Nuclear Magnetic Resonance Spectrometry Chap 19

1. Nuclear Magnetic Resonance SpectrometryChap 19

2. 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 fieldBo 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. 19-12 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. 19-14 Bo = Bapplied – σBapplied

7. Chemical Shifts and Electronegativity of Halogens • Shielding • ∝ electron density • Shielding • ∝ 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. 19-16 (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. 19-15 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