1. Chem 125 Lecture 17 10/10/2005 Projected material 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.

2. CH 3 SOMO PlanarBent

3. Structural Isotope Effect: CH 3 spends more time more bent than CD 3 (thus uses more s-character for SOMO electron) CH 3 CD 3 36 Gauss 38 Gauss

4. CF 3 Repulsion between F atoms?  Flatter than CH 3 Since Fluorine holds the lion's share of the bonding electron pair, Carbon has less reason to use its valuable s-character in the bonding orbitals. Use it for the SOMO.  More Bent than CH 3

5. CF 3 SOMO 271 Gauss!  20% s (vs. 38 for CH 3 )  sp 4

6. Tension! Goals Computer Chem 125 Student Minimize kinetic plus coulomb energies of electrons and nuclei by “settling down” Minimize kinetic plus coulomb energy using Schrödinger equation with “realistic” constraints Understand structure and reactivity with the simplest “realistic” model Experimental Molecule e.g. limited set of AOs, SCF, some correlation e.g. hybridization, bonds, lone pairs, E-match/overlap HOMO/LUMO Validation with Experiment & Computer Validation with Experiment Structure e-Density (X-Ray) Energies (IR) Nuclear Density (EPR) Dipole Moment, etc.

7. Dunitz et al. (1981) Experiment: Pathological Bonding Missing Bond ! Bent Bonds ! Best Overlap Possible for 60° Very Poor Overlap

8. Computer: B H H H Electron Cloud of by "Spartan"

9. BH 3 Total e-Density 0.30 e/Å 3 Mostly 1s Core of Boron B H H H

10. BH 3 Total e-Density 0.15 e/Å 3

11. BH 3 Total e-Density (0.05 e/Å 3 ) Dimple H atoms take e-density from valence orbitals of B B H + H B

12. BH 3 Total e-Density 0.02 e/Å 3

13. BH 3 Total e-Density 0.002 e/Å 3 van der Waals surface

14. BH 3 Total e-Density 0.002 e/Å 3 Electrostatic Potential Energy of a + probe on the surface low (-) high (+) H  

15. Computer Partitions Total e-Density into Symmetrical MOs (à la Chladni)

16. BH 3 8 low-energy AOs  8 low-energy MOs B : 1s, 2s, 2p x, 2p y, 2p z 3  H : 1s

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

18. 1s 1s Boron Core

19. 2s Radial Node

20. 2p x

21. 2p y

22. 2p z

23. 3s

24. 3d x 2 - y 2

25. 3d xy

26. Computer Partitions Total e-Density into Symmetrical MOs (à la Chladni)

27. We Partition 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 RESONANCE

28. 2p z   B H H B Same Total e-Density Same Total Energy    B H H B For Many Purposes Localized Orbitals are Not Bad Boron Core

29. 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

30. The Localized 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

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

32. 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

33. Which MO Mixings Matter for Reactivity? etc. UMOs etc. UMOs OMOs B A Nothing Weak Net Repulsion Negligible mixing Bad E-match

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

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

36. Acid-Base Theories Lavoisier (1789) Oxidized Substance Substance to be Oxidized Arrhenius (1887) H + SourceOH - Source Increasing Generality THEORYACIDBASE 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

37. Unusual: Compared to What?

38. End of Lecture 17