Nuclear Magnetic Resonance Spectroscopy

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Nuclear Magnetic Resonance Spectroscopy
Dr. Clower Chemistry 2412L
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Introduction

• NMR is the most powerful technique for organic
  structure determination
• Number and type of atoms in a molecule
• Connectivity of atoms
• Used to study a wide variety of nuclei
• 1H
• 13C
• 15N, 19F, 31P
• Radio-frequency radiation used to transition
  between energy states
• 30 900 MHz
• Transition nuclear spin
  

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Nuclear Spin

• A nucleus with an odd atomic number or an odd
  mass number has a nuclear spin
• The spinning charged nucleus generates a magnetic
  field

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External Magnetic Field

• When placed in an external field, spinning
  protons act like bar magnets

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Two Energy States

• The magnetic fields of the spinning nuclei will
  align either with the external field, or against
  the field
• A photon with the right amount of energy can be
  absorbed and cause the spinning proton to flip
• Spin flip resonance
• Detected and recorded by the spectrometer as a
  signal

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Magnetic Shielding

• If all protons absorbed the same amount of energy
  in a given magnetic field, not much information
  could be obtained
• But protons are surrounded by electrons that
  shield them from the external field
• Circulating electrons create an induced magnetic
  field that opposes the external magnetic field
• Effective magnetic field

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Shielded Protons

• Magnetic field strength must be increased for a
  shielded proton to flip at the same frequency
• Differences detected by machine, cause
  differences in signals (chemical shift, d)

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Protons in a Molecule

• Depending on their chemical environment, protons
  in a molecule are shielded by different amounts
• Chemically equivalent nuclei
• Interchanged through bond rotation or element of
  symmetry
• Have same absorption
• Chemically different nuclei have different
  absorption

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NMR Signals

• The number of signals shows how many different
  kinds of protons are present
• The location of the signals shows how shielded or
  deshielded the proton is
• The intensity of the signal shows the number of
  protons of that type
• Signal splitting shows the number of protons on
  adjacent atoms

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The NMR Spectrometer
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The NMR Graph
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Tetramethylsilane

• TMS is added to the sample
• Since silicon is less electronegative than
  carbon, TMS protons are highly shielded
• Signal defined as zero
• Organic protons absorb downfield (to the left) of
  the TMS signal
• Deuterated solvent signal

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Chemical Shift

• Measured in parts per million
• Ratio of shift downfield from TMS (Hz) to total
  spectrometer frequency (Hz)
• Same value for 60, 100, or 300 MHz machine
• Called the delta (d) scale

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Delta Scale
downfield
upfield
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Location of Signals

• More electronegative atoms deshield more and give
  larger shift values (downfield)
• Effect decreases with distance
• Additional electronegative atoms cause increase
  in chemical shift
  

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Hydrogen and Carbon Chemical Shifts
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13C-NMR

• 12C has no magnetic spin
• 13C has a magnetic spin, but is only 1 of the
  carbon in a sample
• Signals are weak, get lost in noise
• Hundreds of spectra are taken, averaged
• Signal one sharp line for each different type
  of carbon
  

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3-Pentanone

• How many signals?
• Chemical shifts
• sp3 C upfield
• sp, sp2 C downfield
• C adjacent to en atom downfield

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2-Butanone

• How many signals?
• Chemical shifts?

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How is 13C useful for reactions we have studied?

• Zaitsev vs. non-Zaitsev

7 signals
5 signals
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1H-NMR

• More info than 13C-NMR
• Given a structure, how many signals are expected?
• How many sets of H in each molecule?

• Isomers
• Same molecular formula
• Same IR stretches
• Different NMR

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Another example
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Chemical shifts in 1H-NMR

• Info about type of H giving rise to signal
• Strongly shielded upfield (to the right)
• Less shielded downfield (to the left)
• Most common shifts
• Wade Appendix 1A
• Wade, Table 13-3
• -CH2-O-C(O)- ranged from 3.7-4.7 ppm

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Typical Values
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O-H and N-H Signals

• Chemical shift depends on concentration
• Hydrogen bonding in concentrated solutions
  deshield the protons, so signal is around ?3.5
  for N-H and ?4.5 for O-H

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Using chemical shifts

• Given a structure, predict d
• Use to distinguish between two structures
• Example
• Constitutional isomers
• Each with 2 sets of Hs

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Which isomer best fits this spectrum?
or
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Which isomer best fits this spectrum?
or