© S. Lievens January, 2012
University of California, Davis For use with UCD Chem 118 Series
1
Introduction to Spectroscopy I:
Infrared Spectroscopy
Basic Theory:
Covalent bonds exist as sharing of electrons between non-metal atoms.
These bonds act in many ways like springs with the atoms as weights that can
vibrate back and forth along the bond length or even wag off the bond axis.
These vibrations can be symmetrical, asymmetrical, they can stay in a plane or
twist out of a plane, and they can be between two bonded atoms or multiple
bonded atoms to give more complex harmonic oscillations.
symmetric stretch
stretching wagging asymmetric stretch
rocking scissoring wagging (out of plane) twisting (out of plane)
At ground state the atoms are at the lowest possible energy (with the
spring in a neutral state) and are at a particular distance aka bond length apart
(the length of the spring). If the bond absorbs energy the atoms can move a small
distance closer together or further apart as the bond (spring) compresses or
expands. The more energy that is absorbed the larger the compression and
expansion. If sufficient energy is absorbed the bond will break. Breaking bonds
generally requires absorption of ultraviolet light, but infrared is sufficient energy to
get bonds vibrating back and forth.
0
Energy
Distance between Nucleii
Bond
Energy
Bond Length
Excited
Vibration States lowest energy
(ground state)
vibration state - A
Atoms can move back and forth between bond
distances when there is sufficient energy. The higher
the energy the greater the degree of freedom.
vibration state - B
vibration state - C
Covalent Bond Diagram
