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Spectroscopy: Absorbance (Or Emission) of Electromagnetic Radiation

Spectroscopy techniques like absorbance and emission of electromagnetic radiation or nuclear magnetic resonance can be used to identify and characterize molecules. Absorbance/emission measurements provide information on molecular energy levels, while NMR gives insights into bond lengths, geometry, purity, and presence of isomers, tautomers, or impurities. NMR specifically relies on nuclear spin and magnetic fields to measure properties like chemical shift and spin-spin interactions that reveal details about a molecule's nuclear environment and connectivity.

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Nalini Rooplal
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62 views14 pages

Spectroscopy: Absorbance (Or Emission) of Electromagnetic Radiation

Spectroscopy techniques like absorbance and emission of electromagnetic radiation or nuclear magnetic resonance can be used to identify and characterize molecules. Absorbance/emission measurements provide information on molecular energy levels, while NMR gives insights into bond lengths, geometry, purity, and presence of isomers, tautomers, or impurities. NMR specifically relies on nuclear spin and magnetic fields to measure properties like chemical shift and spin-spin interactions that reveal details about a molecule's nuclear environment and connectivity.

Uploaded by

Nalini Rooplal
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PPTX, PDF, TXT or read online on Scribd
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Spectroscopy

Absorbance (or emission) of


electromagnetic radiation

On the molecular scale energy is


quantized.
Atoms and molecules have many
energy levels by measuring the
separation of these levels we can
identify and characterise molecules.
Identification
Bond lengths
Geometry
Purity, isomers, tautomers,

Nuclear Magnetic Resonance NMR


Nuclear energy levels (NOT
radioactivity)
Need a magnet
Resonance low frequency? Radio
Frequency
Nucleus must have spin for the
technique to work. 1H has a spin of
, but 12C has a spin of zero.

H (a proton) in a magnetic
field

Phys. Rev., 1946,69,37.

Chemical Shift
Electrons (surrounding the nuclei) try
to circulate in a manner that causes
an opposing magnetic field.
It is very small typically measured in
ppm (parts per million)
Shielded protons see a lower field o
0ppm
De-shielded higher field
10
ppm
Chemical shift is proportional applied

Spin-spin interactions
Nuclear spins can sense the state of
neighbouring nuclear spins.
The interaction does not depend on
magnetic field
It is measured in frequency units (Hz)

Phys. Rev., 81(1951),20

Nitroethene

How many chemically different hydrogens?

Nitroethene (400 MHz)

Nitroethene (100 MHz)

Nitroethene (25 MHz)

Methylphosphine

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