Question about organic spectroscopy (IR)

In summary, the conversation discusses the different types of vibrational modes in molecules, specifically in water. The question is raised about how to determine which type of mode a molecule has, and the topic of different types of bending modes is also brought up. The conversation ends with a comment about the accuracy of the information in the book.
  • #1
Rujano
17
0

Homework Statement



Hi. I'm starting to read about IR spectroscopy and I have a question. I know that there are 2 kinds of vibrational modes in molecules: bending and stretching, so in a molecule like water for example, how can I tell which one of the vibrational does it have?

What I mean is that I also know that there different types of modes within the bending category (scissoring, rocking, wagging and twisting), so which one of them is the right one (in the book that I'm reading it says that it can only have scissoring... why?)?

Thanks in advance!
 
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  • #2
I don't think your book is right, but I could be wrong.

A non-linear molecule with three atoms should have 3 degrees of freedom, because 3n-6 = 3(3)-6 = 3, where n = number of atoms involved in the molecule.
 

1. What is organic spectroscopy (IR)?

Organic spectroscopy (IR) is a technique used to study the chemical structure of organic compounds. It involves the use of infrared (IR) radiation to measure the absorption of energy by the molecules in a sample, providing information about the functional groups present in the compound.

2. How does IR spectroscopy work?

In IR spectroscopy, a beam of infrared light is passed through a sample of the compound. The molecules in the sample will absorb specific wavelengths of the light, depending on the types of bonds present in the compound. The resulting spectrum, or pattern of absorption peaks, can then be analyzed to determine the functional groups present in the compound.

3. What information can be obtained from an IR spectrum?

An IR spectrum can provide information about the functional groups present in a compound, such as the presence of specific types of bonds (e.g. C-H, O-H, C=O) and their locations within the molecule. It can also give insight into the structure and identity of the compound.

4. What are some limitations of IR spectroscopy?

One limitation of IR spectroscopy is that it cannot provide information about the arrangement of atoms within a molecule. It also cannot differentiate between compounds with similar functional groups. Additionally, the sample must be in the gas or liquid phase, so solid samples may need to be dissolved or melted for analysis.

5. How is IR spectroscopy used in research?

IR spectroscopy is commonly used in research to analyze and identify organic compounds. It can be used to determine the purity of a sample, identify unknown compounds, and monitor chemical reactions. It is also used in various fields such as pharmaceuticals, forensics, and environmental science.

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