MadSci Network: Chemistry |
Spectroscopy is the study of the interaction of electromagnetic radiation with a chemical substance. The frequencies that these substances will absorb are unique for each molecule. Generally, the wavenumber is used to measure the position of an infrared (IR) absorption. IR absorption bands are common in most organic and inorganic compounds. When IR light in incident on the molecule, the frequency, which matches the natural frequency of that molecule, is absorbed resulting on molecular vibrations. These vibrations can either be stretching or bending. As only a few molecules also exhibit rotation transitions upon IR absorption the vibrational transitions are of more practical interest. For diatomic molecules: v = 1/(2*pi*c) * sqrt{[k(m1+m2)]/(m1*m2)} Where, v = the wavenumber [cm-1], pi = 3.1415926…, c = speed of light in a vacuum [cm/s], k = the force constant [dynes/cm], and m1 & m2 are the masses of the vibrating atoms within the molecule [g]. The force constant (k) is proportional to the bond energy for stretching and the overall resistance to motion for bending. Specific vibrational wavenumbers can be observed from any IR absorption spectrum. By knowing these wavenumber and the molecule (atoms) involved in the scan, the previous equation can be solved for the force constant. Following are several additional references related to IR spectroscopy and bond strength: www.scar.utoronto.ca/~jthompso/chmc31/c31irtes.pdf chemlearn.chem.indiana.edu/c343/08-C343IR.pdf willson.cm.utexas.edu/Teaching/Chem618B/Files/PDF_Lectures/ lecture6.pdf www.vuw.ac.nz/scps/students/chem206/c206spectroscopy.pdf
Try the links in the MadSci Library for more information on Chemistry.