# Lesson09 - Pairwise Orbital Combinations Adopt a convention...

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Unformatted text preview: Pairwise Orbital Combinations Adopt a convention for combining orbital pairs Identify all the sigma-type combinations Cylindrical Symmetry: The top and bottom of a cylindrically symmetrical object may be different, but everything is the same around its cylindrical axis. Sigma orbitals have cylindrical symmetry SHEET #1 - The Pairwise Orbital Combinations z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x atom 2 atom 1 Instructions : (Part 1) For each symmetry- allowed combination, draw the orbital that results when the two atoms are brought together along the the x-axis (calling the internuclear axis the x-axis). See the example below: z x y z x y atom 1 atom 2 x 2s –2s –2p z 2p z –2p y 2p y –2p x 2p x 2s –2s 2p x –2p x 2p y –2p y 2p z –2p z atom 1 atom 2 Bringing orbitals together from two different atoms → making MOs • some orbital combinations interact in such a way as to c o n c e n t r a t e the electron density between the nuclei ( bonding interactions ) • some orbital combinations interact in such a way as to d e p l e t e the electron density between the nuclei ( antibonding interactions ) • some orbital combinations interact in such a way as to neither concentrate nor deplete the electron density between the nuclei ( nonbonding interactions ) See: Molecular Orbitals (N 2 ) at http://www.shef.ac.uk/chemistry/orbitron/ Instructions : (Part 2) Draw an outline around the group of orbital combinations that are n o t allowed to interact because of symmetry reasons. Write the label "symmetry forbidden" in large letters inside this zone. Instructions : (Part 3) For each allowed combination, add one of the following labels ( σ s , σ s *, σ p , σ p *, σ s/p , σ s/p *, ! y , ! y *, ! z , ! z *) where * denotes an antibonding interaction. SHEET #1 - The Pairwise Orbital Combinations z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y z x y x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x atom 2 atom 1 Instructions : (Part 1) For each symmetry- allowed combination, draw the orbital that results when the two atoms are brought together along the the x-axis (calling the internuclear axis the x-axis). See the example below: z x y z x y atom 1 atom 2 x 2s –2s –2p z 2p z –2p y 2p y –2p x 2p x 2s –2s 2p x –2p x 2p y –2p y 2p z –2p z atom 1 atom 2 Bringing orbitals together from two different atoms → making MOs • some orbital combinations interact in such a way as to c o n c e n t r a t e the electron density between the nuclei ( bonding interactions ) • some orbital combinations interact in such a way as to d e p l e t e the electron density between the nuclei ( antibonding interactions ) • some orbital combinations interact in such a way as to neither concentrate nor deplete the electron density between the nuclei (...
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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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Lesson09 - Pairwise Orbital Combinations Adopt a convention...

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