Lecture 8

Lecture 8 - Molecular Orbitals of H2 Molecular orbitals are...

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Molecular Orbitals of H 2 Molecular orbitals are derived by a superimposition of the atomic orbitals belonging to the valence shells of all atoms, as illustrated by H 2 . As two atoms approach each other, the orbitals overlap and interfere either constructively (addition) or destructively (subtraction). Note that color is often used to indicate phase (+/–). subtraction 1s A – 1s B 1s A 1s B The derived orbitals are called a linear combination of atomic orbitals (LCAO). addition 1s A + 1s B
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Molecular Orbital Diagrams The energies of the σ and σ * orbitals depend on the amount of overlap. σ –orbital becomes more stable as distance decreases due to increased overlap E σ –orbital becomes less stable due to internuclear repulsion σ *–orbital becomes more unstable as distance decreases due to increased internuclear repulsion r
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Molecular Orbital Diagram for O 2 Oxygen has 6 valence electrons, i.e. there are 12 electrons in MOs. Two electrons in π * orbitals are unpaired, i.e. O 2 is paramagnetic . The electron configuration is: ( σ 2s ) 2 ( σ * 2s ) 2 ( σ 2p ) 2 ( π 2p ) 4 ( π * 2p ) 2 . Bond order = 8 – 4 2 = 2
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Valence-bond theory is modified to explain the bonding in CH 4 by assuming that a 2s electron is first promoted into the 2p level, i.e. an excited state, thereby giving rise to four unpaired electrons . promotion energy Each bond releases energy as it forms, and this is sufficient to overcome the promotion energy. However, what structure would we expect for CH 4 using this model? two
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This note was uploaded on 11/03/2011 for the course MATH 1090 taught by Professor Greenwood during the Spring '08 term at MIT.

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Lecture 8 - Molecular Orbitals of H2 Molecular orbitals are...

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