Organic Chemistry Semester 1 LABORATORY MANUAL - Moravian ...

Part V. Techniques and Theory Appendix C1 Infrared Spectroscopy
Appendix C. Infrared Spectroscopy:
See also See Padías pp. 64 – 74 and CGWW pp. 65-72
Source of the IR Spectrum
Vibrational Energy Levels
Covalent bonds link atoms together to form molecules. Though these bonds have normal
average lengths, the relative positions of the atoms are constantly changing due to bond
vibrations. A bond can be thought of as a spring with atoms attached to each end.
A
B
bending
stretching
bond
stretching
As with other types of molecular changes, bond vibrations occur only at certain frequencies
(ν) and each of these ν's is associated with a vibrational energy level of energy = hν. Thus, each
bond has a series of vibrational energy levels.
Transitions
When electromagnetic radiation (light) is shone upon a molecule, one of its bonds can absorb a
quantum of the energy and pass from a lower vibrational energy level to a higher one if the
radiation contains light of a proper frequency (ν).
e.g.
light (! 1 )
E 3
!E o -> 2
E = h! 1
E 2
E 1
The Spectrum:
" !E 0 -> 2 = h! 1
E 0
IR absorption bands are measured as a function of the ν of the incident light. The units of ν are
cm -1 .
Note: Since E = hν for light, the energy of absorbed light is proportional to ν, that is, the energy
of the photons of light increase as their ν's increase.