CHEMISTRY

Chemistry



Chemistry

This report is intended to familiarize you with the identification of functional groups in organic compounds using infrared spectra. Before you can use this technique, you need to have an introduction to infrared spectroscopy and to what an IR spectrum is. Infrared spectroscopy deals with the interaction of infrared light with matter. The energy of an infrared photon can be calculated using the Planck energy relation.

= h?

Where h = 6.6 x 10-34 joule second and ????frequency of the photon. This shows that high energy photons have high frequency.

The frequency, ?? and speed of light, c, are related through the relation

c = ??

where c = 3.0 x 108 meter/second and ? = wavelength for the light

These two equations can be used to identify a common spectroscopic unit called wavenumber, , which is the reciprocal of the wavelength.

E = h? = h c; E = h? = h c;

= wavenumber = has units of (cm-1)

You can see that both frequency and wavenumber are directly proportional to energy. Molecules are flexible, moving collections of atoms. The atoms in a molecule are constantly oscillating around average positions. Bond lengths and bond angles are continuously changing due to this vibration. A molecule absorbs infrared radiation when the vibration of the atoms in the molecule produces an oscillating electric field with the same frequency as the frequency of incident IR "light".

All of the motions can be described in terms of two types of molecular vibrations. One type of vibration, a stretch, produces a change of bond length. A stretch is a rhythmic movement along the line between the atoms so that the interatomic distance is either increasing or decreasing.

The second type of vibration, a bend, results in a change in bond angle. These are also sometimes called scissoring, rocking, or "wig wag" motions.

Each of these two main types of vibration can have variations. A stretch can be symmetric or asymmetric. Bending can occur in the plane of the molecule or out of plane; it can be scissoring, like blades of a pair of scissors, or rocking, where two atoms move in the same direction. Different stretching and bending vibrations can be visualized by considering the CH2 group in hydrocarbons. The arrows indicate the direction of motion. The stretching motions require more energy than the bending ones.

 

Note the high wave number (high energy) required to produce these motions.

The bending motions are sometimes described as wagging or scissoring motions.

You can see that the lower wavenumber values are consistent with lower energy to cause these vibrations. A molecule absorbs a unique set of IR light frequencies. Its IR spectrum is often likened to a person's fingerprints. These frequencies match the natural vibrational modes of the molecule. A molecule absorbs only those frequencies of IR light that match vibrations that cause a change in the dipole moment of the molecule. Bonds in symmetric N2 and H2 molecules do not absorb IR because stretching does not change the dipole moment, and bending cannot occur with only ...