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CH11_MO_notes.W12 - Energy MOLECULAR ORBITAL THEORY Chapter...

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Unformatted text preview: Energy MOLECULAR ORBITAL THEORY Chapter 11, M&B ° more understanding: why oxygen is paramagnetic, why H; exists ° eliminates need for resonance structures ° rather than orbitals localized on an atom, uses molecular orbitals which are delocalized over several, if not all, of the atoms in a molecule _ . ° recognizes that conjugated 7r electron systems have added stability from delocalization of electrons Molecular Orbital (MO) Theory of Diatomic Molecules molecular. orbital formed from a linear combination (i.e., combining by adding or subtracting) of atomic orbitals (AOs) centered on the nuclei being bonded — for a homonuclear diatomic (atom 1 = atom 2) the MO (wmolecular) is formed from a superposition (overlap) of waves (atomic orbitals) on atom 1 and atom 2 LIJmolecular orbital : Cl Tatomic orbital on atom 1 + CZWatomic orbital on atom 2 where the coefficient cl = icz 1. number of MOS is equal to the number of atomic orbitals combined 2. when c1 = c2 there is in phase constructive interference and a bonding molecular orbital results which is lower in energy than the parent atomic orbitals; when c1 = —cZ there is out of phase destructive interference and'an antibonding molecular orbital results which is higher in energy than the the parent atomic orbitals as there is a node between the nuclei being bonded 3. electrons assigned to MOs in increasing energy (aufbau principle) according to Hund’s rule ,(equal energy orbitals first half fill with parallel electron spins before pairing) and Pauli exclu— _ sion principle (no more than two electrons per MO) 4. most effective combination is when the parent atomic orbitals are of similar energy (the combination of a 13 A0 and a 2s A0 is not effective) relative energies and parentage of MOs are visualized in an M0 energy diagram where the bond order = 1/2 (# e_’s in bonding MOs - # e"s in antibonding MOS) Homonuclear Diatomics (Hf; — Beg) FIG I - 0', 0* M05 Formed from s AOs ‘ In phase (constructive interference) ing) - 13, A ls, B Molecular orbltal‘s (a) /F\ Atomic orbitals Molecular orbitals Atomic orbital / 1% \ Atomic orbital (Atom A) / ”1s \ (Atom B) Out of phase Nodal plane / \ (destructive interference) 1s—-—/\T/)——-——-<\ />—( l )——1s _ ‘ \\ / V . \ r“ I ,. till—4 . . V Is, A —ls, B 61, (antibonding) “’15 (b) ‘/ ‘ FIG" II — MO Energy Diagram for FIG I 2s significantly higher in energy than ls so there is no combination of 1s and 23 to form a molecular orbital - What is the MO energy diagram for Liz? What about B2? 1) Remember how two p orbitals can combine. FIG III - 0' MOS formed from sp AOs In phase ~ (constructive interference) 2pz, A ~21)” B 62p: (bonding) (3) Atomic orbitals Molecular orbitals Out of phase (destructive interference) Nodal plane 217:) A 2pz, B 034,2 (antibonding) (b) FIG IV - 7: M05 Formed from p A08 In phase 2px! A 211;, B 1E2“ (bonding) (a) Nodal plane Out of phase 162x (antibonding) (b) 2) Remember that atomic size decreases across a period so energy increases, separation between 2s and 2 p orbitals increases, sp mixing decreases FIG V - MO Diagram for 02, F2, Nez Without 25 - 2p mixing Energy AO MO AO FIGAVI - MO Diagram for z s 7, ' Most Heteronuclear Diatomics With 25 - 2p mixing AO .MO AO Heteronuclear Diatomics _ FIG VII - MO Diagram for CO FIG VIII - MOS for CO electrons of lriple bond Bond Order = 3. ENEHGV No bonds: bonding and amibonding eleclrons c: C CO 0 Atomic Molecular Marnie 01bilals orbitals Orbllals Summary for diatomics — homonuclear/heteronuclear 1) draw full molecular orbital energy diagram atomic orbitals of more electronegative element lower in energy ' MOS: as<as-*<0'p<7rp<n'p *<0'p*for228 Sp mixing causes 75p <- 0'], for Z S 7, most heteronuclear diatomics 2) fill with valence electrons obeying Pauli principle and Hund’s rule 3) determine bond order, etc 7: Electron Systems CH2 = CH2 FIG IX - '7: MOS of Butadiene CH2 = (mm = CH2 _4_ ozone, 03 FIG X-fl' MOS of Ozone Anflbondlng 7r orbital n and 17 bonding in ozone benzene, C6H6 FIG XI - 7r MOS of Benzene Mm Bananas Summary for 7: electron systems beyond diatomics I) draw Lewis structure + all resonance structures 2) determine number of 0', 7t, and lone electrons 3) determine sigma bonding, orbitals containing lone electrons, and sketch structure 4) determine number of adjacent or conjugated sp2 and/or Sp atoms = number of 7: MOS 5) draw MOS, note nodes, energy order, label bonding, nonbonding, antibonding orbitals 6) draw MO energy diagram, fill with electrons obeying Pauli principle and Hund’s rule ...
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