03-25-2011 Lecture

03-25-2011 Lecture - σ 2p σ * 2p π 2p π * 2p σ 2s σ *...

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The Expected MO Diagram for B 2 Indicates that all of the electrons in the B 2 molecule are paired = Diamagnetic Problem: B 2 is known to be a paramagnetic molecule with two unpaired electrons
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The Correct MO Diagram for B 2 p-s mixing increases the energy of the σ 2p orbital But why?
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In B 2 , C 2 , N 2 , σ 2s and σ 2p orbitals both show significant probability near the area occupied by the nuclei of the bonded atoms. Electrons in σ 2s and σ 2p orbitals repel one another. σ 2s wins out, making electrons in the σ 2p orbital less stable (higher in energy) than they would ordinarily be. Thus, this pushes the σ 2p orbital above the π 2p orbitals. Z eff increases as you go across a row in the periodic table. The greater nuclear charge pulls in the σ 2s electrons closer. Therefore, the σ 2s orbital gets smaller, and interacts less with the σ 2p orbital. This causes the energy of the σ 2p orbital to drop below the π 2p orbitals by the time you get to O 2 . What is p‐s Mixing?
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σ 2s σ * 2s
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Unformatted text preview: σ 2p σ * 2p π 2p π * 2p σ 2s σ * 2s σ * 2p π 2p π * 2p σ 2p B 2 : 6e C 2 : 8e N 2 : 10e O 2 : 12e F 2 : 14e Ne 2 : 16e Bond Order 1 2 3 2 1 0 paramagnetic diamagnetic diamagnetic paramagnetic diamagnetic diamagnetic Determine the bond orders for the following species: F 2 2-, F 2-, F 2 , F 2 + , F 2 2+ and arrange in order of increasing bond strength (and decreasing bond length.) Concept Ques?on Using energy level diagrams for the MO’s of N 2 , N 2 + , O 2 and O 2 + , explain the following data which show that removing an electron from N 2 weakens the bond and lengthens it, but removing an electron from O 2 strengthens the bond and shortens it: N 2 N 2 + O 2 O 2 + bond energy / kJ mol-1 945 841 498 623 bond length / pm 110 112 121 112 Concept Ques?on Which of the following species has the largest dissociation energy? 1) 2) 3) 4) 5) O 2 O 2- O 2 2- O 2 + O 2 2+ Concept Ques?on...
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This note was uploaded on 10/13/2011 for the course CHEM 101 taught by Professor Fitts during the Fall '08 term at UPenn.

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03-25-2011 Lecture - σ 2p σ * 2p π 2p π * 2p σ 2s σ *...

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