Molecular Vibrations and IR Spectroscopy. Vibrations. What is a vibration in a molecule?. Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bonds http://academic.pgcc.edu/~ssinex/VibMol/form.html.
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What is a vibration in a molecule?
Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bonds
Can a vibration change the dipole moment of a molecule?
Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active.
Symmetrical stretching/bending does not. Not IR active
Infrared (IR) electromagnetic radiation causes vibrations in molecules (wavelengths of 2500-15,000 nm or 2.5 – 15 mm or mcm)
For a vibration at 4111 cm-1 (the stretch in H2), how many vibrations occur in a second?
120 trillion vibration per second!!!!
120 x 1012 vibrations/sec or a vibration every 8 x 10-15 seconds!
MM =2 g/mole
MM =254 g/mole
The greater the mass - the lower the wavenumber
How much movement occurs in the
vibration of a C-C bond?
For a C-C bond with a bond length of 154 pm, the variation is about 10 pm.
For C-C-C bond angle a change of 4o is typical.
This moves a carbon atom about 10 pm.
Energy, frequency, and wavenumber are directly proportional to each other.
Let’s examine the IR spectrum of pentane.
Increasing absorption of IR radiation
Increasing wavenumber (energy, frequency)
Shift of peak due to replacement of H with D (2x mass)
* Spartan ’02 AM1 minimization
W = 6.3286BE + 401.38
r2 = 0.7979
How does the C=O stretching energy compare for these three molecules?
The carbonyl group has a range of 1700-3000 cm-1.