Nuclear Magnetic Resonance Spectrometry Chap 19
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Transcript of Nuclear Magnetic Resonance Spectrometry Chap 19
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Nuclear Magnetic Resonance Spectrometry
Chap 19
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Environmental EffectsEnvironmental Effects
(1) Chemical Shift
• Nearby electrons and nuclei generate small B fields which tends to oppose Bapplied:
Bo = Bapplied – σBapplied
where σ ≡ screening constant
It is the local field Bo that interacts with magnetic moments!
• Now, resonance condition:
Common to hold ν constant (e.g., 100 MHz) and sweep Bo
)1(2
oLarmor B
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Abscissa Scales for NMR Spectra
• In terms of chemical shift, δ
• Almost impossible to measure absolute Bo
• Measure change in Bo relative to internal standard: Tetramethylsilane (TMS)
ppm10 x ν
ννδ 6
ref
sampleref
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High Resolution NMR Spectrum of High Resolution NMR Spectrum of EthanolEthanolFig. 19-12Fig. 19-12
Bo
High field
High shieldLow field Low shield
in ppm
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Chemical Shift (cont’d)
• Diamagnetic currents by electrons tend to
oppose Bapplied
• Nucleus is then “shielded” from Bapplied
• ∴ Bapplied must be increased to cause resonance
• Shielding proportional to electron density
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Diamagnetic Current Shielding of a NucleusDiamagnetic Current Shielding of a Nucleus
Fig. 19-14Fig. 19-14
Bo = Bapplied – σBapplied
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Chemical Shifts and Electronegativity of Halogens
• Shielding ∝ electron density
• Shielding ∝ 1/electronegativity
of adjacent halogen
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Effect of Magnetic AnisotropyEffect of Magnetic Anisotropy
• Unsaturated hydrocarbons
• Local diamagnetic effects do not explainproton chemical shifts
e.g.: CH3 - CH3 (δ = 0.9)
CH2 = CH2 (δ = 5.8)
CH ≡ CH (δ = 2.9)
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Deshielding of Ethylene and Shielding of AcetyleneDeshielding of Ethylene and Shielding of Acetylene
Brought About by Electronic CurrentsBrought About by Electronic Currents
Fig. 19-16Fig. 19-16
(δ = 5.8)
(δ = 2.9)
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Magnetic Anisotropy Combined withMagnetic Anisotropy Combined withElectronegative Group ResultsElectronegative Group Resultsin Very Large in Very Large δδ For Protons For Protons
δ ≈ 10 – 11
Far downfield
Aldehydes:
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Ring Current Deshielding of Aromatic ProtonsRing Current Deshielding of Aromatic Protons
Fig. 19-15Fig. 19-15
δ ≈ 7 – 13
• Far down field
• Effect is absentor self-cancellingin other ringorientations
Aromatics:
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(2) (2) Spin-Spin SplittingSpin-Spin Splitting
• Result of coupling interaction betweenResult of coupling interaction between2 groups of protons 2 groups of protons
TMS
Multiplicity
The fine structure
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• The ± magnetic effect transmitted to methyl protons
• Methyl peak split into a triplet by methylene
• Triplet with 1:2:1 intensity ratio
Effect of methylene protons on resonance of methyl protonsEffect of methylene protons on resonance of methyl protons
Enhances Bapplied
Resonance atlower Bapplied
Opposes Bapplied
Resonance athigher Bapplied
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Effect of methyl protons on resonance of methylene protonsEffect of methyl protons on resonance of methylene protons
• The ± magnetic effect transmitted to methylene protons
• Methylene peak split into a quartet by methyl protons
• Quartet with 1:3:3:1 intensity ratio
Enhances Bapplied
Resonance atlower Bapplied
3:1 intensity
ratio
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Rules Governing Spin-Spin SplittingRules Governing Spin-Spin Splitting
• Equivalent nuclei do not interact
• Coupling constants decrease with separationof groups (< 4 bond lengths)
• Multiplicity = n+1 where n = mag equivalentprotons on adjacent atoms
• Approximate relative areas of a multiplet aresymmetric about midpoint of band
• Coupling constant J is independent of Bo
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Summary of Information from NMRSummary of Information from NMR
• The screening constant (σ) determined from the chemical shift (δ)
• The spin-spin coupling constant (J) determined from the fine structure (unaffected by Bapplied)
• Motional information determined from the nuclear spin relaxation times, T1 and T2