FTIR stands for Fourier Transform Infrared Spectroscopy. FTIR involves the vibrations of molecules. Molecules can bend or stretch at their bonds. They can also wiggle around in a wag, or twist. I kind of liketo think of the molecule as if it is gettin' down and groovin'!.
Molecules vibrate in certain modes depending on the symmetry properties of the molecule's shape. The energy it takes to excite a vibrational mode varies depending on the strength of the bond and the weight of the molecule.
Here are the basic priniciples of FTIR Spectroscopy:
FTIR involves the conversion of energy to molecular vibrations. Infrared radiation (wavenumbers of 4800-400 cm-1) can be converted to vibrations in the molecule, which causes the molecule to go from a ground vibrational state to an excited vibrational state. Samples are run either as pure substance or in KBR pellets. Pure samples are run in NaCl plates as liquid and are called NEAT samples. KBR pellets are made by combining a solid sample with potassium bromide in a mortar/pestle. The sample is pressed manually into a clear disc inside the wingnut. A beam of infrared radiation is passed through the sample. A detector generates a plot of percent transmission of radiation versus the wavenumber or wavelength of the transmitted radiation. When the precent transmission is below 100, some of the the light is being absorbed by the sample. Each peak in the spectrum represents absorption of light energy, and is called an absorption band. Gas samples can also be run in many IR instruments.
The amount of energy required to stretch a bond depends on many things, one of which is the strength of the bond and the masses of the bonded atoms. The stronger the bonds and the smaller the atoms, the higher the wavenumbers.
Click here to see spectra of my unknowns....
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