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 Chemical Reactivity: SOMO, HOMO, and LUMO Synchronize when the speaker finishes saying “…what holds molecules together, that is on bonding.” Synchrony can be adjusted by using the pause(||) and run(>) controls. For copyright notice see final page of this file

Perspectives: Molecule (Reality) Computer (Approximate Schroedinger) Chemist (Understand Bonds)

Missing Bond ? (e.g. 32 nd of 33 occupied MOs) Cf. Lecture 7 - Dunitz et al. (1981) Experiment: Pathological Bonding Bent Bonds ? Would a Computer’s MOs Provide Understanding? No! Far too complicated to answer “Why?”

But analysis in terms of pairwise bonding overlap of hybrid AOs provides clear explanations. Experiment: Pathological Bonding Missing Bond ? Bent Bonds ? Best Overlap Possible for 60° C-C-C  Very Poor Overlap >90°? p sp 4.1 sp 1.4 Because sp 4.1 extends to give best overlap Why not p orbitals (90°) ? Rehybridizing to strengthen this bond would weaken six others.

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

B H H H Electron Cloud of by "Spartan"

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

BH 3 Total e-Density 0.15 e/Å 3

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

BH 3 Total e-Density 0.02 e/Å 3

BH 3 Total e-Density e/Å 3 van der Waals surface (definition)

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

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

BH 3 8 low-energy AOs  8 low-energy MOs B : 1s, 2s, 2p x, 2p y, 2p z 3  H : 1s AO “basis” set MOLECULAR ORBITALS

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

1s 1s Boron Core MOLECULAR ORBITALS

2s Radial Node MOLECULAR ORBITALS

2p x MOLECULAR ORBITALS

2p y MOLECULAR ORBITALS

2p z MOLECULAR ORBITALS

3s MOLECULAR ORBITALS

3d x 2 - y 2 MOLECULAR ORBITALS

3d xy MOLECULAR ORBITALS

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

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

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

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

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

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

Which MO Mixings Matter for Reactivity? etc. UMOs etc. UMOs OMOs B A Nothing Weak Net Repulsion Negligible Mixing because of Bad E-match molecule

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

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

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

High HOMO- Low LUMO mixing constitutes Reactivity

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

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

End of Lecture 15 Oct. 18, 2008 Copyright © J. M. McBride 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