Nuclear Magnetic Resonance

1
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)

3
Electromagnetic Radiation

• Wavelength

4
Molecular Spectroscopy

• We study three types of molecular spectroscopy

5
A pictorial view of UV/Vis
UV/Vis radiation is measured in nm (wavelength)
6
IR Spectroscopy

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

7
NMR Spectroscopy

• NMR uses radiowaves, measured in MHz

8
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

9
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

10
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.

13
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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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

• 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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Electrons Shield
What causes differences? Electrons shield.
Remove electrons they de-shield.
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Electron Withdrawing groups deshield by removing
electron density
I suck
19
Electron density can be added or removed through
the p or s systems
20
Field currents in benzene
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Ring currents usually deshield
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acetylene
23
Alkenes
24
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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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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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
31
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

34
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
39
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

42
Ethyl acetate
43
Isopropyl alcohol
44
13C-NMR Spectroscopy

• Each nonequivalent 13C gives a different signal
• A 13C is split by the 1H bonded to it according
  to the (n 1) rule
• Coupling constants of 100-250 Hz are common,
  which means that there is often significant
  overlap between signals, and splitting patterns
  can be very difficult to determine
• The most common mode of operation of a 13C-NMR
  spectrometer is a hydrogen-decoupled mode

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13C-NMR Spectroscopy

• In a hydrogen-decoupled mode, a sample is
  irradiated with two different radio frequencies
• one to excite all 13C nuclei
• a second is a broad spectrum of frequencies that
  causes all hydrogens in the molecule to undergo
  rapid transitions between their nuclear spin
  states
• On the time scale of a 13C-NMR spectrum, each
  hydrogen is in an average or effectively constant
  nuclear spin state, with the result that 1H-13C
  spin-spin interactions are not observed they are
  decoupled

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Chemical Shift - 13C-NMR
47
C8H10
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C7H12O4
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C7H14O
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Interpreting NMR spectra

• Alkanes all 1H-NMR signals fall in the narrow
  range of d 0.8-1.7. 13C signals fall in the
  considerably wider range of d 0-60
• Alkenes vinylic hydrogens typically fall in the
  range d 4.6-5.7
• coupling constants are generally larger for trans
  vinylic hydrogens (J 11-18 Hz) compared with
  cis vinylic hydrogens (J 5-10 Hz)
• the sp2 hybridized carbons of alkenes give
  13C-NMR signals in the range d 100-150, which is
  downfield from the signals of sp3 hybridized
  carbons

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Interpreting NMR spectra

• Alcohols
• the chemical shift of the hydroxyl hydrogen is
  variable. It normally falls in the range d
  3.0-4.5, but may be as low as d 0.5.
• hydrogens on an sp3 hybridized carbon adjacent to
  the -OH group are deshielded by the
  electron-withdawing inductive effect of the
  oxygen and their signals appear in the range d
  3.4-4.3.
• Ethers a distinctive feature in the 1H-MNR
  spectra of ethers is the chemical shift, d
  3.3-4.0, of hydrogens on carbon attached to the
  ether oxygen.

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Index of H Deficiency

• Index of hydrogen deficiency (IHD) the sum of
  the number of rings and pi bonds in a molecule
• To determine IHD, compare the number of hydrogens
  in an unknown compound with the number in a
  reference hydrocarbon of the same number of
  carbons and with no rings or pi bonds
• the molecular formula of the reference
  hydrocarbon is CnH2n2

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Index of H Deficiency

• For each atom of a Group VII element (F, Cl, Br,
  I) added to the reference hydrocarbon, subtract
  one H
• No correction is necessary for the addition of
  atoms of Group VI elements (O,S) to the reference
  hydrocarbon
• For each atom of a Group V element (N, P) added
  to the reference hydrocarbon, add one hydrogen

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• Nuclear
• Magnetic Resonance
• End Chapter 13