Questions: Which of the following species are IR inactive? Select all that apply. N ≡ N H2O CO2 CCl4 O=0

Transcript text: Which of the following species are IR inactive? Select all that apply. $\mathrm{N} \equiv \mathrm{N}$ $\mathrm{H}_{2} \mathrm{O}$ $\mathrm{CO}_{2}$ $\mathrm{CCl}_{4}$ $\mathrm{O}=0$

Solution

Solution Steps

Step 1: Understanding IR Activity

Infrared (IR) spectroscopy is based on the absorption of infrared light by molecules, which causes changes in their vibrational states. For a molecule to be IR active, it must have a change in dipole moment during its vibration. Molecules that do not have a change in dipole moment during vibration are IR inactive.

Step 2: Analyzing Each Species

$\mathrm{N} \equiv \mathrm{N}$ (Nitrogen gas): This is a diatomic molecule with a triple bond. It is nonpolar and symmetric, meaning there is no change in dipole moment during vibration. Therefore, it is IR inactive.
$\mathrm{H}_{2} \mathrm{O}$ (Water): Water is a polar molecule with an asymmetric shape. It has a change in dipole moment during its vibrational modes, making it IR active.
$\mathrm{CO}_{2}$ (Carbon dioxide): Although $\mathrm{CO}_{2}$ is a linear molecule, it has asymmetric stretching modes that result in a change in dipole moment, making it IR active.
$\mathrm{CCl}_{4}$ (Carbon tetrachloride): This is a tetrahedral molecule that is symmetric. There is no change in dipole moment during its vibrations, so it is IR inactive.
$\mathrm{O}=0$ (Oxygen gas): This is a diatomic molecule similar to nitrogen gas. It is nonpolar and symmetric, with no change in dipole moment during vibration, making it IR inactive.