Nuclear Magnetic Resonance Spectrometry Chap 19

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

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)

High Resolution NMR Spectrum of Ethanol Fig. 19-12 High field High shield Low field Low shield in ppm Bo

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 Nucleus Fig. 19-14 Bo = Bapplied – σBapplied

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

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)

Deshielding of Ethylene and Shielding of Acetylene Brought About by Electronic Currents Fig. 19-16 (δ = 2.9) (δ = 5.8)

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

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

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

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

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

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

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