Post on 21-Dec-2015
Chem 125 Lecture 49/13/06
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.
Equilibrium vs. Resonance
A B A B
Two Real SpeciesOne Real Species
Two “Reasonable”Structural Formulas
Failure of Simplistic NotationUnusually Stable
Equilibrium vs. Resonance
H CO
O
HH C
O
O H••
••••
•• ••
••
••
••
••
H CO
O
H C
O
O
••••
•• ••
••
••
•••
•
Two Species
Two Species?
••
H C
O
O
H C
O
O
••••
•• ••
••
••
•••
•
One Nuclear Geometry!
•
•One Species!(Infrared)
LORE(Electron Paramagnetic Resonance)
O2 / O3
O•• •• •
•
• •O ••
••••O
• ••
•
EquilateralTriangle
O•
• •• •
•O•• •• •
• O•
• •• •
•O•• •• •
•
OO
••
O• •
• •
OO O
OO
O+
DoubleBond
Open
Trivalent O is positive.
Ozone
OO
O
+
_
O O
O
Ring
OO
O
+
_
Open
A Problem in4 Dimensions!(3 distances + energy)
symmetrical single minimum?
Graph Helphttp://classes.yale.edu/chem125a/125/xray/DensityMaps/3din2d.htm
(Be sure you can do the problems,but you don't have to hand them in)
O3 Ruedenberg 1997
Ring
Open
Constrained4-Dimensional
Structure-Energy
Plot
3 /
O3
MoreConstrained4-Dimensional
Structure-Energy
Plot
Distance along Structure Curve
Ene
rgy
(kca
l/m
ol)
Ring
symmetrical"resonant"OPEN
0
8
2 /
Ozone
OO
O
+
_
OO
O
+
_
Open
What of charges “predicted” by
Lewis bookkeeping?symmetrical single minimum
Calculated Suface Potential* of Open Ozone
*) Energy of a proton on the “molecular surface”
HIGH(+ 25 kcal/mole)
(-16 kcal/mole)
LOW
Lewis Dot Structures
Attempt to provide a “physical” basis for valence rules.
New: Reactivity from unshared pairs(both “hooks” from the same atom)
Convenient for electron bookkeeping(molecular charge; “formal” atomic charges)
Stability and “Resonance”?
2. Structures in which all first-row atoms have filled octets are generally important; however, resulting formal charges and electronegativity differences can make appropriate nonoctet structures comparably important.
From a good Text
“empirical rules for assessing the relative importance of the resonance structures of molecules and ions.
1. Resonance structures involve no change in the positions of nuclei; only electron distribution is involved.
3. The more important structures are those involving a minimum of charge separation, particularly among atoms of comparable electronegativity. Structures with negative charges assigned to electronegative atoms may also be important.
(our depiction of) ^
LORE: That which is learned; learning, scholarship, erudition.
Also, in recent use, applied to the body of traditional facts, anecdotes, or beliefs
relating to some particular subject (Oxford English Dictionary)
LORE
Is it True?
Force Laws
In systems governed byinverse-square force laws
there can be no local minimum (or maximum)
of potential energy.
Earnshaw's Theorem(1839)
http://www.phys.psu.edu/people/display/?person_id=202;mode=research;research_description_id=428
Visualizing Earnshaw - Coulomb's Electrostatics
Lines of ForceMagneticElectrostatic
Faraday/Davy/Phillipsyoung
Michael Faraday
Can show magnitude (as well as direction) of Force
2-D (Flatland)
force magnitude
line density
Circumference r2
Force
line density
1/r
Can show magnitude (as well as direction) of Force
3-Dimensions
Surface r2
Force
line density
1/r2
force magnitude
line density
Such Diagrams Work only for Inverse Square Forces!
A positive particle has a local maximum or minimum of energy only at the location of
another charged particle, never in free space.
A positive particle has a local maximum or minimum of energy only at the location of
another charged particle, never in free space.
In systems governed byinverse-square force laws
there can be no local minimum (or maximum)
of potential energy in free space.
Earnshaw's Theorem
The only stationary pointsare saddle points.
Levitator by Martin Simon (UCLA)
Eppur sta fermo
END