Nuclear Magnetic Resonance Spectrometry

Chap 19

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