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NMR Nuclear Magnetic Resonance Chemical Shift – magnetic induction of the pi electrons in an aromatic ring (Fig. 13.11) Anisotropy of Aromatic compounds: in plane and above H H H H H CH3 H H dring 7.27-6.95 ppm d -0.51 ppm CH2 H H CH 3 H H H H H H H H H H dring 8.14-8.64 ppm H H d -4.25 ppm H H Me H H H dOUTSIDE 9.28 ppm H H dINSIDE -2.99 ppm H H Anisotropy: Aromatic Electronic effects Deshielded + H C H2C 2 - O O H C H3C 3 O H H COOEt H COOEt 7.10 ppm O 6.28 ppm 6.83 ppm H H COOEt EtOOC H O O O 5.93 ppm H 6.10 ppm H 7.07 ppm H H 6.38 ppm 7.71 ppm H H 6.28 ppm Electronic effects: conjugation with carbonyl 3 O O 7 5 2 1 6 4 8 7 6 5 4 3 Electronic effects: conjugation with carbonyl O 1 2 deshielded 4 3 6 5 7.75 6.20 8 7 6 5 4 3 2 Electronic effects: conjugation with heteroatom + O O H H - C H H shielded S S 6.06 ppm H H H 5.48 ppm H 5.81 ppm O O 6.22 ppm H H5.78 ppmH 4.82 ppm H Electronic effects: no conjugation with heteroatom O 5 4 3 2 1 6.5 6.0 5.5 5.0 4.5 4.0 3.5 Electronic effects: conjugation with heteroatom O 5 4 3 2 1 shielded 2.65 2.60 2.55 6.35 6.30 4.95 4.90 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 Electronic effects: conjugation with carbonyl o m O CH3 p deshielded 8.0 7.5 deshielded 8 7 6 5 4 3 2 Electronic effects: conjugation with heteroatom H3C O Shielded shielded m o p 7.3 7.2 7.1 7.0 6.9 6.8 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 Electronic effects: conjugation with heteroatom H3C NH m o Shielded p shielded 7.0 6.5 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 Aromatic: inductive effect and resonance effect F 7.5 7.4 7.3 7.2 7.1 7.0 6.9 Cl 7.4 7.3 7.2 7.1 Br 7.6 7.5 7.4 7.3 7.2 7.1 Calculating Shifts for aromatic compounds NMR Common Aromatic Patterns 5/14/2013 15 H H Ortho 6-10 Hz H para 1-4 H Hz H meta 0-2 Hz H H 8-10 Hz H NMR “Activating” and “Deactivating” groups and the impact of the changing electron density in the Benzene ring on Chemical Shift of ortho, meta, para protons P-Chloroaniline (C6H6ClN) Hb&Hb’ Ha&Ha’ Ha Ha’ Hb Hb’ . Para Di-Substituted Benzene ring . Ha & Ha’ have same Chemical Shift . Hb & Hb’ have same Chemical Shift The Molecular Ion peak . Ha is split into doublet by Hb from Mass Spectrometry . Hb is split into doublet by Ha would have indicated the . Two sets of peaks produced by relative presence of the single electronegativity of Amino & Cl groups Chlorine atom and Nitrogen. 2 Amino Protons 5/14/2013 17 Hydrogen bond meta bromo nitro benzene Br H H B A Calculated shifts O dHA=8.44 dHB=7.82 dHC=7.31 dHD=8.19 + H N C - H O D HA HB HD HC 8.5 8.0 7.5 Aromatic substitution pattern: ortho AA’ XX’ Typical spectra for ortho (symmetrical) O CH3 O CH3 8.00 7.90 7.80 7.70 7.60 7.50 7.40 NMR “Activating” and “Deactivating” groups and the impact of the changing electron density in the Benzene ring on Chemical Shift of ortho, meta, para protons 2,4-Dinitroanisole (C7H6N2O5) 3H . The Methoxy group is moderately activating, while the Nitro groups are strongly deactivating (electron withdrawing) . Net effect is to Decrease c the electron density about the ring protons b a . The a & b protons are Ortho to the strongly deactivating Nitro groups, thus, they have reduced electron density and their Chemical Shift is down field relative to the “c” proton a b c . All protons interact to produce Spin-Spin Coupling. 5/14/2013 21 AB-Spectra AMX C6 H4 O5 N2 I = 6 - 4/2 + 2/2 +1 I = 6 Phenyl = 4 I NO2 = 1 I AMX AFMX C5 H4 N Br I = 5 – 4/2 – 1/2 +1/2 +1 I = 4 (aromatic ring) d J Aromatic substituent pattern O CH O CH3 3 NO2 NO t 2 J=8.1 t J=1.8 dt J=7.7, 1.5 ddd J=8.1, 2.2, 1.1 8.5 8.0 Aromatic substituent pattern O CH O CH3 3 NO2 NO2 td dd J=7.4, 1.1 ~td dd J=8.1, 0.7 J=8.1, 1.5 J=7.7, 1.5 8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4.
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