Chem 125 Lecture 16 10/10/08 This material is for the exclusive use of Chem 125 students at Yale and may not be copied or distributed further. It is not.

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Chem 125 Lecture 16 10/10/08 This material is for the exclusive use of Chem 125 students at Yale and may not be copied or distributed further. It is not readily understood without reference to notes from the lecture.

Unusual: Compared to What? sp 3 C 1s H  * CH  CH "usual" LUMO "usual" HOMO (or  * CC ) (or  CC ) I. Unmixed Valence-Shell AOs III. Unusual AO Energy in MO Sources of weirdness: IV. Electrical Charge II. Poor Overlap of AOs

I. Unmixed Valence-Shell Atomic Orbitals  * CH  CH sp 3 C 1s H BH 3 low LUMO CH 3 high HOMO NH 3 high HOMO "usual" LUMO "usual" HOMO OH 2 high HOMO OH high HOMO (or  * CC ) (or  CC ) H + low “LUMO” (energies qualitative only) (Also IV: Electrical Charge)

Acid-Base Reactions H + OH H-OH H + :NH 3 H 3 B OH H 3 B :NH 3 H-NH 3 + H 3 B-OH H 3 B-NH 3 + Curved Arrows Designate e-Pair Shifts. Start arrow at e-pair location in starting material. End arrow at e-pair location in product. (NOT atomic motion)

II. Poor-Overlap MOs  * C=O  C=O pCpC pOpO  * CH  CH sp 3 C 1s H  * C=C  C=C pCpC pCpC high HOMO low LUMO "usual" LUMO "usual" HOMO  Bent  (energies qualitative only)

III. Unusual AO Energy in MO low LUMO  * CH  CH sp 3 C 1s H  * C-F  C-F sp 3 C pFpF high HOMO  * H-B  H-B 1s H sp 2 B "usual" LUMO "usual" HOMO (energies qualitative only)

Is BH 3 an Acid or a Base?

HOMO H B H H and a Base H B H H LUMO BH 3 is an Acid 3-Center-2-Electron " Y " Bond BB H H HH HH LUMO H B H H HOMO H B H H s Answer to the puzzle about Lewis structures from Lecture 2

Compared to What? Low-Energy MOs  Bonding Lowered-Energy MOs  Bonding Its Separated Components True or False? False

HOMO-LUMO mixing for Reactivity & Resonance (between molecules) (within molecule)

 * LUMO Low LUMO H-F (4 valence e-pairs; 5 valence AOs) 2p F HOMOs Shape of "Frontier" Orbitals (ABN) Antibonding Node Atomic Orbital Node (AON) 1s H 2p F 1s H 2s F 1s H 2sp F n Alternative format from an early MO catalogue An Organic Chemist’s Book of Orbitals Salem & Jorgensen, 1973

Shape of "Frontier" Orbitals F-CH 3 (7 valence pairs) AON ABN Compare their Energies HOMOs ≈> &  C-H pFpF

LUMO F-CH 3 Low LUMO (7 valence pairs) HOMOs ABN AON Why unusually low? Low LUMO ABN AON Shape of "Frontier" Orbitals

F H Direction of best overlap :OH FH OH "Acid-Base" ABN Curved Arrows Designate e-Pair Shifts Start arrow at e-pair location in starting material. End arrow at e-pair location in product. LUMO Reaction Analogies

End of Lecture 16 Oct. 10, 2008