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CH2206 - Symmetry and bonding (CH2206)


To introduce the aims of Group Theory and its application (i) to the interpretation of vibrational spectra of inorganic and organic molecules, and (ii) to introduce Molecular Orbital (MO) descriptions of bonding in simple molecules and complex ions.

General description

This module will extend the foundation work on symmetry from first year giving additional practice at point group assignments and extending the ideas to the use of symmetry representations. Examples will be drawn from inorganic and organic chemistry with descriptions of isomerism including chiral isomers and meso-compounds. Reducible and irreducible representations will be defined along with the reduction formula, basis sets and matrices in group theory. Applications to chemical bonding via group orbitals, determination of symmetry for LCAO and normalisation. Examples such as BH3 , CH4, SF6, ML6 (+ BeH2 /H2O and Walsh diagram) will be included.

Syllabus content

Symmetry elements and operations. Assignment of point group. Descent in symmetry. Simple group theoretical approaches to chirality, with examples of chiral compounds, diastereomers and meso compounds.

Reducible and irreducible representations. Reduction formula.

Selection of basis set. Derivation of group orbitals (symmetry adapted linear combinations) for simple molecules (BeH2, BH3, CH4, SF6).

Derivation of normal modes of vibration of simple molecules (H2O, CO2, NH3, CH4), group theoretical approach to infrared and Raman activity of normal modes.

LCAO applied to the construction of molecular orbital diagrams for heteronuclear diatomics (HF, CO), and polyatomics (BeH2, BH3, CH4, SF6) by use of group orbitals. Delocalised molecular orbitals. Normalisation constants.

Photo-electron spectroscopy. Comparison of molecular orbital and valence bond approaches to chemical bonding.

Walsh diagrams for H2O and BeH2 - MO approach to molecular geometry.

Extension of LCAO/MO approach to co-ordination complexes of Oh, Td and D4h symmetry. Symmetry descriptors of relevant orbitals, relationship to/differences from CFT.

Practical work :

This will include assigning symmetry elements and point groups to various molecules, and the production of reducible representations and their reduction in a variety of point groups, leading to problems in the application of symmetry to spectroscopy and molecular orbital theory.