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Characterisation of molecules (CH2108)


This module aims to give a broad view of aspects of molecular shape and symmetry in inorganic and organic compounds, with particular reference to the spectroscopic techniques used to determine structure, and to the role of symmetry as a central unifying theme in chemistry.

General description

This module provides a basic understanding of the importance of symmetry considerations in chemistry, with particular reference to molecular shape, motion and spectroscopic behaviour, and also brings together a variety of techniques used in the determination of molecular structure and shape.

This includes the use of VSEPR to predict molecular shape and the various types of isomerism in inorganic and organic compounds. It describes infrared and microwave spectroscopy, with an emphasis on the structural correlations and their use.

Syllabus content

Isomerism in inorganic compounds: optical, linkage, geometric etc.

Resonance structures of inorganic compounds containing unsaturated p-block ligand systems: formal charge, energetics of resonance forms etc.

VSEPR (mainly applying knowledge learnt in CH2103 to workshop and lab examples). Multi method problem solving at workshop level.

The use of the potential energy surfaces to understand molecular geometry; basic understanding of infra-red absorption and the spectroscopy of molecules.

Simple harmonic oscillator and energy levels. Definitions of reduced mass and force constant. Vibrational transitions and selection rules. Molecular rotations; energy levels; the rotational constant; estimating bond lengths; rotational transitions and selection rules. Group frequencies for the identification of organic functional groups.

Nature of chromophores and extinction coefficients in UV analysis. Selection rules. Mass spectrometry.

Symmetry elements and operations (Identity, rotations, reflections, inversion, improper rotations.) Classification of molecules, description of some point groups (Cn, Cnv, Dn, Dnh, Dnd,). Chirality and isomerism.

Practical work :

Students will practice using VSEPR to define molecular shape, the analysis of simple spectra based on the simple harmonic oscillator, the deduction of chemical composition from spectra (IR & UV). They will also gain additional practice in assigning point groups to molecules.