Chem 125 Lecture 49/13/06

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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