IR and Raman Spectroscopy With Vibrational Modes

In summary, the infrared spectra measurements of CF4 and CF2Cl2 have one and two peaks, respectively, indicating one and two IR active vibrational modes. The symmetries associated with each peak are A1 for CF4 and A1 and E for CF2Cl2.
  • #1
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Homework Statement

The question states to identify which graph is associated with which molecule, which are CF4 and CF2Cl2. The graphs are measurements of infrared spectra, one has one peak while the other has two. It then asks to determine which symmetry labels are associated with each peak.



Homework Equations


I'm given to understand that each peak with be associated with a specific vibrational mode, correct? And that using the 3(N)-6 rule, we have a total of 6 modes for each molecule.


The Attempt at a Solution

The only thing that I can gather from this is that whichever molecule has only one vibrational mode which is IR active, it will be attributed to the graph with one peak whereas the molecule with two will go to the graph with two peaks. The way we've covered this segment has been very broken and indirect so it's really hard to understand what's going on.
 
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Hello! You are correct in your understanding that each peak on the infrared spectrum is associated with a specific vibrational mode. In order to determine which graph is associated with which molecule, we need to consider the symmetries of the molecules.

CF4 has a tetrahedral geometry, meaning it has four symmetric C-F bonds and no asymmetric bonds. This results in only one IR active vibrational mode, which corresponds to stretching and bending of the C-F bonds. This molecule will have a single peak on the infrared spectrum.

CF2Cl2, on the other hand, has a planar geometry with two symmetric C-F bonds and two asymmetric C-Cl bonds. This results in two IR active vibrational modes, corresponding to stretching and bending of both the C-F and C-Cl bonds. This molecule will have two peaks on the infrared spectrum.

As for the symmetry labels, the peak in the CF4 graph will correspond to the A1 symmetry label, while the peaks in the CF2Cl2 graph will correspond to the A1 and E symmetry labels. I hope this helps clarify things for you. Let me know if you have any other questions.
 

What is IR and Raman Spectroscopy?

IR and Raman spectroscopy are two techniques used in analytical chemistry to study the molecular structure of a substance. Both techniques involve the interaction of light with a sample, which causes the molecules to vibrate. By measuring the frequencies and intensities of these vibrations, information about the chemical bonds and functional groups present in the sample can be obtained.

How do IR and Raman spectroscopy work?

In IR spectroscopy, infrared light is passed through a sample and the absorption of the light by the molecules is measured. The absorbed energy causes the molecules to vibrate, and the resulting spectrum shows peaks at specific frequencies corresponding to the molecular vibrations. Raman spectroscopy works by measuring the scattered light when a sample is illuminated with a laser. The scattered light is shifted in energy due to the molecules' vibrations, resulting in a spectrum that can be used to identify the sample's chemical composition.

What are vibrational modes in IR and Raman spectroscopy?

Vibrational modes refer to the different ways in which molecules can vibrate. In IR and Raman spectroscopy, these vibrations can be categorized as stretching or bending modes, depending on the type of motion exhibited by the molecule. Each functional group and chemical bond has a unique set of vibrational frequencies, allowing for the identification of specific components in a sample.

What are the differences between IR and Raman spectroscopy?

One of the main differences between IR and Raman spectroscopy is the type of radiation used. IR spectroscopy uses infrared light, while Raman spectroscopy uses visible or near-infrared light. Additionally, IR spectroscopy is more sensitive to the polar nature of molecules, while Raman spectroscopy is more sensitive to the non-polar aspects of a molecule. This means that the two techniques can provide complementary information about a sample's molecular structure.

What are some applications of IR and Raman spectroscopy?

IR and Raman spectroscopy are commonly used in a variety of fields, including materials science, pharmaceuticals, forensics, and environmental analysis. They can be used to identify unknown substances, monitor chemical reactions, and determine the purity of a sample. These techniques are also useful in studying the structure and function of biomolecules such as proteins and DNA.

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