Spectroscopy 3:Spectroscopy 3:Magnetic ResonanceMagnetic Resonance
CHAPTER 15CHAPTER 15
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
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
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
Fig 15.4 Layout of a typical NMR spectrometer
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
Fig 15.5(a) Range of typical chemical shifts for 1H
TMS
Deshieldednuclei
(low field)
Shieldednuclei
(high field)
Fig 15.5(b) Range of typical chemical shifts for 13C
TMS
Deshieldednuclei
(low field)
Shieldednuclei
(high field)
Fig 15.6 The 1H-NMR spectrum of ethanol
Integrated signal
singlet
quartet
triplet
1
3
2
Fig 15.7 Variation of the chemical shift with electronegativity
Trend due to
magnetic anisotropy
Trend due to
electronegativity
P 523: Magnetic anisotropy shields proton
P 523: Magnetic anisotropy shields proton
P 523: Magnetic anisotropy shields proton
H
Bloc
Fig 15.9 Ring current deshields ring protons and
shields substituent protons
• Special case of neighboring group effect in aromatics
deshielded
shielded
Fig 15.6 The 1H-NMR spectrum of ethanol
Integrated signal
singlet
quartet
triplet
1
3
2
Fine structure
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
Margin pg 526 n equivalent nuclei split adjacent spin(s) into n+1 lines with intensity distribution given by Pascal’s triangle:
Fig 15.15 Origin of the 1:2:1 triplet in the proton
resonance of a –CH3 species
e.g., CH3CH2OH
⇇⇉⇆
⇄B0
Fig 15.16 Origin of the 1:3:3:1 quartet in the proton
resonance of a -CH2- species
e.g., CH3CH2OHB0
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