2.3 Molecular Orbital Theory

Molecular Orbital Theory explains how atomic orbitals combine to form molecular orbitals. It helps us understand bonding, electronic structure, and properties of molecules. This theory is crucial for grasping how atoms join to create more complex structures.

By looking at how electrons are shared between atoms, we can predict a molecule's shape, stability, and behavior. This knowledge is key for understanding chemical reactions and designing new materials with specific properties.

Molecular Orbital Formation

Linear Combination and Orbital Types

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• Linear combination of atomic orbitals (LCAO) forms molecular orbitals by adding or subtracting atomic wavefunctions
• Bonding orbitals result from constructive interference between atomic orbitals, leading to increased electron density between nuclei
• Antibonding orbitals arise from destructive interference, creating a node between nuclei and decreasing electron density in the bonding region
• Non-bonding orbitals maintain their atomic character without significant contribution to bonding
• Symmetry and overlap determine the formation and stability of molecular orbitals
  • Proper symmetry allows for effective orbital overlap
  • Greater overlap leads to stronger bonds and more stable molecular orbitals

Bond Types and Characteristics

• Sigma (σ) bonds form through head-on overlap of atomic orbitals along the internuclear axis
  • Can be formed by s-s, s-p, or p-p orbital combinations
  • Exhibit cylindrical symmetry around the bond axis
• Pi (π) bonds result from side-by-side overlap of p orbitals perpendicular to the internuclear axis
  • Formed by p-p orbital combinations
  • Have a nodal plane containing the internuclear axis
• Multiple bonds consist of one σ bond and one or more π bonds (double bond, triple bond)

Molecular Orbital Diagrams and Properties

Diagramming and Bond Order

• Molecular orbital diagrams visually represent the energy levels and electron configurations of molecules
  • Typically show atomic orbitals on the sides and molecular orbitals in the center
  • Electrons fill molecular orbitals from lowest to highest energy, following the Aufbau principle
• Bond order calculated using the formula: $(number of bonding electrons - number of antibonding electrons) / 2$
  • Indicates the strength and stability of a chemical bond
  • Fractional bond orders possible in some molecules
• Homonuclear diatomic molecules consist of two identical atoms (O₂, N₂)
  • Exhibit symmetrical molecular orbital diagrams
  • Energy levels of atomic orbitals match on both sides
• Heteronuclear diatomic molecules contain two different atoms (CO, HCl)
  • Display asymmetrical molecular orbital diagrams
  • Atomic orbital energy levels differ between atoms

Magnetic Properties

• Paramagnetism occurs in molecules with unpaired electrons
  • Attracted to magnetic fields
  • Observed in oxygen (O₂) due to its two unpaired electrons in π* orbitals
• Diamagnetism characterizes molecules with all paired electrons
  • Weakly repelled by magnetic fields
  • Nitrogen (N₂) exhibits diamagnetism with its fully paired electron configuration

Advanced Molecular Orbital Concepts

Delocalized Bonding and Applications

• Delocalized bonding involves electrons spread over multiple atoms or an entire molecule
  • Occurs in conjugated systems and aromatic compounds (benzene)
  • Lowers overall energy and increases stability of the molecule
• Resonance structures represent different electron distributions in delocalized systems
  • Actual molecular structure is a hybrid of all possible resonance forms
• Hückel molecular orbital theory applies to planar, conjugated systems
  • Predicts aromaticity based on the number of π electrons (4n+2 rule)
• Molecular orbital theory explains conductivity in metals and semiconductors
  • Overlapping orbitals create energy bands allowing electron movement
• Photochemical processes understood through molecular orbital transitions
  • Electron excitation from HOMO to LUMO upon light absorption
• Frontier molecular orbital theory uses HOMO-LUMO interactions to predict reactivity
  • Explains regioselectivity and stereoselectivity in organic reactions