Lesson08 - Orbital Interaction Theory Blowing Up the Lewis...

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Orbital Interaction Theory Blowing Up the Lewis Model The concepts of quantum mechanics and orbitals are abstract and difficult to grasp. While these concepts are grounded in a rigorous mathematical formalism, a more qualitative “back-of-the- envelope” approach known as orbital interaction theory offers practical interpretations and physical insight for chemists. For example, orbital interaction theory will serve to rationalize and guide our thinking about problems in molecular geometry, conformation, nucleophilicity, electrophilicity, stabilization, destabilization, and reactivity. In this lesson we introduce some basic concepts about orbitals. This lesson will improve your skills in combining orbitals for the purpose of constructing and interpreting orbital interaction diagrams.
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Shortcomings of the Lewis Model 9 Problems Not Explained by the Lewis Model of Bonding C C C 2 O 2 has unpaired electrons O O R H R H R H H R H R H H H R H H R H H R H H H CH 3 CH 3 H 3 C less stable more stable H H H H H H H H more stable less stable N R H R O planar R 1 Br R 2 H Nu R 1 R 2 H Nu + Br S N 2 inversion O Nu O Nu O Nu NOT O Nu
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The Orbital Model of Bonding Electron Density Distributions A bond in the Lewis model becomes an electron density map in the MO model H H Red - electron rich regions Blue - electron deficient regions The electron density map describes where the electrons are ( nucleophilic sites ) and where they are not ( electrophilic sites ) H H Quantum mechanics is the branch of physical chemistry concerned with calculating a molecule’s electron density distribution. Electron density and how electron density varies in space is key to understanding molecular bonding and geometry. The forces holding nuclei together are Coulombic (i.e., electrostatic) interactions between electrons and nuclei. These attractive forces are balanced by repulsive interactions (i.e., electron-electron repulsion and nuclear- nuclear repulsion). A quantitative expression of these ideas is given by the Hellman-Feynman theorem which states that the total force on a nucleus in a molecule is the sum of the Coulombic forces exerted by the other nuclei and by the electron density distribution, ρ . The “bottom line” is that electrostatic forces are the only forces that operate in a molecule to hold the atoms together; there are no mysterious quantum mechanical forces. The electron density distribution, ρ , together with the positions of the nuclei determines if a particular arrangement of atoms (i.e., a molecule) is stable or not. For details see: R. J. Gillespie, and P. L. A. Popelier, Chemical Bonding and Molecular Geometry , Oxford University Press, New York, 2001; pp. 134-136.
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The Glue that Holds Molecules Together +q +q -q -q +q +q -q -q attractive net force +q +q -q -q repulsive net force
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This note was uploaded on 01/25/2012 for the course CHEM 232 taught by Professor Miller during the Spring '08 term at University of Illinois, Urbana Champaign.

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Lesson08 - Orbital Interaction Theory Blowing Up the Lewis...

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