Nuclear Magnetic Resonance (NMR) Spectroscopy

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Nuclear Magnetic Resonance (NMR) Spectroscopy

• Part 1
• Carbon 13 NMR

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Theory of NMR

• The positively charged nuclei of certain elements
  (e.g., 13C and 1H) behave as tiny magnets.
• In the presence of a strong external magnetic
  field (Bo), these nuclear magnets align either
  with ( ) the applied field or opposed to ( )
  the applied field.
• The latter (opposed) is slightly higher in energy
  than aligned with the field.

DE is very small
3
Theory of NMR

• The small energy difference between the two
  alignments of magnetic spin corresponds to the
  energy of radio waves according to Einsteins
  equation Ehn.
• Application of just the right radiofrequency (n)
  causes the nucleus to flip to the higher energy
  spin state
• Not all nuclei require the same amount of energy
  for the quantized spin flip to take place.
• The exact amount of energy required depends on
  the chemical identity (H, C, or other element)
  and the chemical environment of the particular
  nucleus.

4
Theory of NMR

• Our departments NMR spectrometer (in Dobo 245)
  has a superconducting magnet with a field
  strength of 9.4 Tesla. On this
  instrument, 1H nuclei absorb (resonate) near a
  radiofrequency of 400 MHz
  13C nuclei absorb around 100 MHz.
• Nuclei are surrounded by electrons.
  The strong applied magnetic field
  (Bo) induces the
  electrons to circulate
  around the nucleus (left hand rule).

(9.4 T)
5
Theory of NMR

• The induced circulation of electrons sets up a
  secondary (induced) magnetic field (Bi) that
  opposes the applied field (Bo) at the nucleus
  (right hand rule).
• We say that nuclei are shielded from the full
  applied magnetic field by the surrounding
  electrons because the secondary field diminishes
  the field at the nuclei.

6
Theory of NMR

• The electron density surrounding a given nucleus
  depends on the electronegativity of the attached
  atoms.
• The more electronegative the attached atoms, the
  less the electron density around the nucleus in
  question.
• We say that that nucleus is less shielded, or is
  deshielded by the electronegative atoms.
• Deshielding effects are generally additive. That
  is, two highly electronegative atoms (2 Cl atoms,
  for example) would cause more deshielding than
  only 1 Cl atom.

C and H are deshielded C and H are more
deshielded
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Chemical Shift

• We call the relative position of absorption in
  the NMR spectrum (which is related to the amount
  of deshielding) the chemical shift. It is a
  unitless number (actually a ratio, in which the
  units cancel), but we assign units of ppm or d
  (Greek letter delta) units.
• For 1H, the usual scale of NMR spectra is 0 to 10
  (or 12) ppm (or d).
• The usual 13C scale goes from 0 to about 220
  ppm.
• The zero point is defined as the position of
  absorption of a standard, tetramethylsilane
  (TMS)
• This standard has only one type
  of C and only one
  type of H.

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Chemical Shifts
9
Chemical Shifts

• Both 1H and 13C Chemical shifts are related to
  three major factors
• The hybridization (of carbon)
• Presence of electronegative atoms or electron
  attracting groups
• The degree of substitution (1º, 2º or 3º).
  These latter effects are most important in 13C
  NMR, and in that context are usually called
  steric effects.
• First well focus on Carbon NMR spectra
• (they are simpler)

10
CMR Spectra

• Each unique C in a structure gives a single peak
  in the spectrum there is rarely any overlap.
• The carbon spectrum spans over 200 ppm chemical
  shifts only 0.001 ppm apart can be distinguished
  this allows for over 2x105 possible chemical
  shifts for carbon.
• The intensity (size) of each peak is NOT directly
  related to the number of that type of carbon.
  Other factors contribute to the size of a peak
• Peaks from carbon atoms that have attached
  hydrogen atoms are bigger than those that dont
  have hydrogens attached.
• Carbon chemical shifts are usually reported as
  downfield from the carbon signal of
  tetramethylsilane (TMS).

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13C Chemical Shifts
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Predicting 13C Spectra

• Problem 13.6 Predict the number of carbon
  resonance lines in the 13C spectra of the
  following ( unique Cs)
• 
  
• 4 lines
• plane of symmetry

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Predicting 13C Spectra

• Predicte the number of carbon resonance lines in
  the 13C spectra of the major product of the
  following reaction
• 
  
• 7 lines
• 5 lines
• plane of symmetry

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Predicting 13C Spectra
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C6H12O2
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