Background

Use the Back Arrow to return to a spectroscopy problem

The region of the infrared spectrum which is of greatest interest to organic chemists is the wavelength range 2.5 to 15 micrometers (µ). In practice, units proportional to frequency, (wave number in units of cm-1) rather than wavelength, are commonly used and the region 2.5 to 15 µ corresponds to approximately 4000 to 600 cm-1.

Absorption of radiation in this region by a typical organic molecule results in the excitation of vibrational, rotational and bending modes, while the molecule itself remains in its electronic ground state. Movie files demonstrating vibrational and bending modes for water (HO) are available by clicking on the icon shown below:

Symmetric Stretch Assymmetric Stretch Symmetric Bend
Molecular asymmetry is a requirement for excitation by infrared radiation and fully symmetric molecules do not display absorbances in this region unless asymmetric stretching or bending transitions are possible.

For the purpose of routine organic structure determination, using a battery of spectroscopic methods, the most important absorptions in the infrared region are the simple stretching vibrations. For simple systems, these can be approximated by considering the atoms as point masses, linked by a 'spring' having a force constant k and following Hooke's Law. Using this simple approximation, the equation shown below can be utilized to approximate the characteristic stretching frequency (in cm-1) of two atoms of masses m and m2, linked by a bond with a force constant k:

where µ = m1m2/(m1+m2) (termed the 'reduced mass'), and c is the velocity of light. The stretching vibrations of typical organic molecules tend to fall within distinct regions of the infrared spectrum, as shown below:
Since most organic molecules have single bonds, the region below 1500 cm-1 can become quite complex and is often referred to as the 'fingerprint region': that is, if you are dealing with an unknown molecule which has the same 'fingerprint' in this region, that is considered evidence that the two molecules may be identical.

Because of the complexity of the region below 1500 cm-1, in this review, we will focus on functional group stretching bands in the higher frequency region. You should note that for many of these bands, the IR spectrum may give equivocal structural information; quite often the absence of a band is as informative as the presence of a particular band. A correlation table showing many of the common IR absorbance bands is given in the next section.

Use the Back Arrow to return to a spectroscopy problem
Copyright 1996, Paul R. Young, University of Illinois at Chicago, All Rights Reserved