NMR Spectroscopy Chapter 13

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NMR SpectroscopyChapter 13

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Contents of Chapter 13

• Proton NMR Spectroscopy
• Number of Signals and Position of Resonance
• Chemical Shifts and Splitting of Signals
• Coupling Constants
• 13C NMR Spectroscopy
• Advanced NMR Techniques

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Introduction to NMR Spectroscopy
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Effective Magnetic Field

• Exact energy at which various protons flip
  depends on effective rather than applied magnetic
  field
• Diamagnetic shielding caused by electrons
  diminishes effective field
• Deshielding caused by electronegative groups
  pulls electrons away from nuclei and makes nuclei
  feel stronger effective magnetic fields
• Neighboring hydrogens enhance and diminish
  applied field leading to signal multiplicity

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Shielding and Deshielding
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The Number of Signals in the 1H NMR Spectrum
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Signals vs. Lines or Peaks

• Each set of equivalent hydrogens makes one signal
• Each signal appears as a set of one or more
  lines or peaks
• The number of peaks produced by one signal is
  called its multiplicity

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The Position of the 1H NMR Signals

• Tetramethylsilane (TMS) is used as reference
  standard because its protons are shielded more
  than protons in almost all other organic compounds

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The Chemical Shift

• The chemical shift is measured in Hz (i.e.
  frequency units) or ppm downfield from the TMS
  signal
• Typically these differences are reported in parts
  per million, ppm

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The Chemical Shift

• The advantage of measuring chemical shift in ppm
  is that signal positions are independent of the
  frequency of the spectrometer
• Coupling constants are measured in Hz rather than
  ppm for the same reason

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Characteristic Values of Chemical Shifts

• Its useful to know that acetone Hs a to
  carbonyl normally show up at about 2.1 ppm

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Chemical Shifts

• Note that d for a CH3 (0.9 ppm) is farther
  upfield than for a CH2 (1.3 ppm) which is upfield
  of CH (1.4 ppm)
• In all illustrations in our textbook, protons are
  labeled a, b, c etc. beginning with the proton
  farthest upfield

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Splitting of Signals

• Signal splitting is described by the N1 Rule
• N is the number of equivalent protons bonded to
  the adjacent carbon

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Splitting of Signals

• It is important to remember that it is the number
  of proton on the adjacent carbon which determines
  the multiplicity NOT the number of protons giving
  rise to the signal
• In CH3CHCl2, the methyl signal at ? 2.8 is
  split into a doublet by the single proton on the
  adjacent carbon

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Splitting of Signals
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Splitting of Signals

• Equivalent protons never split each other

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Deshielding and Splitting Analogy

• Protons like gangs which make peaks to kill rival
  gang members living in carbon house next door
• Every hydrogen in identical chemical situation is
  in same gang even if it lives in different house
• Gang makes n1 peaks to kill n rival gang members
  and have one spare peak left over
• If one gang lives next door to two or more gangs
  it gets confused about how many peaks to make and
  makes multiple peaks

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Deshielding and Splitting Analogy

• All peaks for a single gang stored in same
  location-2 sets of peaks stored in same spot
  merge and become one set of peaks
• Electronegative landlords suck electrons (money)
  from gangs to pay the rent
• Rental office is downtown (downfield) at higher
  numbered address, furthest from waterfront (TMS),
  which has lowest ppm address number (0)
• Gang which runs rental office furthest downtown

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Deshielding and Splitting Analogy

• Poorer gangs which live closer to the rental
  office (lectronegative group) store their peaks
  downtown since they cant afford an uptown crib
• Gangs with less members per house are poorer than
  gangs with more members in a house, so they store
  peaks further downtown

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Splitting of Signals
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Splitting of Signals

• Nonequivalent protons split each other if they
  are on adjacent carbons, i.e. separated by 3 s
  bonds
• Protons separated by 4 bonds will split if one of
  the bonds is a double or triple bond

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Splitting of Signals

• The spectrum of isopropyl butanoate shows five
  signals

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Splitting of Signals
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Coupling Constants

• The distance between two peaks in a split NMR
  signal is called the coupling constant and is
  symbolized by Jab
• The signals of coupled protons have the same
  coupling constant, i.e. Jab Jba

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Coupling Constants

• The magnitude of the coupling constant is
  independent of the frequency of the spectrometer
• The frequency difference between split peaks in a
  60 MHz spectrum is the same as the frequency
  difference between the same peaks in a 300 MHz
  spectrum
• In the latter case the peaks appear closer
  together because while the Hz difference is the
  same, it represents a smaller fraction of the
  spectrometer frequency

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Coupling Constants
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Coupling Constants

• For complex splitting patterns, a splitting
  diagram is used

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Coupling Constants

• Note the difference between a quartet and a
  doublet of doublets

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Coupling Constants

• For 1-chloro-3-iodopropane, Jab Jac and what
  should have been a triplet of triplets collapses
  into a simple quintet

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Coupling Constants

• Always do highest multiplicity splitting first

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Integration of NMR Signals

• For 1H NMR spectra, the area under each
  resonance peak is proportional to the number of
  protons that absorb at that particular energy

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Integration of NMR Signals

• Height of integration line is proportional to the
  area under the peak
• If you are given a molecular formula its best to
  calculate the total Hs divided by the total
  units of integration height, H/U and use this to
  calculate number of Hs in each signal.
• In previous slide H/U 11H/8.6U 1.28 H/U
• Signal a is 7 U tall and 1.28 x 7 8.96 H (ie. 9
  H)
• Signal b is 1.6 U tall, 1.6 U x 1.28 H/U 2 H

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13C NMR Spectroscopy

• Alkyl 0 60 ppm
• C-O 50 80 ppm
• CC/Ph 100-170 ppm
• Amide/ester/acid 175
• Aldehyde lt 200 ppm
• Ketone gt 200 ppm

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13C NMR Spectroscopy
When the 13C spectrum is taken with
spin-coupling, only H atom bonded to the C atoms
split signals
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13C DEPT NMR spectrum of Citronellal
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COSY Spectrum of Ethyl Vinyl Ether
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Unsaturation Analysis

• Whenever a molecular formula for an unknown
  compound is given an unsaturation analysis should
  be done
• First rewrite formula so that each halogen
  replaced by an H and each N atom removes an H
  from formula
• Write rmaining formula ignoring all O atoms and
  NH groups (both have two bonds)
• Calculate Hs missing from CnH2n2 sat formula
• Each missing pair of Hs 1 unit of unsaturation
• Aromatic ring 4 units of unsaturation

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1H NMR Alkyl Group Patterns
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1H NMR Alkyl Group PatternsMethyl and t-Butyl
Groups
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1H NMR Alkyl Group PatternsEthyl Group
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1H NMR Alkyl Group PatternsPropyl Group
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1H NMR Alkyl Group PatternsButyl Group
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1H NMR Alkyl Group PatternsIsopropyl Group
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1H NMR Alkyl Group Patternsn-Butyl Group
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1H NMR Alkyl Group Patternssec-Butyl Group
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1H NMR Alkyl Group PatternsPhenyl Group
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1H NMR Alkyl Group Patterns1,4-Disubstituted
Aromatics