🧶Inorganic Chemistry I Unit 3 – Molecular Symmetry and Group Theory

Key Concepts and Definitions

• Symmetry involves the self-similarity or invariance of a molecule under certain transformations (rotations, reflections, inversions)
• Symmetry elements are geometrical entities (points, lines, planes) with respect to which symmetry operations are performed
  • Include rotation axes, mirror planes, and inversion centers
• Symmetry operations are actions that leave the molecule in an indistinguishable orientation from its original state
  • Comprise rotations, reflections, inversions, and improper rotations
• A point group is a collection of symmetry elements and operations that characterize the symmetry of a molecule
• Character tables summarize the behavior of molecular orbitals and vibrational modes under symmetry operations
  • Consist of irreducible representations, characters, and symmetry species
• Molecular orbital theory describes the electronic structure of molecules using symmetry-adapted linear combinations of atomic orbitals (SALCOs)

Symmetry Elements and Operations

• Identity operation (E) leaves the molecule unchanged and is present in all point groups
• Proper rotation axis (Cn) involves rotating the molecule by 360°/n around an axis, where n is the order of rotation (C2, C3, C4, etc.)
• Reflection plane (σ) is a mirror plane that bisects the molecule, creating a mirror image
  • Can be horizontal (σh), vertical (σv), or dihedral (σd)
• Inversion center (i) involves the inversion of all atoms through a central point, resulting in a molecule indistinguishable from the original
• Improper rotation axis (Sn) combines a proper rotation with a reflection perpendicular to the rotation axis
• Rotation-reflection axis (Sn) involves a proper rotation followed by a reflection in a plane perpendicular to the rotation axis
• The combination of symmetry elements and operations determines the overall symmetry and point group of a molecule

Point Groups and Their Classification

• Point groups are classified based on the presence and arrangement of symmetry elements
• The Schoenflies notation is commonly used to denote point groups (Cn, Cnv, Cnh, Dn, Dnh, etc.)
• Cn point groups have an n-fold rotation axis (C2, C3, C4, etc.)
• Cnv point groups have an n-fold rotation axis and n vertical reflection planes (C2v, C3v, C4v, etc.)
  • Example: NH3 belongs to the C3v point group
• Cnh point groups have an n-fold rotation axis and a horizontal reflection plane (C2h, C3h, C4h, etc.)
• Dn point groups have an n-fold rotation axis and n two-fold rotation axes perpendicular to the principal axis (D2, D3, D4, etc.)
  • Example: H2O belongs to the C2v point group
• Dnh point groups have an n-fold rotation axis, n two-fold rotation axes, and a horizontal reflection plane (D2h, D3h, D4h, etc.)
• Cubic point groups (T, Th, Td, O, Oh) have high symmetry and are associated with cubic structures
  • Example: CH4 belongs to the Td point group

Character Tables and Their Interpretation

• Character tables provide a concise summary of the symmetry properties of a point group
• Each row in a character table represents an irreducible representation (IR) of the point group
• IRs are labeled according to their symmetry species (A, B, E, T) and subscripts (g, u, 1, 2) indicating additional symmetry properties
  • Example: A1g represents a totally symmetric IR
• The characters in a character table indicate the behavior of the IR under each symmetry operation
  • Characters can be +1, -1, or 0, depending on whether the IR is symmetric, antisymmetric, or unaffected by the operation
• The reducible representation (Γ) of a molecule can be decomposed into a sum of IRs using the reduction formula
• The IRs of molecular orbitals and vibrational modes can be determined by comparing their characters with those in the character table

Molecular Orbital Theory and Symmetry

• Molecular orbital theory describes the electronic structure of molecules using a linear combination of atomic orbitals (LCAO) approach
• Symmetry-adapted linear combinations of atomic orbitals (SALCOs) are constructed by applying symmetry operations to the atomic orbitals
  • SALCOs transform according to the IRs of the point group
• The symmetry of molecular orbitals can be determined by the direct product of the IRs of the constituent atomic orbitals
• Molecular orbitals with the same symmetry can mix and interact, leading to bonding and antibonding combinations
• The number and symmetry of molecular orbitals can be predicted using the SALCOs and the character table
• Symmetry considerations help in understanding the electronic transitions, spectroscopic properties, and reactivity of molecules

Applications in Spectroscopy

• Symmetry plays a crucial role in interpreting and predicting spectroscopic transitions in molecules
• Selection rules for electronic, vibrational, and rotational transitions are based on the symmetry of the initial and final states
  • Example: In IR spectroscopy, only vibrational modes that change the dipole moment are IR-active
• The symmetry of vibrational modes can be determined using the character table and the reducible representation of the normal modes
• Raman spectroscopy probes vibrational modes that change the polarizability of the molecule
  • Raman-active modes have symmetric polarizability tensors
• Electronic transitions are governed by the symmetry of the molecular orbitals involved
  • Allowed transitions occur between orbitals with compatible symmetry
• Symmetry-forbidden transitions can become weakly allowed through vibronic coupling or symmetry breaking
• Group theory helps in assigning and interpreting spectroscopic bands and understanding the electronic structure of molecules

Problem-Solving Strategies

• Identify the symmetry elements and operations present in the molecule
• Determine the point group of the molecule based on the combination of symmetry elements
• Construct the reducible representation (Γ) of the molecule by considering the characters under each symmetry operation
• Use the reduction formula to decompose Γ into a sum of irreducible representations (IRs)
  • The reduction formula involves the product of the characters and the order of the point group
• Assign the symmetry species to molecular orbitals, vibrational modes, or other properties based on the IRs
• Utilize the character table to predict the number and symmetry of orbitals, modes, or transitions
• Apply selection rules and symmetry considerations to determine allowed or forbidden transitions in spectroscopy
• Analyze the direct product of IRs to understand the mixing and interaction of orbitals or modes
• Use symmetry arguments to rationalize chemical bonding, reactivity, and other molecular properties

Real-World Applications and Examples

• Symmetry is essential in understanding the structure and properties of crystals and solid-state materials
  • Example: The electronic band structure of semiconductors is influenced by the symmetry of the crystal lattice
• Molecular symmetry is crucial in designing and synthesizing compounds with desired properties
  • Example: The symmetry of ligands affects the stability and reactivity of metal complexes
• Symmetry considerations are used in developing chiral drugs and understanding their biological activity
  • Example: The different enantiomers of a chiral drug can have distinct pharmacological effects
• Group theory is applied in analyzing the vibrational spectra of molecules and identifying functional groups
  • Example: The C=O stretching vibration in carbonyl compounds has a characteristic IR absorption band
• Symmetry is exploited in the design of optical materials and devices, such as polarizers and liquid crystals
  • Example: The liquid crystalline phase of some materials arises from their anisotropic molecular symmetry
• Symmetry principles are employed in studying the electronic structure and reactivity of transition metal complexes
  • Example: The d-orbital splitting and ligand field stabilization energy depend on the symmetry of the complex
• Group theory is used in computational chemistry to simplify calculations and interpret results
  • Example: Symmetry-adapted perturbation theory (SAPT) is a powerful tool for studying intermolecular interactions