The Localized Electron Model Chapter 9 Covalent Bonding: Orbitals
Several tricks (“Z-effective” and “Self Consistent Field”) allow one to correct...
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![Page 1: Several tricks (“Z-effective” and “Self Consistent Field”) allow one to correct approximately for the error in using orbitals when there is electron-electron.](https://reader035.fdocuments.in/reader035/viewer/2022062516/56649d785503460f94a5af11/html5/thumbnails/1.jpg)
Several tricks (“Z-effective” and “Self Consistent Field”) allow one to correct approximately
for the error in using orbitals when there is electron-electron repulsion. Residual error is
hidden by naming it “Correlation energy.” J.J. Thomson’s Plum-Pudding model of the atom
can be modified to visualize the form of molecular orbitals. There is a close analogy in form
between the molecular orbitals of CH4 and NH3 and the atomic orbitals of neon, which has the
same number of protons and electrons. The underlying form, dictated by kinetic energy, is
distorted by pulling protons out of the Ne nucleus to play the role of H atoms.
Synchronize when the speaker finishes saying
“we’ve been looking at atoms.” Synchrony can be adjusted by using the pause(||) and run(>) controls.
Chemistry 125: Lecture 11
Orbital Correction and Plum-Pudding Molecules
For copyright notice see final page of this file
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What's Coming for Next Exam?
MoleculesPlum-Pudding Molecules (the "United Atom" Limit)
Understanding Bonds (Pairwise LCAO)"Energy-Match & Overlap"
Reality: Structure (and Dynamics) of XH3 Molecules
AtomsOrbitals for Many-Electron Atoms (Wrong!)Recovering from the Orbital Approximation
Payoff forOrganic
Chemistry!
ReactivityHOMOs and LUMOs
Recognizing Functional Groups
How Organic Chemistry Really Developed (Intro)
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2-e Wave Function
(r1,1,1,r2,2,2)
a(r1,1,1) b(r2,2,2)
=?
Multiply 1-e Wave Functions
2
2 2
No way can electrons be independent!
They repel one another.
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Tricks for SalvagingOrbitals
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Pretend that the other electron(s) just reduce the nuclear charge for the orbital of interest.
"Clementi-Raimondi" values for Zeff(best fit to better calculations)
Atom Z Zeff 1s
He 2 1.69
2s
2p
Z - effective
Zeff 2s Zeff 2p
C 6 5.67 3.22 3.14
Zeff 3s
Na 11 10.63 6.57 6.80 2.51
!
!
2s slightlyless screened
than 2p
vice versa for Na
Pretty
Crude
r2Znao
1s = K e-/2
(subtle)
1s
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Self-Consistent Field (SCF)1. Find approximate orbitals for all electrons
(e.g. using Zeff)
2. Calculate potential from nuclei and. .. fixed clouds for all electrons but one.
3. Use this new potential to calculate an. an..improved orbital for that one electron.
4. Repeat steps 2 and 3 to improvethe orbital for another electron.
. . . Improve all orbitals one by one.
Quit When orbital shapes stop changing
Cycle back to improve 1st orbital further, etc. etc.
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Still Wrong!because real electronsare not fixed clouds.
They keep out of each other’s way by correlating their motions.
True Energy < SCF Energy
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"Correlation Energy"
Hide the residual error after full SCF calculation to the “Hartree-Fock” limit
by giving it a fancy name:
Where to get correct energy (& total electron density)?
by experiment
or by a whopping calculation:
e.g. “Configuration Interaction” (CI)
or
“Density Functional Theory” (DFT)
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If we’re really lucky, "Correlation Energy"might be Negligible.
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"Non-bonded" Contacts (1-20)
+
++ ++ +C+6
-- ---
Energy Magnitudes
Should Chemists care about the error in Orbital Theory?
-2log
(Ene
rgy
Cha
nge
kca
l / m
ole
C Core (2 104)
1/2 4 Single Bonds (2 102)
He•He @ 52Å! (2 10-6)
Changes in "correlation energy"
can be ~10-15% of Bond Energy.
