W2013CHM2311 Part 4c Notes

These are nonbonding orbitals and contain nonbonding

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Unformatted text preview: 2p 2s F H F 2p 2s F H F Combining Group Orbitals and the Central Atom Symmetry requirements for bonding overlap: The only fluorine 2p GOs with the correct symmetry for overlap with the hydrogen 1s AO are the 2pz combina0ons. Combining Group Orbitals and the Central Atom What are the resul3ng MOs? MOs are formed from both posi1ve and nega1ve combina0ons. # MOs = # AOs + # GOs The only bonding combina0on of orbitals is the hydrogen 1s with the fluorine 2pz in ­phase combina0on GO. These orbitals can be added (bonding) or subtracted (an0bonding). Bonding MO An3bonding MO The MO Diagram can be drawn in the same fashion as for a diatomic. →  Bonding orbitals are lower in energy than the lowest ­energy basis orbital →  An0bonding orbitals are higher in energy than the highest ­energy basis orbital →  Nonbonding orbitals keep their original energies and shapes  ­ MO Diagram for FHF Energy 1s 2p GOs 2s GOs H FHF ­ F F GOs  ­ MO Diagram for HFH Summary of the MO Descrip3on of FHF ­ Bonding: 1) Seven of the eight fluorine group orbitals do not interact with hydrogen. These are nonbonding orbitals and contain nonbonding electrons. 2) The 2pz fluorine GO interacts with the 1s orbital of hydrogen to give two molecular orbitals: one bonding and one an0bonding. 3) The bonding orbital contains one electron pair. This means that only 2 electrons are used in total to bond the two fluorine atoms to the hydrogen atom (3 ­center, 2 ­electron bond). 4) The bonding MO is very low in energy. Generally, the more atoms that are covered by a MO (i.e., the more delocalized the electron density), the more stable the MO will be. A MO covering three atoms will be more stable than a MO covering only two atoms. Determining the MO Diagram for Linear Triatomic AB2 Molecules 1. Determine the valence atomic orbitals for each atom 2. Determine the group orbitals (GOs) for the terminal atoms. 3. Based on rules of symmetry and orbital energy, determine which GOs are able to interact with the atomic orbitals of the central atom. 4. Carry out addi0on and subtrac0on of the appropriate atomic orbitals and GOs to determine the molecular orbitals that are formed. 5. Use a molecular orbital energy level diagram to show the rela0ve energies of the molecular orbitals. 6. Place electrons in the MOs to determine bonding characteris0cs of the molecule. Group Theory Method for Determining Molecular Orbitals FHF ­ is a simple example of a linear AB2 triatomic molecule since there is only one possible orbital interac0on (H...
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