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Symmetry Lab

From ChemWiki


Staff Contacts

Theoretical Methods in Chemistry course co-ordinator: Prof. Nic Harrison
Symmetry Lab: Dr. Mike Bearpark
(This lab supports the Molecular Orbitals in Inorganic Chemistry course (2.I1): Dr. P. A. Hunt)

Timetable

In ground floor chemistry computer room, and also the first floor 232A study area.
4 afternoons, 2-5pm
2 group of about 78 students each (demonstrators TBC):

Group B
Introduction: LTC, Monday 19th October 11-12
Tuesday 20th October (Rob Felstead / Lee Thompson)
Friday 23rd (Rob Felstead / Michael Inkpen)
Tuesday 3rd November (Lee Thompson)
Friday 6th (Hin Chun Yau)

Group A
Introduction: LTC, Monday 23rd November, 11-12
Tuesday 24th November (Rob Felstead / Michael Inkpen)
Friday 27th (Hin Chun Yau / Tim Wilson)
Tuesday 8th December (Michael Inkpen)
Friday 11th December (Tim Wilson)

Hand-in Dates

Group B: Friday 13th November
Group A: Friday 18th December

Printed copies in the box outside 160a, C2.



Exercise: Molecular Symmetry

Objectives

This ‘experiment’ consists of two exercises. Allow roughly equal time for each.

Exercise 1: Determine the symmetry operations that can be applied to several molecules, and hence the molecular point group.

Exercise 2: Determine the symmetry properties of some molecular vibrations and molecular orbitals.

By the end
You should be able to

  • Determine the point group of any molecule
  • Determine which irreducible representation of a point group labels the symmetry of a particular molecular vibration or molecular orbital

You will also have a better understanding of symmetry and some of its applications in chemistry.


Introduction

The exercises run in the computer labs using Gaussview3 for Windows.
(Programs menu - use Gaussview version 3, not 5).

The data files you need are available here:

Copy the files extracted from the ex1 and ex2 zip archives (linked above) to your network home directory (or somewhere else you can write to) first. The files contained in these directories should be opened from within Gaussview: you will not be able to open them by double-clicking (in Windows).

As part of the exercise, you will need to copy images from the Gaussview display. Two suggested ways of doing this:

  • Alt-Print Screen copies the current window to the clipboard. This can be pasted into e.g. PowerPoint or Word.
  • Gaussview can save graphics files in TIFF, JPEG or BMP format, which can be imported into another application. JPEG is recommended, as the files are compressed and smaller than TIFF or BMP.

Find a method that works for you before starting your write-up.

For exercise 1, you’ll be opening Gaussian input files (extension .com) and viewing molecular structures. Some instructions and suggestions for using Gaussview are included here. There is also a built-in help menu.

For exercise 2, you’ll be opening Gaussian formatted checkpoint files (extension .fchk) and viewing molecular orbitals and vibrations.

Background reading

Atkins and de Paula, Physical Chemistry 8ed, Chapter 12 (Molecular Symmetry) and references cited therein. Some websites may also be useful:
A local symmetry tutorial
Interactive group theory tables

Concerning the report

The marking scheme is here for reference: Media:Mark_page.doc

A data sheet is available here : Media:DATA SHEET_ex1_ex2.doc
You will find it helpful to record data at points indicated by ==> Complete data sheet in the exercises.

Report

Suggested guide/template for write up 10-15 pages, using the headings suggested below. You should start by reading the discussions in e.g. Atkins and this wiki. Marks are given for correct results, but also for demonstrating that you understand how the results are obtained, and what they mean. A few carefully constructed diagrams with clear explanations are more likely to demonstrate this. Marks will be lost for unthinking copy/paste!

Exercise 1:

Theory
Briefly discuss the concepts of symmetry elements and symmetry operations, groups of symmetry operations (point groups) in your own words.

Results and Discussion
For the six molecules being tested, give the point group, flowchart assignment, and additional symmetry elements (from the data sheet). All symmetry elements should be illustrated for each molecule. Comment on the symmetry elements that are present in both cyclohexane (demo) and chlorocubane, and those that are only present in one or other of the two molecules.

Exercise 2:

Theory
Briefly discuss the concept of an irreducible representation of a point group.

Results and Discussion
Tabulate and summarise the results you collected in completing the data sheet for each of the four examples. What is a degenerate irreducible representation, and what symmetry element(s) must be present for these to occur? Discuss this, referring to the results you obtained for BFBr2 and BCl3. For CrCO6, discuss the relationship between the correlation diagram, the orbitals you obtained (data sheet / diagrams) and their bonding characteristics. E.g. the T2g orbitals are nonbonding and located mainly on the central metal atom.

Conclusions

Exercise 1 : Point groups - symmetry elements and operations

Exercise 2 : Symmetry properties of molecular vibrations and molecular orbitals