Question

Explain what is meant by the terms symmetric and antisymmetric bond-stretching vibrations and by bending vibrations.

Short answer

Symmetric vibrations involve simultaneous bond stretching in the same direction, antisymmetric involve opposite directions. Bending vibrations change the angle between bonds.

Step-by-step solution

Understand Bond-Stretching Vibrations

Bond-stretching vibrations involve changes in the internuclear distance between atoms in a molecule. These vibrations can be symmetric or antisymmetric. Symmetric stretching occurs when two bonds stretch at the same time in the same direction, meaning the bond length increases and decreases synchronously. Antisymmetric stretching occurs when bonds stretch in opposite directions, so while one bond length increases, the other decreases.

Visualize Symmetric Stretching

In symmetric stretching, consider a molecule like the water molecule (H₂O). The two O-H bonds increase or decrease their lengths simultaneously, causing the molecule to symmetrically stretch in a way that preserves the center of mass of the molecule.

Visualize Antisymmetric Stretching

For antisymmetric stretching, think again of a molecule such as water. During antisymmetric stretching, one O-H bond becomes shorter while the other becomes longer, causing a change that does not symmetrically preserve the molecule's configuration but rather alternates it.

Introduce Bending Vibrations

Bending vibrations do not involve changes in bond length but rather involve a change in the angle between bonded atoms. For a water molecule, a bending vibration would occur when the bond angle between the hydrogen atoms and the oxygen changes, causing the bonds to 'bend' rather than stretch.

Key concepts

Symmetric Stretching

Symmetric stretching is a type of molecular vibration where bonds in a molecule stretch in unison. This is akin to two people stretching a rubber band together at the same pace. Each bond in a symmetric stretch changes its length simultaneously in the same manner. In a symmetrical molecule like water (H₂O), the two O-H bonds extend or contract at the same time. This synchronized movement maintains the balance of the molecule's structure, ensuring that its center of mass does not shift.

Symmetric stretching is crucial in identifying molecular vibrations in spectroscopy. Without a shift in the center of mass during the vibration, these stretches can be detected and analyzed to better understand a molecule's behavior and characteristics.

In summary, symmetric stretching involves:

• Simultaneous bond length change
• Preservation of structural symmetry
• No change in the molecule's center of mass

Antisymmetric Stretching

Unlike symmetric stretching, antisymmetric stretching involves an asymmetric change in bond lengths. Imagine again our example of a water molecule. Here, when one O-H bond stretches, the other compresses. This means, as one bond becomes longer, the other shortens, creating an alternating pattern.

Antisymmetric stretching results in a shift of the molecule's configuration. This type of vibration is crucial for providing insights into molecular dynamics and is often involved in more complex vibrational modes.

The key characteristics of antisymmetric stretching include:

• Alternating change in bond lengths
• Changes in the molecular configuration
• A shift in the molecule's center of mass

Bending Vibrations

Bending vibrations are a different form of molecular motion compared to stretching vibrations. Instead of changes in bond lengths, it's the angles between bonds that vary. These bending movements can be visualized as the 'flapping' of wings or the movement of a scissor's blades as they open and close.

In a molecule like water, bending vibrations alter the angle between the hydrogen and oxygen atoms. As these angles change, the molecule undergoes a type of deformation that can be crucial for identifying the energy states and bonding characteristics of the molecule.

Bending vibrations are characterized by:

• Alterations in bond angles
• No changes in bond lengths
• Potential to reveal information about molecular energy states