MOVBTheory(1) - MOLECULAR ORBITAL AND VALENCE BOND THEORY...

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MOLECULAR ORBITAL AND VALENCE BOND THEORY EXPLAINED (HOPEFULLY) Quantum Mechanics is a very difficult topic, with a great deal of detail that is extremely complex, yet interesting. However, in this Organic Chemistry Class we only need to understand certain key aspects of Quantum Mechanics as applied to electronic theory. What follows is an outline of many of the important concepts, color coded to help you. The statements in red are items you need to know . Items in black are for your information, but it is not essential that you know them. Items in purple describe what you need to be able to do, namely describe organic molecules in terms of overlap of hybridized orbitals . Keep this in mind as you go through the following. QUANTUM OR WAVE MECHANICS Electrons have certain properties of particles and certain properties of waves. Electrons have mass and charge like particles. Because they are so small and are moving so fast, electrons have no defined position. Their location is best described by wave mechanics (i.e. a three- dimensional wave) and a wave equation called the Schrödinger equation. Solutions of the Schrödinger equation are called wave functions and are represented by the Greek letter psi. Each wave function describes a different orbital. There are many solutions to the Schrödinger equation for a given atom. The sign of the wave function can change from positive (+) to negative (-) in different parts of the same orbital. This is analogous to the way that waves can have positive or negative amplitudes. The sign of the wave function does not indicate anything about charge. [This can be confusing. Make sure that you understand it before you go on.] The value of the square of the wave function is proportional to the probability of finding electron density at a given point in an orbital. Note that the sign of square of the wave function is always positive, because the square of even a negative value is still positive. In a 2p orbital, it is just as probable to find electron density in the negative lobe as it is to find electron density in the positive lobe. [Make sure you understand this statement.] A node is any place in an orbital at which the value of the wave function is zero. A nodal surface or nodal plane are surfaces or planes where the value of the wave function is zeor. There is absolutely no electron density at a node, a nodal surface, or
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a nodal plane. The Schrödinger equation can in principle describe covalent bonding, but, even with powerful computers the equation is too complicated to be solved exactly for large molecules. MOLECULAR ORBITAL THEORY OF COVALENT BONDING Molecular orbital theory assumes that individual electron pairs are found in molecular orbitals that are distributed over the entire molecule.
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This note was uploaded on 01/25/2012 for the course CH 310 M taught by Professor Iverson during the Spring '09 term at University of Texas at Austin.

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MOVBTheory(1) - MOLECULAR ORBITAL AND VALENCE BOND THEORY...

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