Spectroscopy 3: Magnetic Resonance CHAPTER 15
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Transcript of Spectroscopy 3: Magnetic Resonance CHAPTER 15
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Spectroscopy 3:Spectroscopy 3:Magnetic ResonanceMagnetic Resonance
CHAPTER 15CHAPTER 15
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Conventional nuclear magnetic resonance Energies of nuclei in magnetic fields
Typical NMR spectrometer
The chemical shift (effect of nearby nuclei)
Fine structure (nuclear spin-spin coupling)
Pulsed techniques in FT-NMR
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Fig 15.1 Interactions between ms states of an
electron and an external B field
precessionνL ≡ the Larmor freq
π
γν
2
BoeL mmss = +1/2 = +1/2
mmss = −1/2 = −1/2
where γe ≡ magnetogyric ratio
Bo ≡ applied magnetic field
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Fig 15.3 Nuclear spin states of a spin-1/2 nucleus
(e.g., 1H or 13C) in a magnetic field
= hνradio
Typically:
• A 100 MHz NMR employs a 2.35 T field
• Resonance is achieved
when νradio = energy separation between levels
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Fig 15.4 Layout of a typical NMR spectrometer
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The Chemical Shift
• Nuclear magnetic moments interact with the local field
• In most cases, Bloc ≠ B0 due to electronic orbital ang momentum
• The Larmor frequency νL (frequency of precession)
differs for nuclei in different environments
• Resonance frequencies expressed as the chemical shift
21
20B
)(Bloc
L
610
TMS
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Fig 15.5(a) Range of typical chemical shifts for 1H
TMS
Deshieldednuclei
(low field)
Shieldednuclei
(high field)
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Fig 15.5(b) Range of typical chemical shifts for 13C
TMS
Deshieldednuclei
(low field)
Shieldednuclei
(high field)
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Fig 15.6 The 1H-NMR spectrum of ethanol
Integrated signal
singlet
quartet
triplet
1
3
2
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Fig 15.7 Variation of the chemical shift with electronegativity
Trend due to
magnetic anisotropy
Trend due to
electronegativity
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P 523: Magnetic anisotropy shields proton
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P 523: Magnetic anisotropy shields proton
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P 523: Magnetic anisotropy shields proton
H
Bloc
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Fig 15.9 Ring current deshields ring protons and
shields substituent protons
• Special case of neighboring group effect in aromatics
deshielded
shielded
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Fig 15.6 The 1H-NMR spectrum of ethanol
Integrated signal
singlet
quartet
triplet
1
3
2
Fine structure
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Fine Structure
• Each magnetic nucleus may contribute to the local field ofadjacent nuclei
• ∴ Resonance frequencies are modified
• Strength of interaction given by the coupling constant, J (Hz)
• J is independent of applied mag field, Bo
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Margin pg 526 n equivalent nuclei split adjacent spin(s) into n+1 lines with intensity distribution given by Pascal’s triangle:
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Fig 15.15 Origin of the 1:2:1 triplet in the proton
resonance of a –CH3 species
e.g., CH3CH2OH
⇇⇉⇆
⇄B0
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Fig 15.16 Origin of the 1:3:3:1 quartet in the proton
resonance of a -CH2- species
e.g., CH3CH2OHB0