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Lecture 23

CHM 114 Lecture Notes - Lecture 23: Valence Bond Theory, Sigma Bond, Rotational Symmetry


Department
Chemistry
Course Code
CHM 114
Professor
George Wolf
Lecture
23

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Lecture 20
โ— *Review
โ—‹ same rotational symmetry
โ—‹ Hybridization
โ–  sigma vs pi type bonds
โ–  overlap of orbitals
โ— *sigma ๎€much
๎€
stronger than pi
โ—‹ sigma = head to head
โ—‹ pi = side to side
โ— no bond formed
โ—‹ these two atomic orbitals have different rotational
symmetries when viewed down the internuclear axis
โ–  Multiple Bonds:
โ— pi and sigma
โ— takes energy to rotate bonds
โ— internal rotation to break bond
โ—‹ break pi bond
โ—‹ takes A LOT of energy
โ—‹ locks structure into ๎€planar
๎€
structure
โ—‹ To internally rotate the C-C pi-bond must be broken
(C2H2)
โ—‹ But C2H6 โ†’ all sigma bonding no pi
โ—‹ free rotation around C-C (donโ€™t have to break sigma
bond)
โ–  only limit is energetically overlap of H bumping
โ— low interaction barrier
โ— Resonance
โ—‹ C6H6 Delocalized pi electrons
โ—‹ all C atoms are sp2 hybridized
โ—‹ delocalization (resonance) ๎€lowers
๎€
๎€
the energy of
orbital and ๎€increases
๎€
stability of molecule
โ— NO3-
โ—‹ 24 Total VEโ€™s
โ—‹ 3 structures for resonance
โ—‹ Valence Bond Theory
โ–  Triple Bonds
โ— N2
โ— 2 electron domains around each N atom
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