Nuclear Magnetic Resonance

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Nuclear Magnetic Resonance

• Chapter 13

2
Electromagnetic Radiation

• Electromagnetic radiation light and other forms
  of radiant energy ??? c E h?
• Wavelength (l) the distance between consecutive
  identical points on a wave
• Frequency (n) the number of full cycles of a
  wave that pass a point in a second
• Hertz (Hz) the unit in which radiation
  frequency is reported s-1 (read per second)

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Electromagnetic Radiation

• Wavelength

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Molecular Spectroscopy

• We study three types of molecular spectroscopy

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A pictorial view of UV/Vis
UV/Vis radiation is measured in nm (wavelength)
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IR Spectroscopy

• IR radiation is measured in cm-1
• This is actually a frequency. Remember that
  frequency and wavelength are inversely
  proportional.

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NMR Spectroscopy

• NMR uses radiowaves, measured in MHz

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Nuclear Spin States

• Any atomic nucleus that has an odd mass, an odd
  atomic number, or both has a spin and a resulting
  nuclear magnetic moment.
• The allowed nuclear spin states are determined by
  the spin quantum number, I , of the nucleus.
• A nucleus with spin quantum
  number I has 2I 1spin
  states. If I 1/2, there are
  two allowed spin states

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Molecular Spectroscopy

• Nuclear magnetic resonance (NMR) spectroscopy a
  spectroscopic technique that gives us information
  about the number and types of atoms in a
  molecule, for example, about the number and types
  of
• hydrogens using 1H-NMR spectroscopy
• carbons using 13C-NMR spectroscopy
• phosphorus using 31P-NMR spectroscopy

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Nuclear Spin States

• Spin quantum numbers and allowed nuclear spin
  states for selected isotopes of elements common
  to organic compounds

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Nuclear Spins in B0

• Within a collection of 1H and 13C atoms, nuclear
  spins are completely random in orientation
• When placed in a strong external magnetic field
  of strength B0, however, interaction between
  nuclear spins and the applied magnetic field are
  quartered, with the result that only certain
  orientations of nuclear magnetic moments are
  allowed

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Nuclear Spins in B0

• For 1H and 13C, only two orientations are allowed.

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Nuclear Spins in B0

• In an applied field strength of 7.05T, which is
  readily available with present-day
  superconducting electromagnets, the difference in
  energy between nuclear spin states for
• 1H is approximately 0.0286 cal/mol, which
  corresponds to electromagnetic radiation of 300
  MHz (300,000,000 Hz)(300MHz)
• 13C is approximately 0.00715 cal/mol, which
  corresponds to electromagnetic radiation of 75MHz
  (75,000,000 Hz)(75 MHz)

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Population in high vs low

• ?E 0.0286 cal/mol RT582cal/mol
• If pop in high E state is 1,000,000 then pop in
  low energy state is 1,000,049

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Nuclear Magnetic Resonance

• If we were dealing with 1H nuclei isolated from
  all other atoms and electrons, any combination of
  applied field and radiation that produces a
  signal for one 1H would produce a signal for all
  1H. The same is true of 13C nuclei
• But hydrogens in organic molecules are not
  isolated from all other atoms they are
  surrounded by electrons, which are caused to
  circulate by the presence of the applied field

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NMR Spectroscopy

• NMR uses radiowaves, measured in MHz
• The energy transitions depend on the strength of
  the magnetic field which is different from
  machine to machine
• We define the machine independent ppm as

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Nuclear Magnetic Resonance

• The circulation of electrons around a nucleus in
  an applied field is called diamagnetic current
  and the nuclear shielding resulting from it is
  called diamagnetic shielding
• The difference in resonance frequencies among the
  various hydrogen nuclei within a molecule due to
  shielding/deshielding is generally very small

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Nuclear Magnetic Resonance

• The difference in resonance frequencies for
  hydrogens in CH3Cl compared to CH3F under an
  applied field of 7.05T is only 360 Hz, which is
  1.2 parts per million (ppm) compared with the
  irradiating frequency

