Question

Oxygen \(\left(\mathrm{O}_{2}\right)\) does not exhibit an IR spectrum because: (A) it has no molecular motions. (B) it is not possible to record IR spectra of a gaseous molecule. (C) molecular vibrations do not result in a change in the dipole moment. (D) molecular oxygen contains four lone pairs overall.

Short answer

C

Step-by-step solution

- Understand Infrared (IR) Spectroscopy

IR spectroscopy is a technique used to identify molecules based on their absorption of infrared light, which causes molecular vibrations. For a molecule to absorb IR radiation, these vibrations must lead to a change in the dipole moment of the molecule.

- Analyze the Options

Let's analyze the given options: (A) 'it has no molecular motions'; this is incorrect because all molecules have some form of motion or vibration.(B) 'it is not possible to record IR spectra of a gaseous molecule'; this is incorrect because gaseous molecules can indeed have IR spectra.(C) 'molecular vibrations do not result in a change in the dipole moment'; this addresses the requirement for IR absorption, indicating that the vibrations of O2 do not alter the dipole moment. (D) 'molecular oxygen contains four lone pairs overall'; this is true for the electron configuration but irrelevant to IR absorption.

- Identify the Correct Answer

Given the requirement for IR absorption being a change in dipole moment and knowing that \(\text{O}_2\) is a homonuclear diatomic molecule (same atoms), which does not have a dipole moment change during vibration, the correct answer is: \(\text{(C)}\).

Key concepts

molecular vibrations

Molecular vibrations are the movements of atoms within a molecule that occur when a molecule absorbs energy. These vibrations can happen in various ways, such as stretching, bending, or twisting of the bonds between atoms.

There are two main types of molecular vibrations:

• Stretching vibrations: These occur when the distance between two bonded atoms increases and decreases, like stretching and compressing a spring.
• Bending vibrations: These occur when the angle between two bonds changes, akin to bending a piece of flexible material back and forth.

Vibrations are crucial for understanding IR spectroscopy because they determine how molecules interact with infrared light.

dipole moment

The dipole moment of a molecule is a measure of the separation of electrical charges within the molecule. It tells us if there is an uneven distribution of electron density.

Here's how it works:

• When two atoms have different electronegativities, the shared electrons are pulled more towards the more electronegative atom, creating a partial negative charge.
• The less electronegative atom becomes partially positive.
• A dipole moment occurs due to this separation of charge, creating a polar molecule.

For a molecule to absorb IR light, its vibrations must lead to a change in the dipole moment. If the molecule's vibrations don't change this separation of charge, the molecule won't absorb IR radiation effectively. This is why O₂, a homonuclear diatomic molecule with no permanent dipole moment change during vibration, does not exhibit IR absorption.

IR absorption

IR absorption refers to the process by which a molecule absorbs infrared light, leading to an increase in the energy of certain molecular vibrations. For a molecule to absorb IR radiation, these vibrations must cause a change in the molecule's dipole moment.

Steps to IR Absorption:

• Infrared light interacts with a molecule.
• The energy from the light is absorbed by the molecule, which leads to increased vibrational motion.
• This absorption can only happen if the molecule's vibration changes its dipole moment.

Gaseous molecules, liquids, and solids can all exhibit IR spectra. This technique is incredibly useful for identifying substances and studying molecular structures, as each molecule has a unique set of vibrations that show up as distinct absorption bands in an IR spectrum. This uniqueness allows scientists to identify specific molecular components within a sample based purely on their IR absorption characteristics. For instance, because O₂ lacks a change in its dipole moment during vibration, it does not absorb IR light, hence it does not show up in IR spectroscopy.