Infrared Spectroscopy An Instrumental Method for Detecting Functional Groups

15 Infrared Spectroscopy An Instrumental Method for Detecting Functional Groups 

An infrared spectrometer (Fig. 2.10) operates by passing a beam of IR radiation through a sample and comparing the radialion transmitted through the sample with that transmitted in the absence of the sample. Any frequencies absorbed by the sample will be apparent by the difference. The spectrometer plots the results as a graph showing absorbance 

In their vibrations covalent bonds behave as if they were tiny springs connecting the atoms. When the atoms vibrate, they can do so only at certain frequencies, as if the bonds were tuned. Because of this, covalently bonded atoms have only particular vibrational energy levels that is, the levels are quantized. 

The excitation of a molecule from one vibrational energy level to another occurs only when the compound absorbs IR radiation of a particular energy, meaning a particular wavelength or frequency. Note that the energy ( ) of absorption is directly proportional to the 

Molecules can vibrate in a variety of ways. Two atoms joined by a covalent bond can undergo a stretching vibration where the atoms move back and forth as if joined by a spring. Three atoms can also undergo a variety of stretching and bending vibrations. 

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INFRARED SPECTROSCOPY AN INSTRUMENTAL METHOD FOR DETECTING FUNCTIONAL GROUPS... 

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