Post on 21-Dec-2015
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
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
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
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
Diamagnetic Current Shielding of a NucleusDiamagnetic Current Shielding of a Nucleus
Fig. 19-14Fig. 19-14
Bo = Bapplied – σBapplied
Chemical Shifts and Electronegativity of Halogens
• Shielding ∝ electron density
• Shielding ∝ 1/electronegativity
of adjacent halogen
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)
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)
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:
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:
(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
• 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
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
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
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