1. Nuclear Magnetic Resonance Spectrometry Chap 19

2. Bo = Bapplied – σBapplied
Environmental Effects
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

3. 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)

4. High Resolution NMR Spectrum of Ethanol
Fig
High field
High shield
Low field
Low shield
in ppm Bo

5. 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

6. Diamagnetic Current Shielding of a Nucleus
Fig
Bo = Bapplied – σBapplied

7. Chemical Shifts and Electronegativity of Halogens
Shielding
∝ electron density
∝ 1/electronegativity
of adjacent halogen

8. Effect of Magnetic Anisotropy
Unsaturated hydrocarbons
Local diamagnetic effects do not explain
proton chemical shifts
e.g.: CH3 - CH3 (δ = 0.9)
CH2 = CH2 (δ = 5.8)
CH ≡ CH (δ = 2.9)

9. Deshielding of Ethylene and Shielding of Acetylene
Brought About by Electronic Currents
Fig
(δ = 2.9)
(δ = 5.8)

10. Magnetic Anisotropy Combined with Electronegative Group Results
in Very Large δ For Protons
Aldehydes:
δ ≈ 10 – 11
Far down
field

11. Ring Current Deshielding of Aromatic Protons
Fig
Aromatics:
δ ≈ 7 – 13
Far down field
Effect is absent
or self-cancelling
in other ring
orientations

12. (2) Spin-Spin Splitting Result of coupling interaction between
2 groups of protons
Multiplicity
TMS
The fine structure

13. Effect of methylene protons on resonance of methyl protons
Opposes Bapplied
Enhances Bapplied
Resonance at
higher Bapplied
Resonance at
lower Bapplied
The ± magnetic effect transmitted to methyl protons
Methyl peak split into a triplet by methylene
Triplet with 1:2:1 intensity ratio

14. Effect of methyl protons on resonance of methylene protons
Enhances Bapplied
3:1 intensity
ratio
Resonance at
lower Bapplied
The ± magnetic effect transmitted to methylene protons
Methylene peak split into a quartet by methyl protons
Quartet with 1:3:3:1 intensity ratio

15. Rules Governing Spin-Spin Splitting
Equivalent nuclei do not interact
Coupling constants decrease with separation
of groups (< 4 bond lengths)
Multiplicity = n+1 where n = mag equivalent
protons on adjacent atoms
Approximate relative areas of a multiplet are
symmetric about midpoint of band
Coupling constant J is independent of Bo

16. Summary 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