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Chemistry LibreTexts

12: Infrared Spectroscopy and Mass Spectrometry

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  • Learning Objectives

    After reading this chapter and completing ALL the exercises, a student can be able to

    • correlate regions of the electromagnetic spectrum to spectroscopic techniques - refer to section 11.1
    • explain how an IR spectrometer works and the IR region interacts with organic compounds - refer to section 11.2
    • explain the role of asymmetry in IR absorption - refer to section 11.3
    • interpret IR spectra - refer to section 11.4, 11.5, and 11.6
    • expalin how a mass spectrometer works - refer to section 11.7
    • explain the source of the base peak and molecular ion in a mass spectrum - refer to section 11.7
    • correlate bond strength to fragmentation patterns - refer to section 11.8
    • use fragmentation patterns to elucidate structural features of organic compounds - refer to section 11.9
    • explain how high-resolution mass can be used to determine chemical formulas - refer to section 11.10 

    • 12.1: The Electromagnetic Spectrum
      Spectroscopy is an experimental method used by chemists to elucidate structural information.  The interaction between a compound or sample and a selected region of the electromagnetic spectrum can be measured both qualitatively and quantitatively.
    • 12.2: Infrared (IR) Spectroscopy
      The infrared region of the electromagnetic spectrum causes asymmetric bonds to stretch, bend, and/or vibrate.  This interaction can be measured to help elucidate chemical structures.
    • 12.3: IR-Active and IR-Inactive Vibrations
      Asymmetry and polarity increase the strength of IR absorption (infrared active).   Symmetrical carbon-carbon double and triple bonds will not absorb IR light and are called "infrared inactive".
    • 12.4: Interpretting IR Spectra
      The analysis and interpretation of the IR spectra for several compounds are explained.
    • 12.5: Infrared Spectra of Some Common Functional Groups
      One of the most common applications of infrared spectroscopy is the identification of organic compounds. The IR spectra for the major classes of organic molecules are shown and discussed.
    • 12.6: Summary and Tips to Distinguish between Carbonyl Functional Groups
      This summary includes the minimum information that needs to be memorized to interpret IR spectra in first year organic chemistry along with some tips on how to distinguish between the different functional groups that all contain at least one carbonyl structural feature.
    • 12.7: Mass Spectrometry - an introduction
      Mass spectrometry is an analytic method that employs ionization and mass analysis of compounds in order to determine the mass, formula and structure of the compound being analyzed. A mass analyzer is the component of the mass spectrometer that takes ionized masses and separates them based on charge to mass ratios and outputs them to the detector where they are detected and later converted to a digital output.
    • 12.8: Fragmentation Patterns in Mass Spectrometry
      When interpreting fragmentation patterns, as you might expect, the weakest carbon-carbon bonds are the ones most likely to break.
    • 12.9: Useful Patterns for Structure Elucidation
      Pattern recognition is a chemistry student's best friend, especially when analyzing and interpreting mass spectra.
    • 12.10: Determination of the Molecular Formula by High Resolution Mass Spectrometry
      High resolution mass spectrometry can determine molecular formulas by distinguishing between the masses of compounds based on the isotopic distribution of each element in the compound.