NMR Handout

NMR Handout - NMR Spectroscopy Chemical Shifts Reich Chem 345 Chemical shifts have their origin in the circulation of electrons induced by the

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NMR Spectroscopy Chemical Shifts Chemical shifts are always reported in δ units: δ = (Downfield shift from Me 4 Si in Hz) (Spectrometer frequency, MHz) 10 9 8 7 6 5 4 3 2 1 0 δ ppm H R O H H X H X=O,Cl,Br X H X=N,S Alkanes H H 1 H Chemical Shifts The ranges above provide an estimate of the chemical shift for simple molecules, but don't help very much when there are multiple substituents. A simple scheme can be used to estimate chemical shifts of protons on sp 3 carbons. Use a base shift of δ 0.9 for methyl groups, δ 1.2 for CH 2 groups, and δ 1.5 for CH groups, and add to these the following values for each α substituent: OC(=O)R 3.0 OR 2.3 Br 2.2 Cl 2.4 Aryl 1.4 C(=O)R 1.0 C=C Ph Cl OH Base shift CH: 1.5 α Ph: 1.4 α OH: 2.3 5.3 Base shift CH 2 : 1.2 α Cl: 2.4 3.6 1.0 Calculated: Calculated: Observed: 4.8 Observed: 3.65 B o increases ν o decreases Upfield Shielded B o decreases ν o increases Downfield Deshielded High frequency Low frequency e - B e B o B = B o - B e ν o = γ B/2 π H A (magnetic field at nucleus) (Larmor precession frequency of H A ) Chemical shifts have their origin in the circulation of electrons induced by the magnetic field, which reduces the actual field at the nucleus. Thus a higher magnetic field has to be aplied to achieve resonance. Reich Chem 345 H O H H PLT j:\_345\345-nmr-handout.plt p. 1/19 Scale: 0.65
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-2 -2.1 -2.2 -2.3 -2.4 -2.5 -2.6 -2.7 -2.8 -2.9 -3 -3.1 -3.2 -3.3 -3.4 -3.5 -3.6 -3.7 -3.8 -3.9 -4 Li H Ph-Te- H Me-Te- H -5.5 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1 -1.1 -1.2 -1.3 -1.4 -1.5 -1.6 -1.7 -1.8 -1.9 -2 H -Se-CH 2 Ph 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 (CH 3 ) 4 Si (CH 3 ) 4 Sn (C H 3 ) 3 CH (C H 3 ) 4 C (C H 3 CH 2 ) 2 CO C H 3 CH 2 OH CH 3 -C H 2 -CH 3 H H (C H 3 ) 3 COH (C H 3 ) 3 CCl C H 3 CH 2 Br (C H 3 ) 3 CBr C H 3 CH 2 I C H 3 C H 3 C H 3 -CO 2 Me 4 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 C H 3 -C N N C C H 3 C H 3 C H 3 O (C H 3 ) 2 S (C H 3 ) 3 N H C H 3 O Ph-C H 3 C H 3 I C H 3 Br (CH 3 C H 2 ) 3 N H -C C- H (CH 3 C H 2 ) 2 CO C H 3 Cl H -C C-Ph (C H 3 ) 2 N-Ph CH 3 C H 2 I Ph-C H 2 -CH 3 C H 3 CCl 3 ( Me 2 N) 3 P=O (C H 3 ) 2 O C H 3 OH (C H 3 O) 2 CH 2 CH 3 C H 2 OH O O Br-C H 2 C H 2 -Br CF 3 C H 2 OH C H 3 O-Ph H CH 3 -CO 2 C H 3 Ph-C O -C C- H N N H H H -C C-S O O -Ph HO H H H 6 5.9 5.8 5.7 5.6 5.5 5.4 5.3 5.2 5.1 5 4.9 4.8 4.7 4.6 4.5 4.4 4.3 4.2 4.1 4 HO H H H Me-C O -OC H 2 CH 3 C H 3 F C H 3 NO 2 (CH 3 ) 2 C H Cl C H 2 Cl 2 PhC H 2 Br O H Ph H H H Ph H H H H C H 2 Br 2 PhC H 2 Cl C H Cl 2 CO 2 Me H 2 SiPh 2 H C(OEt) 3 Representative Proton Chemical Shift Values ( d -4 to 6) C H 3 Li (C H 3 ) 2 Mg H (CH 3 O) 2 C H 2 ClC H 2 C H 2 Cl OAc H H H H OAc H H H H H SiMe 3 H H H Me 3 Si O H N 2 H -SnMe 3 (Me 3 Si) 3 Si-Te- H -8.8 (Me 3 Si) 3 Si-Se- H (Me 3 Si) 3 Si-S- H PLT j:\_345\345-nmr-handout.plt p. 2/19 Scale: 0.57
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8 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7 6.9 6.8 6.7 6.6 6.5 6.4 6.3 6.2 6.1 6 HO H H H 10 9.9 9.8 9.7 9.6 9.5 9.4 9.3
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NMR Handout - NMR Spectroscopy Chemical Shifts Reich Chem 345 Chemical shifts have their origin in the circulation of electrons induced by the

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