Structure & Reactivity

Introduction to Spectroscopy

The structure of a compound has a big effect on its properties.  But how do we know what that structure is? 

The most useful methods of determining molecular structure involve the interaction of electromagnetic radiation, or light, with matter.  Visible light, ultraviolet and infrared radiation, and even microwaves and radio waves interact with matter.  They can each tell us different kinds of information about the materials they interact with.


How does light interact with matter?

Light has wave properties, much like the waves you could see at the ocean shore.  In physics, waves can be described in a number of different ways.  Waves have amplitude: there are waves that rise very tall, and others that are low.   Waves also have wavelength: they may have long wavelengths, with big distances from the peak of one wave to the peak of the one coming behind it.  They may have short wavelengths, with one following very closely behind another.  Wavelength gives rise to a complementary property, which is frequency.  WHen waves are close together, you can see or hear them crashing to the shore very frequently.  When they are farther apart, they seem to crash to the shore with a much lower frequency.

The different colors of light that we see have different wavelengths; blue light has a shorter wavelength than red light, for example.  These different wavelengths of light have different amounts of energy.  This idea is described in the Planck-Einstein relation:

E = h ν

(where E = energy, h = Planck's constant, n = frequency)


E = h c / λ

(where c = speed of light, λ = wavelength)

This equation means:


There are a couple of important and surprising points about the interaction of photons with matter:


When ultraviolet and visible light are absorbed, the energy from the light is transferred to an electron.  The electron is excited to a higher energy level.  Only certain energy levels are available in a material, and so the material can only absorb certain photons.  That means:

 The same sort of event can happen "backwards": an electron can lose energy by falling to a lower energy level.  The lost energy can be given up by the electron as a photon of light.  The wavelength or frequency of the photon corresponds to the difference between electron energy levels.  This phenomenon, in which light is absorbed by a material and then given off again, is called "fluorescence".

There are many kinds of electromagnetic radiation.

Many of these kinds of "light" can provide different kinds of information about structure.  For example:


This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License

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