Return to Contents Chapter 13: Spectroscopy Ch 13 contents
Theoretical Background

Since spectroscopy is based on the interaction of electromagnetic radiation (EMR) with a molecule, an understanding of electromagnetic radiation is a must.

Spectroscopy monitors the changes in energy states of a molecule, so one must be familiar with the important energy states and concept of quantisation of energy within a molecule.

Electromagnetic Radiation (EMR)

The part of the electromagnetic radiation spectrum that you are most familar with is "visible light" but this is just a small portion of all the possible types.

: Can you think of common applications of other regions of the electromagnetic spectrum ? ANSWER

the electromagnetic spectrum

Electromagnetic radiation has both particle and wave properties.

QUESTION Can you think of an example of each ? ANSWER

Wave-like properties:
It is important to understand wavelength, l ,and frequency, n, and how they relate to each other : cl n
(c = speed of light)

Particle-like properties:
A particle of energy is called a photon.
Each photon has a discrete amount of energy : a quantum, E = h n = h c / l
(h = Planck's constant)

Energy States
There are many energy 'states' in a molecule.  Of particular interest to the organic chemist will be those relating to the energy associated with the nuclear spin state, the vibration of a bond or an electronic energy levels (orbitals)

Quantisation of Energy

schematic diagram of molecular energy levels The absorption of energy causes an atom or molecule to go from an initial energy state (the ground state) to another higher energy state (an excited state).  The energy changes are frequently described using an energy level diagram. 

The energy states are said to be quantised because there are only certain discrete values that are possible, there is not a continuous spread of energy levels available. 


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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary