A.3.3 Explain what occurs at a molecular level during absorption of IR by molecules IB Chemistry SL

Summary

This video explains the concept of infrared spectroscopy, focusing on the importance of detecting molecules with moving dipoles. It introduces two types of molecules, such as water and sulfur dioxide, where their vibrational modes lead to infrared activity. The script explains how symmetrical stretching does not involve dipole movement, while asymmetrical stretching and bending do. The key takeaway is that a moving dipole in a molecule, like in certain organic compounds, is essential for infrared detection, providing insight into molecular structure and behavior.

Takeaways

• 😀 Infrared spectroscopy can detect molecules with a moving dipole during vibration.
• 😀 A moving dipole is essential for a molecule to be infrared active, meaning it can be detected by an IR spectrometer.
• 😀 Common examples of molecules with a moving dipole include water, sulfur dioxide, and parts of larger organic molecules like CH2.
• 😀 There are three main vibrational modes: symmetrical stretching, asymmetrical stretching, and bending, all of which can cause dipole movement in certain molecules.
• 😀 The symmetrical stretching mode in molecules like water and sulfur dioxide does not result in a moving dipole.
• 😀 The asymmetrical stretching and bending modes of molecules such as CH2 do result in a moving dipole, making them infrared active.
• 😀 Carbon dioxide (CO2) is another molecule with vibrational modes that do not always involve a moving dipole.
• 😀 In CO2, the symmetrical stretching mode does not involve dipole movement, but asymmetrical stretching and bending do.
• 😀 While the motion of atoms in molecules like CO2 is important, the IB (International Baccalaureate) exam doesn't require students to show that specific motion in diagrams.
• 😀 The key takeaway is that for a molecule to be infrared active, it must have a dipole that moves during vibration, which can be detected by an infrared spectrometer.

Q & A

What is required for a molecule to be detected with an infrared spectrometer?

For a molecule to be detected with an infrared spectrometer, it must have a moving dipole moment when it vibrates.

Which types of molecules have a moving dipole and can be detected in infrared spectroscopy?

Molecules like water, sulfur dioxide, and CH2 (part of a larger organic molecule) have a moving dipole during certain vibrational modes, making them detectable in infrared spectroscopy.

What are the three vibrational modes that can move the dipole in infrared-active molecules?

The three vibrational modes that can move the dipole are symmetrical stretching, asymmetrical stretching, and bending.

What role does the movement of the dipole play in infrared spectroscopy?

When the dipole moves during the vibration of a molecule, it makes the molecule infrared active, allowing it to be detected by an infrared spectrometer.

Does the IB care about the movement of individual atoms in a molecule during vibration?

No, the IB does not care about the detailed movement of individual atoms (such as the red ball in the example), though it is important that the dipole moves.

What is the difference between the vibrational modes of carbon dioxide and the other molecules discussed?

In carbon dioxide, the symmetrical stretching vibrational mode does not move the dipole, while the asymmetrical stretching and bending modes do. This contrasts with other molecules like water and sulfur dioxide, where all vibrational modes move the dipole.

Why is the symmetrical stretching mode in carbon dioxide infrared inactive?

The symmetrical stretching mode in carbon dioxide does not involve a shifting dipole, which is why it is infrared inactive and cannot be detected using infrared spectroscopy.

How does the asymmetrical stretching mode in carbon dioxide affect the dipole?

The asymmetrical stretching mode in carbon dioxide involves a shifting dipole, making it infrared active and detectable by an infrared spectrometer.

Why does the IB not require the drawing of the motion of the carbon atom in the infrared-active modes of carbon dioxide?

The IB does not require the drawing of the motion of individual atoms, such as the carbon atom in carbon dioxide, as long as the presence of a shifting dipole is understood in the vibrational modes.

What is the main takeaway from this segment about molecular vibrations and infrared activity?

The key takeaway is that infrared activity depends on the movement of a dipole during molecular vibrations. If the dipole moves, the molecule can be detected in infrared spectroscopy.