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
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Previous examples of “pathological” bonding and the BH 3 molecule illustrate how a chemist’s use of localized bonds, vacant atomic orbitals, and unshared.
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Previous examples of “pathological” bonding and the BH3 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)
MissingBond ?
(e.g. 32nd of 33 occupied MOs)
Cf. Lecture 7 - Dunitz et al. (1981)
Experiment: Pathological Bonding
BentBonds ?
Would a Computer’s MOsProvide Understanding?
No! Far too complicated to answer “Why?”
But analysis in terms of pairwise bonding overlapof hybrid AOs provides
clear explanations.
Experiment: Pathological Bonding
MissingBond ?
BentBonds ?
BestOverlapPossiblefor 60°
C-C-C
Very PoorOverlap
>90°?
psp4.1sp1.4
Because sp4.1extendsto give best overlap
Why not p orbitals (90°)?
Rehybridizing to strengthen this
bond would weaken six others.
Three Views of BH3
2) Molecular Orbitals
1) Total Electron Density
3) Bonds from Hybrid AOs
(Nature)
(Computer)
(Student)
B
H
HH
ElectronCloud of
by "Spartan"
BH3
Total e-Density0.30 e/Å3
Mostly1s Core
of Boron
B
H
HH
BH3
Total e-Density0.15 e/Å3
BH3
Total e-Density (0.05 e/Å3)
Dimple
H atoms take e-densityfrom valence orbitals of B
BH+••H •
B•
BH3
Total e-Density0.02 e/Å3
BH3
Total e-Density0.002 e/Å3
van der Waalssurface
(definition)
BH3
Total e-Density0.002 e/Å3
ElectrostaticPotential
Energy of a+ probe onthe surface
low (-) high (+)
H
Computer PartitionsTotal e-Density
intoSymmetrical MOs
(à la Chladni)
BH3
8 low-energy AOs 8 low-energy MOs
B : 1s , 2s , 2px , 2py , 2pz
3 H : 1s
AO “basis”
set
MOLECULAR ORBITALS
noccupied
BH3
8 electrons / 4 pairs
B : 5 electrons3 H : 3 1 electron
••••
••
••
OMOs
UMOs
LUMO
HOMO(s) •• ••
ccupied
ighest
owest
MOLECULAR ORBITALS
1s
••••
••
••
1s ••Boron Core
MOLECULAR ORBITALS
2s ••••
••
••
Radial Node
MOLECULAR ORBITALS
2px ••••
••
••
MOLECULAR ORBITALS
2py••
••
••
••
MOLECULAR ORBITALS
2pz
••••
••
••
MOLECULAR ORBITALS
3s
••••
••
••
MOLECULAR ORBITALS
3dx2-y2
••••
••
••
MOLECULAR ORBITALS
3dxy
••••
••
••
MOLECULAR ORBITALS
We PartitionTotal e-Density
intoAtom-Pair Bonds
(and anti-bonds)
& Lone Pairs(and vacant atomic orbitals)
(à la Lewis)
usually
When thisdoesn't work,and we must
use moresophisticatedorbitals, wesay there is
RESONANCE
2pz
••
••
••
•••••• ••BHHB
Same Total e-Density!
Same Total Energy!
BH
HB
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 betweenH-like AOs and Computer MOs
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