Lacey_Che131_F2011_Lect-31p

Lacey_Che131_F2011_Lect-31p - Lec-31: Molecular Orbital...

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Unformatted text preview: Lec-31: Molecular Orbital Theory Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 1 Valence Bond Theory Bonds are formed from the overlap of orbitals of orbitals Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 2 Molecular Orbital Theory (1) During bonding, atomic orbitals from DIFFERENT atoms are transformed are transformed into new orbitals with different shapes, energies, and electron density distridistributions. ons. (2) This is brought about by the overlapoverlapping of atomic orbitals among different diff atoms. Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 3 Molecular Orbital Theory Molecular Orbitals : Allowed states for an electron moving in the electric field generated by two or more nuclei. The The Aufbau principle, the Pauli Exclusion principle, and Hund’s Rule of Maximum Multiplicity Multiplicity are all used to fill Molecular Orbitals. Molecular The total number of molecular orbitals is the same as the number of atomic orbitals combined. Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 4 Molecular Orbitals Bonding molecular orbitals have lower energy than the Bonding molecular orbitals have lower energy than the parent parent atomic orbitals while antibonding orbitals have higher energy than the parent atomic orbitals. orbitals. * σ1s 1s 1s Atomic Orbitals Atomic Orbitals σ1s Molecular Orbitals Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 5 Filling Molecular Orbitals Antibonding orbitals have higher energy have higher energy than than the parent atomic orbitals. orbitals. The The Aufbau principle, the Pauli Exclusion principle, and Hund’s Rule of Maximum Maximum Multiplicity Multiplicity are all used to fill Molecular Orbitals Molecular Orbitals. Bonding molecular orbitals have lower energy than the parent atomic orbitals. orbitals. Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 6 Stable Molecules σ1s* Η 1s Η 1s σ1s A molecule is stable with respect to its atoms whenever the number of bonding electrons is greater than the the number of bonding electrons is greater than the number number of antibonding electrons. electrons. Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 Bonding 7 * σ1s 1s 1s H2 (σ1s) 2 σ1s + H2 - H2 Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 (σ1s) (σ1s) 1 2 (σ *)1 1s 8 Bond Order Bond Order = ½(#bonding electrons Order ½(#bonding electrons # antibonding electrons) Η2 (σ1s)2 B.O. = 1/2(2 - 0) = 1 1/2(2 Η2+ (σ1s)1 B.O. = 1/2(1 - 0) = 1/2 1/2(1 0) 1/2 Η2− (σ1s)2(σ1s*)1 B.O. = 1/2(2 - 1) = 1/2 He2+ (σ1s)2(σ1s*)1 B.O. = 1/2(2 - 1) = 1/2 He2 (σ1s)2(σ1s*)2 B.O. = 1/2(2 - 2) = 0 A molecule with B.O. = 0 is not stable molecule with is not stable Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 9 Magnetic Behavior Paramagnetism - atoms or molecules having unpaired electrons are attracted to having unpaired electrons are attracted to magnetic fields. Diamagnetism - atoms or molecules having all paired electrons are repelled by magnetic fields. Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 10 Pi bonding σp MO is higher due to interaction between the 2s between the 2s MO and the 2pz MO Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 11 MO for Homo Nuclear Molecules Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 12 Pi Bonding σp MO is lower when interaction between the 2s MO and the 2pz MO is small Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 13 MO for Homo Nuclear Molecules The 2s and 2p interactions are strong in Li2 through N2 but weaker in O2 through Ne2. Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 14 MO Diagram Electron configuration for N2: σ2s2σ2s*2σ2p2π2p4 • • Bond order = 1/2 (8 - 2) = 3 N2 has three bonds N2 has no unpaired electrons O2 (σ2s) 2 (σ 2s*) 2 (σ 2p ) 2 (π 2p ) 4 (π 2p * ) 2 (for valence electrons) valence electrons) Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 15 MO for Heteronuclear molecules NO The effective nuclear charge alters the charge alters the symmetry of the diagram. The odd electron is more likely to be found on nitrogen since it is in an orbital closer in energy to gy the atomic orbitals of the nitrogen atom. Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 16 MO for Heteronuclear molecules CO CO MO diagrams can become ca beco complicated quickly Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 17 MO Theory MO theory may provide the most complete picture of covalent bonding, but it is also the most difficult to apply to large molecules NOTE it does not account for molecular shapes. Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 18 ...
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