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NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY

Radio waves do the trick.

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Nuclei can be thought of as tiny magnets.

No external magnetic fieldSpin alignment random

With external magnetic fieldSpins aligned

Bo

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When a nucleus occupying the α spin state is subjected to radio waves, an absorption can take place.

ener

gy DE

α spin state

β spin state

magnetic fieldis applied

ener

gy

Magnetic field strength

DEDE

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1H NMR SPECTRUM

An NMR spectrum is a plot of resonance frequency vs. the intensity of rf absorption by the sample.

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NUMBER OF SIGNALS

homotopic

diastereotopic

enantiotopic

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Exercise 1:Identify the number of signals expected in the 1H NMR spectrum of the following compounds.

1 signal

1 signal

2 signals

2 signals

4 signals

4 signals

5 signals

3 signals

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Exercise 2:Determine whether the two protons shown in red are homotopic, enantiotopic, or diastereotopic.

homotopic

enantiotopic

diastereotopicdiastereotopic

enantiotopic

homotopic

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The x-axis is a frequency scale but is normalized to be independent of the field strength.

POSITION OF SIGNALS

spectrometer operating at 300 MHz

2181 Hz larger than that of TMS

spectrometer operating at 60 MHz

436 Hz larger than that of TMS

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deshielded shielded

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The different degree of shielding experienced by the protons is due to electron density.

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13

14

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The Shoolery’s additivity rules:

0.6 + 0.3 + (20% of 2.5) = 1.4 exp = 1.488 ppm

0.6 + 1.5 + 0.3 = 3.4 exp = 3.505 ppm

C Cl

H

H

C

H

H

H

0.6 + 2.5 + 0.3 = 3.4 exp = 3.505 ppm

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0.6 + 2.5 + (20% of 0.3) = 3.16 exp = 3.242 ppm

H

C HOC

H

H

H H

0.6 + 2.5 + 0.3 + (20% of 0.3, 0.3, 1.6) = 3.84 exp = 3.683 ppm

0.6 + 0.3 + (20% of 0.3, 0.3, 1.6) = 1.34 exp = 1.423 ppm

C

C H 3

C H 3

C Cl

O

C

H

Cl

H 0.6 + 0.3 + (20% of 0.0, 2.5, 0.3) = 1.46

0.6 + 2.5 + 0.3 + 0.3 + (20% of 0.0, 2.5, 0.3) = 4.26

C

C H 3

Cl

HC

C H 3

H

Cl

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INTENSITY OF SIGNALS

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If n is the number of neighboring protons, then the multiplicity will be n + 1.

SPIN-SPIN SPLITTING (COUPLING)

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What causes splitting?

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When signal splitting occurs, the distance between the individual peaks of a signal is called the coupling constant, or J value.

It is measured in hertz and is independent of field strength.