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Nuclear Magnetic Resonance

• It is customary to measure the resonance
  frequency (signal) of individual nuclei relative
  to the resonance frequency (signal) of a
  reference compound
• The reference compound now universally accepted
  is tetramethylsilane (TMS)

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Nuclear Magnetic Resonance

• For a 1H-NMR spectrum, signals are reported by
  their shift from the 12 H signal in TMS
• For a 13C-NMR spectrum, signals are reported by
  their shift from the 4 C signal in TMS
• Chemical shift (d) the shift in ppm of an NMR
  signal from the signal of TMS

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NMR Spectrometer

• Essentials of an NMR spectrometer are a powerful
  magnet, a radio-frequency generator, and a
  radio-frequency detector
• The sample is dissolved in a solvent, most
  commonly CDCl3 or D2O, and placed in a sample
  tube which is then suspended in the magnetic
  field and set spinning
• Using a Fourier transform NMR (FT-NMR)
  spectrometer, a spectrum can be recorded in about
  2 seconds

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NMR Spectrum

• Downfield the shift of an NMR signal to the left
  on the chart paper
• Upfield the shift of an NMR signal to the right
  on the chart paper

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Equivalent Hydrogens

• Equivalent hydrogens have the same chemical
  environment (Section 2.3C)
• Molecules with
• 1 set of equivalent hydrogens give 1 NMR signal
• 2 or more sets of equivalent hydrogens give a
  different NMR signal for each set

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Signal Areas

• Relative areas of signals are proportional to the
  number of hydrogens giving rise to each signal
• All modern NMR spectrometers electronically
  integrate and record the area of each signal

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Chemical Shift - 1H-NMR
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Chemical Shift - 1H-NMR
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Chemical Shift

• Depends on (1) electronegativity of nearby atoms,
  (2) the hybridization of adjacent atoms, and (3)
  magnetic induction within an adjacent pi bond
• Electronegativity

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Chemical Shift

• Hybridization of adjacent atoms

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Chemical Shift

• Magnetic induction in pi bonds of a
• a carbon-carbon triple bond shields an acetylenic
  hydrogen and shifts its signal upfield (to the
  right) to a smaller d value
• carbon-carbon double bond deshields vinylic a
  hydrogens and shifts their signal downfield (to
  the left) to a larger d value

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Field currents in benzene
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acetylene
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Alkenes
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Methyl Acetate
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Signal Splitting (n 1)

• Peak the units into which an NMR signal is
  split doublet, triplet, quartet, etc.
• Signal splitting splitting of an NMR signal
  into a set of peaks by the influence of
  neighboring nonequivalent hydrogens
• (n 1) rule the 1H-NMR signal of a hydrogen or
  set of equivalent hydrogens is split into (n 1)
  peaks by a nonequivalent set of n equivalent
  neighboring hydrogens

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Signal Splitting (n 1)

• Problem predict the number of 1H-NMR signals
  and the splitting pattern of each

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Origins of Signal Splitting

• When the chemical shift of one nucleus is
  influenced by the spin of another, the two are
  said to be coupled
• Consider nonequivalent hydrogens Ha and Hb on
  adjacent carbons
• the chemical shift of Ha is influenced by whether
  the spin of Hb is aligned with or against the
  applied field

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Origins of Signal Splitting
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Origins of Signal Splitting

• Table 13.8 Observed signal splitting patterns
  for an H with 0, 1, 2, and 3 equivalent
  neighboring hydrogens

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Origins of Signal Splitting

• Table 13.8 (contd.)

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Coupling Constants

• Coupling constant (J) the distance between
  peaks in an NMR multiplet, expressed in hertz
• J is a quantitative measure of the magnetic
  interaction of nuclei whose spins are coupled

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Ethyl acetate
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Isopropyl alcohol