Exp 2 Using Molecular Modeling Software for VSEPR Sp08

# Exp 2 Using Molecular Modeling Software for VSEPR Sp08 -...

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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: Semi-empirical 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 PM3-Parametrized Method 3 of the MNDO method Experiment 2 - 1 -

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Using Molecular Modeling Software to Verify VSEPR Molecular Shapes Ab-initio methods : Ab-initio 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 Hartree-Fock approximation, also known as self-consistent field (SCF method). The Hartree-Fock method is broken down into UHF (Unrestricted Hartree-Fock, allows unpaired or odd electrons) and RHF (Restricted Hartree-Fock; 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
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## This note was uploaded on 06/19/2009 for the course CHEM 1312 taught by Professor Bottomley during the Spring '07 term at Georgia Tech.

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Exp 2 Using Molecular Modeling Software for VSEPR Sp08 -...

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