Orbital Theory is fine for Qualitative
Understanding of Bonding.
C "Correlation Energy" (102)
-C••
••C
12C Nucleus (2 109)Loses 0.1 amu (E = mc2)Fortunately nuclear energy
is totally unchangedduring chemistry!
0.001% change in nuclear energy would overwhelm all of Coulomb.
correlation error ≈ bond
8
0
6
2
4
~
C Atom (3 103)
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Orbitals can't be “true”for >1 electron, because of e-e repulsion
butwe'll use themto understand
bonding, structure,energy, and reactivity
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What gives Atomic Orbitals their Shape?
Potential Energyscales r
(via )
Kinetic Energy
creates nodes
4d
2s
double the nuclear charge
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If we use orbitals, how should we reckon total electron density?
Density of electron 1 = 1 2(x1,y1,z1)
Density of electron 2 = 2 2(x2,y2,z2)
Total density (x,y,z) = 1 2(x,y,z) + 2
2(x,y,z)
(Sum, not Product. Not a question of joint probability)
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How Lumpy is the N Atom?
Total = K(r2) e-
(2px)2 = K x2 e-
(2py)2 = K y2 e-
(2pz)2 = K z2 e-
Total = K(x2 + y2 + z2) e-
Spherical ![from an Organic Text]
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TFDCBC
CC
C
F
N
is roundnot clover-leafnor diamond!
C N Triple Bond
2px2 + 2py
2 depends on (x2+y2) It is thus symmetrical
about the z axis
cross section
?
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MoleculesPlum-Pudding MOs (the "United Atom" Limit)Understanding Bonds (Pairwise LCAO-MOs)
“Overlap & Energy-Match"
Atoms3-Dimensional Reality (H-like Atoms)
HybridizationOrbitals for Many-Electron Atoms (Wrong!)Recovering from the Orbital Approximation
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Ways of Looking at an Elephant
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Set of atoms
Atoms withsmall bonding
distortion
Single “United Atom”
Ways of Looking at a Molecule(or a Molecular Orbital)
e-densitycontours
of H2
Whichcontourshould
we use?
Moleculefrom setof atoms
Moleculeas one atom
distorted by afragmented nucleusNuclei embedded in
a cloud of electronsdispersed and “noded”
by kinetic energy
J. J. Thomson'sPlum Pudding!
(backwards)
Moleculeas atoms
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How the PlumsDistort
Electronic Puddings
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Methane&
Ammonia
Spartan 6-31G* calculates good SCF MOs(on my laptop!)
We want to understand them visually.
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4 Pairs of Valence Electrons
H
C HHH
NH H
H
Compare MOsto AOs of Ne
(4 electron pairs with n=2)
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1sCH4 NH3
"Core" OrbitalsLike 1s of C/NTightly Held
Little Distortion
Contour Level 0.001 e/Å3
We'll focus onValenceOrbitals
Boring!
..
.. ......
.... ..
8 valence e-
4 MOs8 valence e-
4 MOs
ene
rgy
Three “degenerate”Molecular Orbitals
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2s.... ....
...... ..
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2s.... ....
...... ..
“Spherical”node
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2px
.. ......
..
CH4 NH3
.. ....
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CH4 NH32py
.. ..
......
CH4 NH3
.. ....
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CH4 NH32py
.. ..
......
CH4 NH3
.. ....
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CH4 NH32pz
HOMOLewis's "unshared pair"
......
CH4 NH3
.... .... ..
+Unoccupied Orbitals +Unoccupied Orbitals
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CH4 NH3
3s LUMO"HUMO?"
..
.. ......
.... ..
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2sCH4 NH3
3s LUMO"HUMO"
...... ..
..
.. ....
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CH4 NH33dx2-y2
..
.. ......
.... ..
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CH4 NH33dx2-y2
...... ..
..
.. ....
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CH4 NH33dxy
..
.. ......
.... ..
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CH4 NH33dxy
...... ..
..
.. ....
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CH4
3dz2 3dz2.... ....
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End of Lecture 11Sept. 29, 2008
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).
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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