104A Sp09 lecture 7 - Valence Bond Theory Reading: DeKock...

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Unformatted text preview: Valence Bond Theory Reading: DeKock and Gray, 3-1 to 3-7 Molecular Orbital Theory Reading: DeKock and Gray, Chap 4 (but not 4-8) Miessler and Tarr, Chap 5 Valence Bond Theory Valence bond theory (VBT) is a localized quantum mechanical approach to describe the bonding in molecules. VBT provides a mathematical justification for the Lewis interpretation of electron pairs making bonds between atoms. VBT asserts that electron pairs occupy directed orbitals localized on a particular atom. The directionality of the orbitals is determined by the geometry around the atom which is obtained from the predictions of VSEPR theory. In VBT, a bond will be formed if there is overlap of appropriate orbitals on two atoms and these orbitals are populated by a maximum of two electrons. bonds : symmetric about the internuclear axis bonds : have a node on the inter-nuclear axis and the sign of the lobes changes across the axis. Valence Bond Theory Valence bond theory treatment of bonding in H 2 and F 2 H A 1s 1 H B 1s 1 A B This gives a 1s-1s bond between the two H atoms. For the VBT treatment of bonding, we ignore the anti-bonding. F 2s 2p F 2s 2p 2p z 2p z z axis This gives a 2p-2p bond between Valence Bond Theory Valence bond theory treatment of bonding in H 2 and F 2 H A 1s 1 H B 1s 1 A B This gives a 1s-1s bond between the two H atoms. For the VBT treatment of bonding, we ignore the anti-bonding. F 2s 2p F 2s 2p 2p z 2p z z axis This gives a 2p-2p bond between the two F atoms. Valence bond theory treatment of bonding in O 2 This gives a 2p-2p bond between the two O atoms. O 2s 2p O 2s 2p 2p z 2p z z axis z axis 2p y 2p y This gives a 2p-2p bond between the two O atoms. In VBT, bonds are predicted to be weaker than bonds because there is less overlap. The Lewis approach and VBT predict that O 2 is diamagnetic this is wrong! (the choice of 2p y is arbitrary) O O Lewis structure Double bond: bond + bond Triple bond: bond + 2 bonds Valence Bond Theory Detailed valence bond theory treatment of bonding in H 2 . H A 1s 1 H B 1s 1 A (1) 1 = A (1) B (2) VBT considers the interactions between separate atoms as they are brought together to form molecules. B (2) Atomic wavefunction on atom B electron 2 = A (2) B (1) + = N ( 1 + 2 ) (bonding, H-H) - = N ( 1- 2 ) (anti-bonding) Quantum mechanics demands that electrons can be interchangeable so we must use a linear combination of 1 and 2 ....
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104A Sp09 lecture 7 - Valence Bond Theory Reading: DeKock...

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