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In-class Questions: Infrared Spectroscopy

  • Page ID
    112632
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    I do not cover the types of IR spectroscopy included in a typical sophomore-level organic chemistry course, but instead focus on specialized applications and Fourier transform IR spectroscopy.

    Background information

    1. Can infrared spectra be recorded in air? If so, what does this say about the major constituents of air?

    Consider the instrument you used in organic chemistry to record IR spectra – did you try to exclude air from the sample chamber or path of the light beam?

    They realize that they did not.

    What are the major constituents of air?

    They realize they are nitrogen and oxygen gas.

    1. Why don’t the major constituents of air absorb infrared radiation? It might be worth noting that a molecule such as hydrogen chloride (HCl) does absorb infrared light.

    They realize that this must have something to do with the fact that oxygen and nitrogen gas are symmetric molecules and hydrogen chloride is not. I then explain that the selection rule for an infrared-absorbing vibration involves a change in the overall molecular dipole.

    1. Describe the vibrations of carbon dioxide (CO2) and determine which ones absorb infrared radiation.

    I go over with them the concept of degrees of freedom and we identify that carbon dioxide has four vibrations. The groups can readily identify the symmetric stretch, asymmetric stretch and bending vibration (I point out how there are two degenerate bending vibrations in two different planes).

    Which bond dipole is larger, the one with a shorter C=O bond length or the one with a longer C=O bond length?

    If they get this wrong or are not sure, I ask them the following:

    What happens if you take this to the limit and actually break the bond, what would happen to any charges or dipoles?

    They can usually reason out that they would have neutral atoms with no charge difference, and then realize that the longer the bond, the smaller the bond dipole.

    With this, they can determine which ones will involve a change in molecular dipole.

    Specialized techniques

    1. One technique is called non-dispersive infrared (NDIR) spectroscopy. NDIR is usually used to measure a single constituent of an air sample. Think what the name implies and consider how such an instrument might be designed.

    They are quite stumped by this.

    What does non-dispersive mean?

    The groups can eventually get to the point that it means that the instrument has no monochromator. I also indicate that a common application of NDIR would be in the analysis of carbon monoxide gas in things like car exhaust or a coal mine.

    How many active vibrations does CO have?

    They can determine that CO only has one stretching vibration and that somehow, without the use of a monochromator, we want to design an instrument that will measure only CO in the presence of other chemicals.

    After giving them a few minutes to discuss this, I might have to indicate that the concept of splitting the IR beam into a sample and reference cell as a possibility. Some then suggest filling the reference cell with CO to absorb all of the light that CO can absorb, but then they are still stumped about how to actually compare the intensity of the two beams.

    What ultimately happens to the radiant energy absorbed by the CO in a closed chamber?

    They can usually reason out that it will eventually get converted to heat.

    As this point, I then show them a diagram of an NDIR and explain how it works.

    I also show them the procedure of attenuated total reflectance spectroscopy.

    What is the purpose of having multiple reflections of the beam of light as it moves through the crystal?

    They can usually reason out that it done that way to increase the path length which then increases the magnitude of the absorption.

    Fourier-transform Infrared Spectroscopy

    1. Consider the light path for a Michelson interferometer and plot the intensity of radiation at the sample versus the position of the moveable mirror for monochromatic radiation of wavelength x, 2x or 4x.

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    Since this is our first introduction to Fourier transform methods, I first explain the difference between frequency and time domain spectra.

    I then explain the design of the instrument and path of the light beam through the Michelson interferometer.

    Plot the intensity as a function of wavelength and mirror movement.

    When the groups work on the intensity, some students do not realize at first that, when the moveable mirror is displaced by some amount (e.g., –½x) that the light traveling to it goes an extra distance of x.

    With their drawings completed, we can examine how different wavelengths (or frequencies) of light have a different time domain to their behavior and how this can be used to generate a spectrum. We also discuss the importance of knowing exactly where the zero point of the mirror is and of the need for reproducible movement of the mirror.

    1. What are the advantages of FT-IR spectrophotometers over conventional IR spectrophotometers that use a monochromator?

    What are typical things we ask about an analytical method?

    They can identify things like sensitivity, resolution and ease of use. I also ask them to think about specific differences in the instrumental design as they might affect things like sensitivity and resolution. This facilitates of discussion of the various advantages of FT-IR over conventional IR spectrophotometers that use a monochromator.


    This page titled In-class Questions: Infrared Spectroscopy is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Thomas Wenzel.