Physical Chemistry : Spectroscopy

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #22 : Nuclear, Quantum, And Molecular Chemistry

Alkynes exhibit a distinctive peak in the  region of an IR spectrum, caused by the stretching of the triple bond. After performing a synthesis of ethyne in the laboratory, she takes an infrared spectrum of the synthesized compound and no such peak appears. Which of the following statements best explains the reason for this phenomenon? 

Possible Answers:

The compound synthesized was not ethyne, and therefore the IR spectrum is incorrect

Ethyne is has a center of symmetry about its triple bond, making it IR inactive

Ethyne does not contain triple bonds

There was not enough sample for the detector to read

The instrument is malfunctioning; ethyne should definitely show a peak at 

Correct answer:

Ethyne is has a center of symmetry about its triple bond, making it IR inactive

Explanation:

Infrared spectroscopes uses dipole changes as a means of recording data. Because ethyne has a center of symmetry about its triple bond, there is no net dipole change when the bond stretches, and therefore no signal is measured by the detector. Such molecules are called IR inactive and a Raman spectrum must be obtained in order to observe the desired peaks. 

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