This preview shows pages 1–3. Sign up to view the full content.
Chem 1312 Exp 2
±
Using Molecular Modeling Software to
Compare to VSEPR Molecular Shapes
by Leigh Bottomley and Demetrio Filho, Georgia Institute of Technology
BRING YOUR LAPTOP TO LAB IF YOU HAVE ONE.
Objective:
The purpose of this lab is to introduce students to molecular modeling software and what
molecular parameters it can solve for and illustrate. This experiment will use ArgusLab
®
molecular modeling software to solve for the energy of simple molecules with varying
geometry and compare to VSEPR (Valence Shell Electron Pair Repulsion) theory.
Introduction:
The Schrödinger equation
, H
Ψ
= E
Ψ
,
states that a Hamiltonian operator acting on a
function,
Ψ
, gives the total energy of the function times the function.
This is applied to a
real atomic or molecular system where the electrons around the nucleus are
mathematically described as waves (wavefunctions in math speak) with boundary
conditions in order to exist. The solution of this differential equation is a family of
functions (wavefunctions) that are denoted by four “counters”; n, l, m
l
, and s which
denotes which wavefunction solution corresponds to a particular electron in the atomic
system.
When a given electron's wave function is graphed in three dimensions, the resulting
graph is a representation of that electron's behavior, or it’s orbital. The
“quantum
derived”
energies together with the orbitals can be used to explain chemical properties
and reactivity of the entire atomic or molecular system.
Solving the Schrödinger equation can, in principle, predict the properties and reactivity of
all atoms and molecules. Unfortunately, the exact mathematical solution for this
differential equation can only be solved for a one electron system.
The mathematical
solutions for systems with more than one electron can be closely approximated by
applying a variety of assumptions and simplifications.
There are many approximation
methods to obtain orbital and energy information but a few of the more popular ones are:
Semiempirical Hamiltonian methods
: here empirical energy data from spectroscopic
experiments is used to evaluate theoretical energies of the atoms iteratively as the
differential equation is being solved.
•
MNDO Modified Neglect of Diatomic Overlap
•
AM1 Austin Model 1 parameterization of the MNDO method
•
PM3Parametrized Method 3 of the MNDO method
Experiment 2
 1 
This preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentUsing Molecular Modeling Software to Verify VSEPR Molecular Shapes
Abinitio
methods
:
Abinitio means
“from first principles” or “from the beginning”.
No empirical
data is used in the calculations and the mathematical solutions are based on simplified solutions
of the Schrödinger equation.
The best known of these is the HartreeFock approximation, also
known as selfconsistent field (SCF method).
The HartreeFock method is broken down into UHF
(Unrestricted HartreeFock, allows unpaired or odd electrons) and RHF (Restricted HartreeFock;
for paired electrons)
Molecular Mechanics Methods (MM);
These methods use classical mechanics from physics to
describe the potential energy of a molecular system. The potential energy is described as a sum
This is the end of the preview. Sign up
to
access the rest of the document.
 Spring '07
 Bottomley
 Mole, VSEPR

Click to edit the document details