ROTATIONAL SPECTRA OF DIATOMIC MOLECULES

ROTATIONAL SPECTRA OF DIATOMIC MOLECULES

The simplest molecules are those made up of only two atoms, such as oxygen (O₂) and hydrogen chloride (HCl). But these structures, even the ones that look simple, can do interesting things, especially when they spin.

1. Rotation: Like a Top That Spins

A diatomic molecule is like a small dumbbell. Its two atoms are linked by a bond, which acts as the "handle." This dumbbell is rotating when it starts to spin around its centre of mass.

Quantised Rotational Energy:

• A diatomic molecule can only rotate within certain energy levels, unlike a large spinning top.
• In other words, the molecule can only spin at specific speeds that match certain amounts of energy.
• Quantum physics is based on the idea that energy is quantised.

Spinning Energy Levels:

• A molecule's energy is based on its spinning speed and moment of inertia, which is how hard it is to change its rotation.
• Higher energy levels correspond to faster rotating speeds.
• A quantum number, "J," represents these energy levels.
• J can be any value from 0 to 9 (inclusive):
• J = 0 means no spin.
• J = 1 represents the lowest rotational energy level.
• Higher J values correspond to higher rotational energy.

2. Energy Absorption and Emission: The Spectrum

When diatomic molecules transition between rotational energy states, they can either absorb or emit energy.

• A molecule can jump to a higher rotational energy level (higher J number) by absorbing energy from an external source, such as light.
• Emission: When a molecule loses energy, it emits photons, transitioning to a lower rotational energy level with a smaller J number.

3. Rotational Spectra: The Fingerprint of a Molecule

The energy difference between two rotational levels determines the frequency of light a molecule absorbs or emits, creating a unique set of spectral lines called the rotational spectrum.

Different Lines:

• The spectrum displays distinct lines for each rotational transition.
• The spacing between these lines provides insights into the molecule’s bond length and moment of inertia.

• Spinning Energy Levels In the electromagnetic spectrum, rotational transitions typically occur in the microwave region.
• Microwave spectroscopy is a powerful tool for analyzing how diatomic molecules rotate and their structural properties.

What Spectra Tell Us:

Scientists can determine the following from rotational spectra:

• The bond length of a diatomic molecule.
• The molecule's moment of inertia.
• The types of bonds within the molecule.
• The masses of the atoms in the molecule.

4. Applications of Rotational Spectroscopy

Understanding rotational spectra has applications in various scientific fields, including:

Chemistry:

• Identifying unknown molecules.
• Studying molecular shapes.
• Understanding chemical reactions.

Astronomy:

• Detecting rotational spectra of molecules in space.
• Learning about the composition of interstellar clouds and planetary atmospheres.

Atmospheric Science:

• Monitoring air pollution.
• Understanding chemical reactions in the Earth's atmosphere.