Chem312_MO_lecture_7 - molecular orbital O 2b2 O H H H H Using Symmetry to Generate Molecular Orbital Diagrams review a few MO concepts generate MO

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Using Symmetry to Generate Molecular Orbital Diagrams • review a few MO concepts • generate MO energy level diagram for XH 2 , H 2 O, SF 6 a 1 3a 1 * 2a 1 b 2 2b 2 * b 1 (nonbonding) H H O H H O H H O O H H O H H O Molecular orbital diagram for H 2 O molecule symmetry label molecular orbital In order to be able to do this, we’ll need to know how to determine a molecule’s symmetry properties A molecule’s symmetry properties can be summarized by one symbol called the “Point Group” symbol Once we know a molecule’s “Point Group”, then we know the symmetry of all of its orbitals Orbitals can only overlap if they have identical symmetry ! ! ! ! Formation of a bond occurs when electron density collects between the two bonded nuclei (ie., ! 2 MO = large in the region of space between nuclei) Spin-pairing Bonding Molecular Orbital Anti bonding Molecular Orbital
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……. the valence electrons draw the atoms together until the core electrons start to repel one other, and that dominates the total number of molecular orbitals ( MO’s ) generated= total number of atomic orbitals ( A O’ s ) combined Atom A (4 AO’s) Atom B (4 AO’s) + Molecule A–B (8 MO’s) Atom A (4 AO’s) Atom B (4 AO’s) + Molecule A–B (8 MO’s) in order to overlap……. .. AO’s must have: identical symmetries similar energies
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Example: overlap of p–orbitals in a diatomic molecule "# type $# type $# type = nucleus p x p y p z p z p z p y p y p x p x + - - + Recall, orbitals have phases bonding MO antibonding MO overlapping in-phase overlapping out-of-phase Atom A Atom B " –orbital doesn’t change sign upon rotation about internuclear axis Atom A Atom B " % –orbital " since still doesn’t change sign upon rotation about internuclear axis overlapping in-phase overlapping out-of-phase bonding MO antibonding MO
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Atom A Atom B doesn’t change sign upon rotation about internuclear axis Atom A Atom B antibonding since changes sign upon rotation about axis perpendicular to internuclear axis " % –orbital overlapping in-phase overlapping out-of-phase bonding MO antibonding MO " –orbital Atom A Atom B $ –bond changes sign upon rotation about internuclear axis bonding MO Atom A Atom B $ % –bond (antibond) $ since still changes sign upon rotation about internuclear axis antibonding since changes sign upon rotation about axis perpendicular to internuclear axis Additional orbital labels, used by your book, describe orbital symmetry with respect to inversion (x,y,z) inversion center i (–x,–y,–z)
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$ g * Additional orbital labels, used by your book, describe orbital symmetry with respect to inversion i g= gerade, if orbital does not change phase upon inversion $ u i u= ungerade, if orbital does change phase upon inversion " g Additional orbital labels, used by your book, describe orbital symmetry with respect to
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This note was uploaded on 03/06/2010 for the course CHEM 312 taught by Professor Staff during the Winter '08 term at University of Washington.

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Chem312_MO_lecture_7 - molecular orbital O 2b2 O H H H H Using Symmetry to Generate Molecular Orbital Diagrams review a few MO concepts generate MO

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