Nuclear Magnetic Resonance (NMR)

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Nuclear Magnetic Resonance (NMR)

• for beginners

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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

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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

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Spin States

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

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Spin States

• In a magnetic field the states have different
  energies

B?
B?
Bo
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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)
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Spin states in a magnetic field

• Even in a very large field (1-20T) the energy
  difference is small (0.1cal/mol)

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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)

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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

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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

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

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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

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NMR data collection
CW NMR of isopropanol
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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

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Simple FT FID and spectrum
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More complex FT FID and spectrum
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Even more complex FT FID
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FT NMR Spectrum
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Pulsed FT NMR of isopropanol
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Chemical shift

• Protons in different environments absorb at
  different field strengths (for the same
  frequency)
• Different environment different electron
  density around the H

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Chemical shift positions
High field, shielded
Low field, deshielded
PPM of applied field (?) from reference
Reference (tetramethylsilane)
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Chemical shift positions
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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

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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

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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

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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?

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Anisotropy

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

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Anisotropy

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

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Typical chemical shifts
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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

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NMR spectrum of dichloroacetaldehyde
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Coupling constants

• Separation between peaks is the coupling
  constant
• Symbol J
• Measured in Hz
• It is the same for both coupled protons

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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
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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)

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Spin coupling example

• Chloroethane CH3CH2Cl

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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

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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)
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Phenolic Protons and Concentration
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Alcoholic protons and coupling
1H NMR spectrum of methanol at various
temperatures
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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

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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
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Chemical Shift Differences and Coupling
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AX system with some distortion
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Ternary systems

• AaMmXx systems exhibit simple splitting with two
  coupling constants

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Ternary Systems
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Ternary systems
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Chemical and magnetic equivalence
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Chemical and magnetic equivalence
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Chemical and magnetic equivalence
NMR spectrum of butane
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Chemical Shift Differences and Coupling

• AaBbXx systems are approximately first order
  (simple splitting)
• AaBbCc systems are complex