Electron Affinity

About

Electron affinity of a molecule refers to the amount of energy released (or sometimes required) when an electron is added to a neutral molecule, forming a negatively charged ion (anion). It quantifies the tendency of a molecule to attract and accept an extra electron, which is an important property in determining the molecule's chemical reactivity, especially in redox reactions.

Key Concepts of Electron Affinity:

1. Definition:

   • Electron affinity (EA) is the energy change that occurs when an electron is added to a neutral molecule or atom in the gas phase to form a negatively charged ion. Mathematically, it is expressed as: [ \text{EA} = E(\text{neutral molecule}) - E(\text{anion}) ]

   • It is often measured in units of electron volts (eV) or kilojoules per mole (kJ/mol).

2. Energy Change:

   • If the process of gaining an electron releases energy, the electron affinity is positive (in energy terms, this means the energy of the system decreases). A high, positive electron affinity indicates that the molecule readily accepts electrons.

   • If the process requires energy input (endothermic), the electron affinity is negative, indicating that the molecule is less likely to gain an electron.

3. Electron Affinity and Stability:

   • A molecule with high electron affinity will release a significant amount of energy when it gains an electron, forming a more stable anion. These molecules are typically electronegative and are good electron acceptors.

   • A molecule with low or negative electron affinity resists gaining an electron because adding an electron increases its energy, making the anion less stable.

4. Electron Affinity vs. Electronegativity:

   • Electron affinity is the actual energy change when an electron is added, while electronegativity refers to the tendency of an atom or molecule to attract electrons in a bond. While both are related, they measure slightly different properties. Electron affinity focuses on adding an electron to a neutral species, whereas electronegativity refers to how atoms share electrons in a covalent bond.

Factors Influencing Electron Affinity:

1. Molecular Structure:

   • Molecules with high electronegativity, such as those containing atoms like oxygen, nitrogen, or halogens, tend to have higher electron affinities because they have a strong ability to stabilize the extra electron.

   • Electron repulsion: In molecules where electron repulsion is high (such as when adding an electron to a small or already negatively charged species), electron affinity may be lower or negative.

2. Conjugation and Resonance:

   • Molecules with extended conjugation or resonance stabilization can distribute the extra electron over several atoms, leading to higher electron affinity. This is common in aromatic compounds or polycyclic molecules like benzene derivatives.

3. Size of the Molecule:

   • In larger molecules, the added electron may be more easily accommodated, leading to a higher electron affinity. Conversely, in smaller molecules or atoms, the electron may experience more repulsion from existing electrons.

Example of Electron Affinity in Molecules:

• Halogen Molecules: Halogens (like chlorine, Cl₂) have high electron affinities because they readily accept an additional electron to complete their outer electron shell. For example: [ \text{Cl} + e^- \rightarrow \text{Cl}^- ] Chlorine has an electron affinity of about 349 kJ/mol, which is a relatively high value, indicating that it strongly attracts electrons to form a chloride anion (Cl⁻).

• Oxygen (O₂): Molecular oxygen also has a significant electron affinity, which plays a key role in its ability to act as an oxidizing agent (accepting electrons) in combustion and biological respiration. The superoxide anion (O₂⁻) is formed when O₂ gains an electron.

• Benzene (C₆H₆): Benzene has a lower electron affinity compared to halogens or oxygen because the extra electron is not as easily stabilized in the π-system of the molecule. However, derivatives of benzene, especially those with electron-withdrawing groups (like nitro groups), can have higher electron affinities due to better stabilization of the added electron.

Applications of Electron Affinity:

1. Redox Chemistry:

   • Electron affinity is crucial in understanding oxidation-reduction reactions. Molecules or atoms with high electron affinity are often strong oxidizing agents because they readily accept electrons from other species.

2. Organic Reactions:

   • In organic chemistry, electron affinity helps predict how molecules will react in processes like electrophilic addition, where a molecule accepts electrons from a nucleophile.

3. Photochemistry and Photophysics:

   • Electron affinity is important in processes like electron transfer in organic semiconductors or in light-absorbing molecules (e.g., in dye-sensitized solar cells). Molecules with high electron affinity can accept electrons excited by photons, leading to energy conversion.

4. Catalysis:

   • Molecules or transition metal complexes with high electron affinity are often used as catalysts in reactions that involve electron transfer, such as in oxidation reactions or fuel cells.

Summary:

• Electron affinity measures the energy change when an electron is added to a molecule. A higher electron affinity indicates that a molecule can more easily accept an electron, often releasing energy and stabilizing the resulting anion. Molecules with high electron affinity tend to be good electron acceptors (oxidizing agents), while those with low electron affinity are less likely to gain electrons. Electron affinity plays a crucial role in determining molecular reactivity, especially in redox processes, catalysis, and photochemical reactions.

Method

Electron Affinity is calculated with --vipea in xTB 6.6.0

Find

The Electron Affinity can be found in the Global property table.