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NUCLEARMAGNETIC

RESONANCE

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INTRODUCTION

Technique used for determining the structure of

organic compounds

Complete analysis and interpretation of the

entire spectrum Larger amounts of sample

Non-destructive

Common types:1. 1H NMR

2. 13C NMR

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Source of energy: radio waves

Charged particles create magnetic field by

spinning on its axis.

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In a magnetic field, energy states for a

proton are:

Lower energy statesame direction as B0

Higher energy stateagainst B0

External energy source matches DE

between the two states -> spin flip of

nucleus

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Frequency at which a particular proton

absorbs is determined by its electronicenvironment.

Only nuclei that contain odd massnumbers or odd atomic numbers give rise

to NMR signals.

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1H NMR

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Spectrum

Sharp peaks

Increasing chemical shift -> right to left

Protons: 0-10 ppm

Upfield: to the right

Downfield: to the left

Reference: TMS

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Chemical shift,

Gives the position of an NMR signal

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FEATURESOF1H NMR SPECTRUM

1. Number of signals

2. Position of signals

3. Intensity of signals

4. Spin-spin splitting of signals

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1H NMR - NUMBEROF SIGNALS

# of signals = # of different types of protons

Equivalent protons give the same NMR signal.

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1H NMR - POSITIONOF SIGNALS

.

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Shielded -> upfieldProtons near EN atoms -> deshielded ->

downfield

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1H NMR - INTENSITYOF SIGNALS

Area under the peak # of absorbing

protons

Height of step area under the peak

Ratio of integrals to one another -> ratio

of absorbing protons

Ratio of protons, NOT the absolute

number

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1H NMR - SPIN-SPIN SPLITTING

Spin-spin splitting

splitting of peaks into multiple peaks due to

magnetic interactions between non-equivalent

protons on adjacent carbons n+1 rule

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RULESFOR SPIN-SPIN SPLITTING

Equivalent protons do not split each other.

Protons that are farther than two carbon atoms apart

do not split each other.

Splitting is not generally observed between protons

separated by more than three s-bonds.

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Whenever two (or three) different sets of adjacent

protons are equivalent to each other, use the n+1 rule to

determine the splitting pattern.

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LIMITATIONS OF H-NMR

Molecules without/minimal C-H bonds

Isomers with similar spectra

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13C NMR

structurally distinct carbon atom

relative strength of carbon nmr signals are not normally

proportional to the number of atoms generating each

one.

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