27-03-2014, 04:20 PM
Molecular Vibrations and IR Spectroscopy
[attachment=60551]
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? (Shockwave animation)
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
Some results
Calculated values using computational software give lower wave numbers
Increasing mass of substituted atoms shifts wave numbers to lower values
(Excel spreadsheet)
Stretching energies > bending energies > internal rotation energies (occur at higher wavelengths)
Use of IR spectra
Identification of functional groups on a molecule – this is a very important tool in organic chemistry
Spectral matching can be done by computer software and library spectra
Since absorbance follows Beer’s Law, can do quantitative analysis
[attachment=60551]
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? (Shockwave animation)
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
Some results
Calculated values using computational software give lower wave numbers
Increasing mass of substituted atoms shifts wave numbers to lower values
(Excel spreadsheet)
Stretching energies > bending energies > internal rotation energies (occur at higher wavelengths)
Use of IR spectra
Identification of functional groups on a molecule – this is a very important tool in organic chemistry
Spectral matching can be done by computer software and library spectra
Since absorbance follows Beer’s Law, can do quantitative analysis