week 6 ppt.pdf - Ways of Looking at a Molecule(or a Molecular Orbital Molecule Molecule asas from one atoms atom set of atoms Set of Single United Atom

week 6 ppt.pdf - Ways of Looking at a Molecule(or a...

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Set of ~normal atoms Atoms with small bonding distortion (~0.05 Lewis) Single “United Atom” Ways of Looking at a Molecule (or a Molecular Orbital) e-density contours of H 2 Which contour should we use? Molecule from set of atoms Molecule as one atom distorted by fragmenting the nucleus Nuclei embedded in a cloud of electrons dispersed and “noded” by kinetic energy J. J. Thomson's Plum Pudding! (backwards) Molecule as atoms (worth a quick look)
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How the Plums Distort Hydrogen-Like Kinetic-Energy Puddings
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Methane & Ammonia Spartan 6-31G* calculates good SCF MOs (on my laptop!) We want to understand them visually.
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4 Pairs of Valence Electrons H C H H H N H H H Compare MOs to AOs of Ne (4 electron pairs with n =2)
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1s CH 4 NH 3 "Core" Orbitals Like 1 s of C/N Tightly Held Little Distortion Contour Level 0.001 e/Å 3 We'll focus on Valence Orbitals Boring! .. .. .. .. .. .. .. .. 8 valence e - Þ 4 MOs 8 valence e - Þ 4 MOs energy Three “degenerate” Molecular Orbitals
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2s .. .. .. .. .. .. .. ..
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2s .. .. .. .. .. .. .. .. “Spherical” node
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2p x .. .. .. .. .. CH 4 NH 3 .. .. ..
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CH 4 NH 3 2p y .. .. .. .. .. .. .. ..
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CH 4 NH 3 2p y .. .. .. .. .. .. .. ..
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CH 4 NH 3 2p z HOMO Lewis's "unshared pair" .. .. .. .. .. .. .. .. +Unoccupied Orbitals +Unoccupied Orbitals
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CH 4 NH 3 3s LUMO "HUMO?" .. .. .. .. .. .. .. ..
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2s CH 4 NH 3 3s LUMO "HUMO" .. .. .. .. .. .. .. ..
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CH 4 NH 3 3d x 2 -y 2 .. .. .. .. .. .. .. ..
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CH 4 NH 3 3d x 2 -y 2 .. .. .. .. .. .. .. ..
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CH 4 NH 3 3d xy .. .. .. .. .. .. .. ..
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CH 4 NH 3 3d xy .. .. .. .. .. .. .. ..
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CH 4 3d z 2 .. .. .. ..
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e-density contours of H 2 Single “United Atom” distorted by a fragmented nucleus Which contour should we use? The Plum-Pudding View of Molecular Orbitals Shows Generality of Kinetic-Energy-Based Clouds Atoms with weak bonding But One Must Probe Harder to Gain a Useful Understanding of Chemical Bonds
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Pairwise LCAO MOs 1 2 ( AO a + AO b ) Y (x 1 ,y 1 ,z 1 ) = SUM ( L inear C ombination ) of AOs (like hybridization, but with two atoms) Why is this form sensible? “True” molecular orbitals extend over entire molecules, but we want to understand local bonds as
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