Question

Is the ability of \(\mathrm{H}_{2} \mathrm{O}\) molecules to absorb photons of IR radiation due to symmetrical stretching or asymmetrical stretching of its \(\mathrm{O}-\mathrm{H}\) bonds, or both? Explain your answer. (Hint: The angle between the two O-H bonds in \(\left.\mathrm{H}_{2} \mathrm{O} \text { is } 104.5^{\circ} .\right).\)

Short answer

Answer: The ability of H2O molecules to absorb photons of IR radiation is due to the asymmetrical stretching of its O-H bonds.

Step-by-step solution

Understand symmetrical and asymmetrical stretching

Symmetrical stretching involves both O-H bonds stretching or compressing in the same direction at the same time. Asymmetrical stretching involves one O-H bond stretching while the other compresses, or vice versa.

Visualize the molecular structure of \(\mathrm{H}_{2} \mathrm{O}\)

It's important to visualize the molecular structure of \(\mathrm{H}_{2} \mathrm{O}\) to understand the impact of stretching on IR radiation absorption. The angle between the two O-H bonds is 104.5°. In the V-shaped structure of water, the oxygen atom is at the vertex, and the two hydrogen atoms are located on either side.

Consider the effect of symmetrical stretching on \(\mathrm{H}_{2} \mathrm{O}\)

During symmetrical stretching, the two O-H bonds are either both stretching or both compressing at the same time in the same direction. In this scenario, the net change in the dipole moment of the molecule would be zero, as the changes in the direction and magnitude of the individual dipoles cancel each other out. This means that symmetrical stretching does not lead to the absorption of IR radiation, as IR radiation absorption relies on the change in the molecule's dipole moment.

Consider the effect of asymmetrical stretching on \(\mathrm{H}_{2} \mathrm{O}\)

During asymmetrical stretching, one O-H bond stretches while the other compresses, or vice versa. This leads to a change in the dipole moment of the molecule, as the direction and magnitude of the individual dipoles do not necessarily cancel out each other. A change in the dipole moment enables the \(\mathrm{H}_{2} \mathrm{O}\) molecule to absorb IR radiation.

Conclude the answer

The ability of \(\mathrm{H}_{2} \mathrm{O}\) molecules to absorb photons of IR radiation is due to the asymmetrical stretching of its \(\mathrm{O}-\mathrm{H}\) bonds. Symmetrical stretching does not lead to IR radiation absorption as it does not result in a change in the molecule's dipole moment.

Key concepts

Molecular Vibrations

When discussing IR spectroscopy, molecular vibrations are a key aspect. Molecules are not static; they are constantly vibrating due to the energy they possess. These vibrations can occur in several modes, such as stretching, bending, twisting, or wagging. Vibrations are crucial in the interaction of a molecule with infrared (IR) radiation, as they can cause a change in the dipole moment of the molecule, allowing it to absorb IR light.

In the case of water (\(\mathrm{H}_2\mathrm{O}\)), the main types of vibrations involved are stretching and bending. The angles and bond lengths of the molecule influence these vibrational modes, enabling certain absorption characteristics. Vibration modes that result in a change in the dipole moment are primarily responsible for IR absorption.

Dipole Moment

The dipole moment is a measure of the polarity of a molecule. It results from the difference in electronegativity between atoms bonded together, which causes an uneven distribution of electrical charges.

In IR spectroscopy, a change in dipole moment is necessary for a molecule to absorb IR radiation. This change occurs during particular vibrational modes. Molecules like water have a permanent dipole moment due to their bent shape and the difference in electronegativity between oxygen and hydrogen.

For water, any vibration that alters the geometry of the molecule or shifts the distribution of electrons may affect its dipole moment. Specifically, asymmetrical stretching often leads to a change in dipole moment, which is key for the IR absorption process.

Asymmetrical Stretching

Asymmetrical stretching involves the simultaneous stretching of one bond while the other contracts in a molecule. This mode of vibration leads to a change in the overall dipole moment of the molecule, making it an active participant in IR absorption.

For water (\(\mathrm{H}_2\mathrm{O}\)), asymmetrical stretching is crucial. When one \(\mathrm{O-H}\) bond extends, and the other contracts, the entire charge distribution within the molecule changes, leading to an altered dipole moment.

This mode of vibration allows water molecules to absorb specific IR wavelengths, as their altered dipole moments interact with the electric field of the incoming IR radiation. Hence, asymmetrical stretching is a primary reason for water's ability to absorb IR radiation.

Symmetrical Stretching

Symmetrical stretching occurs when both bonds in a molecule stretch or compress in the same direction simultaneously. Unlike asymmetrical stretching, this mode does not usually change the dipole moment.

For water (\(\mathrm{H}_2\mathrm{O}\)), when both \(\mathrm{O-H}\) bonds undergo symmetrical stretching, the movement tends to balance out. This is due to equal increases (or decreases) in bond length occurring at the same time, maintaining the net dipole moment of the molecule.

As a result, symmetrical stretching in water does not lead to a change in the dipole moment. Therefore, it does not contribute to the absorption of IR radiation. Symmetrical stretching is generally IR inactive for polyatomic molecules, including \(\mathrm{H}_2\mathrm{O}\).