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Chapter 13. Review.Structure Determination:

Nuclear Magnetic Resonance

Prepared for CHM 247S students

by Dr. Stanislaw SkoniecznyToronto 2010

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Nuclear Magnetic Resonance (NMR)

Three types of information present in an NMR spectrum:

1. Chemical shift - provides information about the nucleusstudied: electron density, magnetic environment.

2. Multiplicity (number of peaks in each group) - providesinformation about other atoms that are near the ones that

produce the peaks.

3. Integral (the area under a group of peaks).

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The 1H NMR spectrum of an organic compound

provides information concerning:

1. The number of different types of hydrogens

present in the molecule2. The relative numbers of the different types

of hydrogens

3. The electronic environment of the differenttypes of hydrogens

4. The number of hydrogen "neighbours"

a hydrogen has

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Equivalent or nonequivalent hydrogen atoms

Homotopic protons are those that, when

substituted for by deuterium, lead to the samestructure. Homotopic protons are alwaysequivalent, and will give one signal in the NMR.

C

Ha

HbCl

ClC

D

HbCl

ClC

D

Ha

Cl

Cl

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

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Equivalent or nonequivalent hydrogen atoms

Enantiotopic protons are those that, whensubstituted for by deuterium, lead to a pair of

enantiomeric structures. Enantiotopic protonsappear to be equivalent (and will usually give onesignal in the NMR)

C

Ha

Hb

Br

ClC

D

Hb

Br

ClC

Ha

D

Cl

Br

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Equivalent or nonequivalent hydrogen atoms

Enantiotopic protons

C

Ha

Hb

BrCl

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

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Equivalent or nonequivalent hydrogen atoms

Diastereotopic protons are those that, when

substituted for by deuterium, lead to a pair ofdiastereomeric structures. Diastereotopicprotons are not equivalent and will usually give

different signals in the NMR.COOH

CH3HO

HbHa

COOH

CH3HO

HbD

COOH

CH3HO

DHa

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Equivalent or nonequivalent hydrogen atoms

Diastereotopic protons

9 8 7 6 5 4 3 2 1 0

3.10 3.00 2.90

Hb Ha

7.60 7.50 7.40 7.30 7.20

COOH

CH3HO

HbHa

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"quartet"(q)

Ha

Hb

X

X

Hb Hb

Jab

Ha

Splitting of Ha signal

by 3 x Hb nuclei:

Ha

1 13

Jab

"doublet"(d)

3

Jab

Hb

(p.p.m)

Peak Splitting

Hb spins

1H

3H

C li

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Coupling

Coupling to equivalent neighbouring nuclei (AXn spin system)

Multiplicity rules:

n is the number of equivalent neighbour nuclei1+= nM

n = 0 1

n = 1 1 1

n = 2 1 2 1

n = 3 1 3 3 1

n = 4 1 4 6 4 1

Th h i l hift d hi ldi

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The chemical shift and shielding

)1( = oobs BB - shielding constant

paradia +=

(i) the contribution from the magnetic anisotropy of neighbouring groups

(ii) the ring current effect in arenes (R),(iii) the electric field effect (e),(iv) effects of intermolecular interactions (i), e.g. hydrogen bonding

and solvent effects.

ieRN

local

para

local

dia +++++=

dia

para

- diamagnetic shielding term (for nuclei with a spherically symmetric

charge distribution, e.g. hydrogen atoms); shielding by s electrons

- paramagnetic shielding term (for nonspherical molecules); deshielding

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HC

OH

HH

Inductive effect

3.97, 3.417 ppm

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HC C

CH2

HH

R

Anisotropic effect

The chemical shift and shielding

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The chemical shift and shielding

(i) the contribution from the magnetic anisotropy of neighbouring groups (N),

ieRN

local

para

local

dia +++++=

The chemical shift and shielding

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The chemical shift and shielding

(ii) the ring current effect in arenes (R),

ieRN

local

para

local

dia +++++=

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7 6 5 4 3 2 1 0

O Cl

Cl Cl

Br

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2.30 2.20 2.10 2.00 1.90 1.80 1.70 1.60 1.50 1.40 1.30 1.20 1.1

Cl Cl

1

2

3

4

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4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

1

23

Br

4.30 4.20 1.70 1.60

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4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

1

2

3

Br

3.50 3.40 2.00 1.90 1.00 0.90 0.80

13C NMR Spectrometry

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C NMR Spectrometry13C isotope constitutes 1.08 % of natural carbon.

1. Carbon-carbon splitting can be ignored.

2. Carbon-hydrogen coupling is observed. The splittingof signals indicates how many protons are bonded

to each carbon atom (N + 1 rule).

3. The number of different absorptions implies how manydifferent types of carbons are present.

4. The chemical shifts of signals suggest what types of

functional groups contain those carbons.

5. The chemical shift scale is large: 0 - 220 ppm.

6. Reference: TMS.

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COOH

CH3O

1

2

3

4

OCH3

COOH

4

2

3

1

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OCH3

COOH

42

3

1

COOH

1

2

3

4OCH3

5

6

6

5

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Proton Chemical Shift Ranges*

*

For samples in CDCl3 solution. The

scale is relative to TMS at=0.

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Carbon Chemical Shift Ranges*

*

For samples in CDCl3 solution. The

scale is relative to TMS at=0.