1. Previous examples of “pathological” bonding and the BH 3 molecule illustrate how a chemist’s use of localized bonds, vacant atomic orbitals, and unshared pairs to understand molecules compares with views based on the molecule’s actual total electron density, and with computational molecular orbitals. This lecture then focuses on understanding reactivity in terms of the overlap of singly-occupied molecular orbitals (SOMOs) and, more commonly, of an unusually high-energy highest occupied molecular orbital (HOMO) with an unusually low-energy lowest unoccupied molecular orbital (LUMO). This generalizes the traditional concepts of acid and base. Criteria for assessing reactivity are outlined and illustrated. Chemistry 125: Lecture 15 October 7, 2009 Chemical Reactivity: SOMO, HOMO, and LUMO (note: to synch with audio file for this lecture press  at 1’18”) For copyright notice see final page of this file

2. Science, 11 August 2000

3. 25 nm (250 Å) >100,000 atoms + hydrogens! http://nobelprize.org/nobel_prizes/chemistry/laureates/2009/

4. 2) Molecular Orbitals 1) Total Electron Density 3) Bonds from Hybrid AOs (Nature) (Computer) (Student) Cf. Course Webpage Three Views of BH 3

5. Computer Partitions Total e-Density into Delocalized MOs (à la Chladni)

6. BH 3 8 low-energy AOs  8 low-energy MOs B : 1s, 2s, 2p x, 2p y, 2p z 3  H : 1s “Minimal.. Basis Set” of AOs MOLECULAR ORBITALS

7. noccupiednoccupied BH 3 8 electrons / 4 pairs B : 5 electrons 3  H : 3  1 electron OMO s UMO s LUMOHOMO( s) ccupiedccupied ighestighest owestowest MOLECULAR ORBITALS

8. 1s 1s Boron Core MOLECULAR ORBITALS Cf. website

9. 2s Radial Node MOLECULAR ORBITALS Cf. website

10. 2p x MOLECULAR ORBITALS Cf. website

11. 2p y MOLECULAR ORBITALS Cf. website

12. 2p z MOLECULAR ORBITALS Cf. website

13. 3s MOLECULAR ORBITALS Cf. website

14. 3d x 2 - y 2 MOLECULAR ORBITALS Cf. website

15. 3d xy MOLECULAR ORBITALS Cf. website

16. RESONANCE We Partition the same Total e-Density into Atom-Pair Bonds (and anti-bonds) & Lone Pairs (and vacant atomic orbitals) (à la Lewis) usually  When this doesn't work, and we must use more sophisticated orbitals, we say there is

17. 2p z For Many Purposes Localized Bond Orbitals are Not Bad Boron Core And they are easy to think about; but beware of resonance. BHBH  H B H B Same Total Energy! (and of properties of individual electrons) H B Same Total e-Density!    B H H B

18. The Localized Bond Orbital Picture (Pairwise MOs and Isolated AOs) is our intermediate between H-like AOs and Computer MOs When must we think more deeply? When mixing of localized orbitals causes Reactivity or Resonance

19. Where are We? Molecules Plum-Pudding Molecules ("United Atom" Limit) Understanding Bonds (Pairwise LCAO) "Energy-Match & Overlap" Structure (and Dynamics) of XH 3 Molecules Parsing Electron Density Atoms 3-Dimensional Reality (H-like Atoms) Hybridization Orbitals for Many-Electron Atoms (Wrong!) Recovering from the Orbital Approximation Recognizing Functional Groups Payoff for Organic Chemistry! Reactivity SOMOs, high HOMOs, and low LUMOs

20. Which MO Mixings Matter for Reactivity? etc. etc. UMOs OMOs B A UMOs Myriad Possible Pairwise Mixings  molecule

21. Which MO Mixings Matter for Reactivity? etc. etc. UMOs OMOs SOMO B A SOMO-SOMO (when they exist) UMOs many atoms "free radicals" e.g. H Cl CH 3  not so common inglyingly molecule

22. Which MO Mixings Matter for Reactivity? etc. UMOs etc. UMOs OMOs B A Nothing Weak Net Repulsion molecule (balances correlation at van der Waals contact) Negligible Mixing & Stabilization because of Bad E-match

23. Which MO Mixings Matter for Reactivity? etc. UMOs etc. UMOs OMOs B A Bonding! Unusually High HOMO with Unusually Low LUMO molecule Negligible Mixing & Stabilization because of Bad E-match

24. Which MO Mixings Matter for Reactivity? etc. UMOs etc. UMOs OMOs B A Bonding! Unusually High HOMO with Unusually Low LUMO BASE ACID molecule

25. Most mixing of MOs affects neither overall energy nor overall electron distribution for one (or more) of these reasons: 1) Electron occupancy 4 or 0 2) Poor energy match 3) Poor overlap BUT

26. High HOMO- Low LUMO mixing constitutes Reactivity

27. Acid-Base Theories THEORYACIDBASE Lavoisier (1789) Oxidized Substance Substance to be Oxidized Arrhenius (1887) H + SourceOH - Source Increasing Generality Brønsted/Lowry (1923) H + DonorH + Acceptor Lewis (1923) e-Pair Acceptor "Electrophile" e-Pair Donor "Nucleophile" HOMO/LUMO (1960s) unusually High HOMO unusually Low LUMO

28. sp 3 C 1s H Unusual:  * 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 Compared to What?

29. 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)

30. 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)

31. 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)

32. 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)

33. Is BH 3 an Acid or a Base?

34. 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

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

36. HOMO-LUMO mixing for Reactivity & Resonance (between molecules) (within molecule) (i.e. between the localized orbitals whose overlap we had overlooked at first glance)

37. End of Lecture 15 Oct. 7, 2009 Copyright © J. M. McBride 2009. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0) Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol. Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0