systems
48
Chemical and magnetic equivalence
49
Chemical and magnetic equivalence
50
Chemical and magnetic equivalence
NMR spectrum of butane
51
Chemical Shift Differences and Coupling
  • AaBbXx systems are approximately first order
    (simple splitting)
  • AaBbCc systems are complex
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