Texas Lutheran University 1 H NMR Interpretation: A New Tool for Students John V. McClusky Texas Lutheran University.

Texas Lutheran University

1H NMR Interpretation: A New Tool for Students

John V. McClusky

Texas Lutheran University

Texas Lutheran University

Why Teach NMR Interpretation?

• Necessary for molecular identification

• Important for teaching problem solving skills– Complex and abstract – Must concentrate on details and big picture– “You guess, you loose”

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The Process

• Students must use– Integration

• Number of hydrogens making the peak

– Multiplicity• Number of adjacent hydrogens

– Chemical shift• Functional groups

• Determine the molecular structure

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The Problem

Doesn’t look like

11 10 9 8 7 6 5 4 3 2 1

300 MHz ¹H NMR C13H16O3

ALL RIGHTS RESERVED © Sigma-Aldrich Co.

OH

O

O

CH3

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Typical Student Errors

• Ignore integration, multiplicity, or chemical shift

• Interpret peak incorrectly– n+1 or n-1?– Switch integration and multiplicity

• Force spectrum to “match” preconceived molecule

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NMR Mosaic Goals

• Make NMR Interpretation more concrete

• Reinforce need to use all data– Chemical shift– Multiplicity– Integration

• Visualize relationship between multiplicity and connectivity

• Retain all problem-solving aspects

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NMR Mosaic

Spectral peaks

Molecular fragments

Molecule

fit mismatch

Finished Revise --

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Advantages of the NMR Mosaic

• Requires use of integration, chemical shift, and multiplicity

• Accentuates student’s interpretation errors

• Eases interpretation of complex multiplets

• Illustrates relationship between connectivity and multiplicity

Helps students understand andvisualize NMR interpretation

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Mosaic Pieces

H | ─C─ | H

CH

3 Fun

ct. G

rp.

CH3

Fun

ct. G

rp

+ + = H | ─C─ | H

Peak is from Splitting indicates Chemical shift Final mosaic piece a CH2 adjacent CH3 indicates adjacent functional group

Students add static clings to generic pieces to produce a custom piece encoding connectivity and

functionality

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

Example: C3H7Cl

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

Example: C3H7Cl

H | ─C─ | H

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

Example: C3H7Cl

H | ─C─ | H

CH2

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

Example: C3H7Cl

H | ─C─ | H

CH2 Fun

ct. G

rp.

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Example: C3H7Cl

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

X-

H | ─C─ | H

CH2 Fun

ct. G

rp.

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Example: C3H7Cl

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

H | ─C─ | H

CH2 Fun

ct.

Grp

.

X-

H | ─C─ | H

CH2

CH3

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Example: C3H7Cl

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

H | ─C─ | H

CH2 Fun

ct.

Grp

.

X-

H | ─C─ | H

CH2

CH3

H |H─C─ | H

CH2

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Example: C3H7Cl

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

H | ─C─ | H

CH2 Fun

ct.

Grp

.

X-

H | ─C─ | H

CH2

CH3

H |H─C─ | H

CH2

1-chloropropane

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NMR Mosaic Simplifies Interpretation of Complex Multiplets

• Adjoining pieces specify connectivity

• Interpretation is no longer “impossible”

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Example: C4H9Cl

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

Texas Lutheran University

Example: C4H9Cl

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1.00

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

X- H | ─C─ |

Fun

ct. G

rp.

CH

3CH2

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Example: C4H9Cl

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1.00

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

X- H | ─C─ |

Fun

ct. G

rp.

CH

3CH2

H | ─C─ | H

Undetermined

Splitting

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Example: C4H9Cl

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5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

X- H | ─C─ |

Fun

ct. G

rp.

CH

3CH2

H | ─C─ | H

Undetermined

Splitting

H |H─C─ | H

CH

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Example: C4H9Cl

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1.00

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

X- H | ─C─ |

Fun

ct. G

rp.

CH

3CH2

H | ─C─ | H

Undetermined

Splitting

H |H─C─ | H

CH

H |H─C─ | H

CH2

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Example: C4H9Cl

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

X-

H | ─C─ |

Fun

ct. G

rp.

CH

3CH2

H | ─C─ | H

Undetermined

Splitting

H

|

H─

C─

|

H

CH

H |H─C─ | H

CH2

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Example: C4H9Cl

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1.00

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

X-

H | ─C─ |

Fun

ct. G

rp.

CH

3CH2

H

|

H─

C─

|

H

CH

H |H─C─ | H

CH2 H | ─C─ | H

CH

CH3

2-chlorobutane

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NMR Mosaic Helps Students Catch Mistakes

• Mosaic pieces no longer match

• Focus students on incorrectly interpreted peaks

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Example: C3H7Cl

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300 MHz ¹H NMRIn CD Cl3

A LL RIG H TS RESERV ED© S igm a-A ldrich Co.

H|

H─C─|H

CH2

H|

H─C─|H

H|

H─C─|H

CH2H|

─C─|H

CH3

CH3

H|

─C─|H

CH3

CH3

H|

─C─|H

CH2 Func

t. G

rp.H

|─C─

|H

CH2 Func

t. G

rp.H

|─C─

|H

CH2 Func

t. G

rp.H

|─C─

|H

CH2 Func

t. G

rp.

Incorrect piece as an example

C3H7Cl

X-

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Example: C3H7Cl

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

H |H─C─ | H

CH2

H | ─C─ | H

CH3

CH3

H | ─C─ | H

CH2 Fun

ct.

Grp

.

X-Incorrect Match ↑

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Example: C3H7Cl

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300 MHz ¹H NMRIn CDCl3

ALL RIGHTS RESERVED© Sigma-Aldrich Co.

H |H─C─ | H

CH2 H | ─C─ | H

CH2 Fun

ct.

Grp

.

X-Corrected Piece

Matches

H | ─C─ | H

CH2

CH3

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Functional Groups

• Easily used for spectra with over 10 carbons plus aromatic rings

• All major functional groups including:– Aromatic rings (mono and di-substituted)– Alcohols, ethers, esters, aldehydes, ketones,

carboxylic acids, amines ……– Alkenes

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Student Experiences

• Students found the NMR Mosaic a great aid in NMR Interpretation

• Much higher percentage of students successfully solved problems

• Allows much more realistic problems on exams

• Better students weaned themselves off NMR Mosaic and only used it when had problems

• Students had a much stronger understanding of NMR interpretation

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NMR Mosaic: Key Features

• Requires use of Integration, Multiplicity, and Chemical Shift

• Accentuates students’ interpretation errors

• Eases interpretation of complex multiplets

• Reinforces relationship between multiplicity and connectivity

• Can be used with thousands of spectra

EMPHASIZES UNDERSTANDING AND PROBLEM SOLVING SKILLS