Rep:Mod:module2dc

Introduction

Predictions of reactions are now possible using computational chemistry. Indeed, by using a molecular modelling, we are able to accurately model many aspects of organic structures and reactivity which appears to be extremely useful. Indeed, computational studies can differentiate between the energy of stable conformers, but more important is the location of transition states and activated complexes that may be difficult or impossible to characterise experimentally. Thermodynamic information is obtained from the energy of stable states, and from barrier heights (energy of a transition state) we can also obtain kinetic data.

In this module, the modelisation and optimisation of the studied molecules is realised by using the software Gaussview, mainly based on quantum mechanics methods.

Part 1: Day 1 and 2

Using Quantum Mechanics, we will look at the structure and the bonding of different molecules, such as BH3 and BCl3. The optimisation and the calculation of the vibrational frequencies will be done with Gaussian. Indeed, when Gaussion runs an optimisation, it effectively calculates the lowest energy structure by solving the Schrödinger equation for the nuclear positions and electrons.

Firstly, we created on Gaussian the BH3 molecule, optimise the structure to determine the lowest energy structure and finally runs calculations in order to determine the vibrational frequencies. The purpose of this manipulation was mainly to be familiar with the software and its different features. We then followed the same procedure and calculations for the BCl3 molecule. We also looked at its molecular orbitals and carried out a Nbo (Natural Bond Orbitals) as well as a vibrational analysis.

Results and Discussion Section:

Part 2: Day 2 and 3

In this part of the module we will essentially study the isomers of Mo(CO)4L2 where L=PPh3. Additionally, since the number of CO vibrational bands active is related to the symmetry of the complex, four carbonyl absorption bands are expected from the compound with cis ligands and only one band is expected from the compound with the trans ligands. We will thus calculate and look at the vibrational frequencies of both isomers after optimising their structure.

Results and Discussion Section:

Part 3: Mini Project

In this part of the module we will use the useful technique learnt in the previous sections on a specific case. Firstly we will optimise the structure to the lowest energy via the SCAN interface. We will then have a close look at the vibrational spectrum of the chosen molecules but also at their Molecular Orbitals in order to get a better understanding of their stability and reactivity. What is expected for this section is to carry out several geometry optimisations and compared the obtained total energy. We will chose a specific molecule and then try to replace ligands but also central atoms and observe what is the actual effect on their geometry, vibrational frequencies and total energy (stability).

Results and Discussion Section: