What is an infrared spectrum

Iq = Intensity in the backgronnd spectrum I = Intensity in the sample spectrum 

Absorbance is also related to the concentration of molecules in a sample via an equation called Beer s Law  

The height or area of a peak in an absorbance spectrum is proportional to concentration, which is why Beer s Law can be used to determine the concentrations of molecules in samples. This topic will be covered in Chapter 5. 

The y-axis of an infrared spectrum can also be plotted in units called percent transmittance (%T), which measures the percentage of light transmitted by a sample. %T spectra are calculated as follows  

Absorbance and %T are mathematically related to each other, and it is easy using FTIR software to convert from one to the other. When this conversion is made only the y-axis changes the peak positions are not affected. In the scientific literature you will see spectra plotted in both absorbance and %T. The spectra in this book will be plotted both ways to give you practice looking at both types of spectra. 

A series of vibrations is possible in a complicated molecule, by which we mean one containing three or more atoms. As a simple example, let us consider the molecule responsible for the bleaching action of common household bleach, hypochlorous acid, H-O-Cl. There are two bonds, H-0 and 0-0, and each can vibrate. Although the vibration of one bond is not wholly independent of the other, we can usually approximate and treat the vibrations as if they were independent. A single molecule can, therefore, have a wide range of frequencies and k values. 

A molecule containing more than one atom is termed polyatomic, since poly is Greek for many.  

The frequency of these two vibration modes will be different because the energy required for each is different. 

Remember an infrared spectroscopist calls these frequencies wavenumbers. 

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