STRAIN, HYPERCONJUGATION, CONJUGATION, AND AROMATICITY by JUDY I-CHIA WU (Under

173 Pages·2011·10.07 MB·English

QUANTIFICATION OF VIRTUAL CHEMICAL PROPERTIES:
STRAIN, HYPERCONJUGATION, CONJUGATION, AND AROMATICITY
by
JUDY I-CHIA WU
(Under the Direction of Paul von Ragué Schleyer)
ABSTRACT
Highly important chemical concepts, like strain, hyperconjugation, conjugation, and aromaticity,
are widely used to interpret the behavior of organic molecules, but are virtual (i.e., not directly
measurable). This dissertation focuses on validating reliable quantum mechanical approaches for
quantitative estimates of these virtual chemical properties and applying them to solve significant
chemical problems. We are concerned with the simplest iconic organic molecules: cyclopropane,
benzene, cyclobutadiene, cyclooctatetraene, and the polybenzenoid hydrocarbons.
Cyclopropane’s unexpectedly low ring strain (almost the same as that of cyclobutane) is due to
substantial CCC geminal delocalization. Cyclooctatetraene (COT) is not representative of an
unconjugated cyclic polyene. Instead, double hyperconjugation stabilizes the tub-shaped COT
(D ), as well as other non-planar conjugated systems, and compensates for their diminished π
2d
conjugation. On the other hand, planar D COT is net stabilized by π conjugation and only
4h
weakly anti-aromatic destabilized. Planar [4]annulenes in general are not very destabilized
antiaromatically. Even cyclobutadiene is only modestly destabilized by antiaromaticity; its high
heat of formation is mainly due to significant angle strain and Pauli repulsion between the pairs
of C=C π bonds. Large 4n π electron polycyclic benzenoids and the higher dihydrodiazaacenes
benefit from substantial π conjugation and can even display aromatic stabilization energies
(ASE), of equal or greater magnitude, than their 4n+2 π electron analogs. Aromaticity is very
robust towards electronic and geometric perturbtations. Hence, perfluorobenzene (C F ) is as
6 6
aromatic as benzene, while perfluorocyclobutadiene (C F ) has only reduced antiaromaticity
4 4
compared to cyclobutadiene, due to the twisting of its π system. This work further shows the
effectiveness of the nucleus independent chemical shifts (NICS) as a “local” probe of aromaticity
for substituted and fused-ring aromatic systems. Our computational findings provide valuable
insight to the development of fundamental organic concepts.

INDEX WORDS: computational chemistry, hyperconjugation, electron delocalization,
resonance energy, aromatic stabilization energy (ASE), substituent effects,
nucleus independent chemical shifts (NICS), block-localized wave
function (BLW)
QUANTIFICATION OF VIRTUAL CHEMICAL PROPERTIES:
STRAIN, CONJUGATION, HYPERCONJUGATION, AND AROMATICITY

by

JUDY I-CHIA WU
B.S., Tung-Hai University, Taichung, Taiwan, 2004

A Dissertation Submitted to the Graduate Faculty of The University of Georgia in Partial
Fulfillment of the Requirements for the Degree

DOCTOR OF PHILISOPHY

ATHENS, GEORGIA
2011
© 2011
Judy I-Chia Wu
All Rights Reserved
QUANTIFICATION OF VIRTUAL CHEMICAL PROPERTIES:
STRAIN, CONJUGATION, HYPERCONJUGATION, AND AROMATICITY

by

JUDY I-CHIA WU

Major Professor: Paul von Ragué Schleyer
Committee: Henry F. Schaefer III
Robert J. Woods

Electronic Version Approved:

Maureen Grasso
Dean of the Graduate School
The University of Georgia
May 2011
DEDICATION
To Keigo, Paul, my Parents,
and in memory of Gnocchi

“It has often been said
there’s so much to be read,
you never can cram
all those words in your head.

So the writer who breeds
more words than he needs
is making a chore
for the reader who reads.

That's why my belief is
the briefer the brief is,
the greater the sigh
of the reader's relief is.

And that's why your books
have such power and strength.
You publish with shorth!
(Shorth is better than length.)”
-Theodor Seuss Giesel
a.k.a. “Dr. Seuss”
iv
“All Hyperconjugation, All the Time.”
v
ACKNOWLEDGEMENTS
My deepest appreciation goes to many people who have helped me greatly in personal and
scientific growth during the past six years. Most of all, I am in debt to Prof. Schleyer for the fine
education he has given me. I thank him especially for changing my mind about chemistry. His
nurturing of a good appetite for chemical discoveries as well as for elegant scientific writing has
made a world of difference for me.
I thank my parents for the love, time, “goodie boxes,” and resources they have devoted to
supporting my ups and downs all the time. Special thanks go to my Dad, whose guidance and
3M tips has been an essential part to my accomplishment. I also thank Keigo and Chad for
wonderful friendship and many cheerful scientific and philosophical conversations. I’ve enjoyed
so much learning and exploring chemistry (and Germany) together with them. I thank Keigo
especially for being a great companion through many difficult times, for sharing my excitement
for every new discovery, and for being a great mentor in helping me improve many
presentations. I am grateful to Prof. Schaefer for his cheerful encouragements and for the
valuable opportunities of meeting and observing many distinguished scientists at the CCQC.
Clémence has been a wonderful role model. I would like to thank her for the time she has spent
guiding me in many ways when I first joined the Schleyer group. I also thank Prof. Mo for many
friendly and insightful discussions. Finally, I want to acknowledge Nattoh, Gnocchi, and Dexter
for being my Athens family. They do a (almost) perfect job of distracting me from work when
stubborn problems and a stubborn mind prevent me from going to bed on time.

vi
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS...........................................................................................................vi
LIST OF TABLES.........................................................................................................................xi
LIST OF FIGURES.....................................................................................................................xiii
CHAPTER
1 INTRODUCTION.........................................................................................................1
2 WHY CYCLOOCTATETRAENE IS HIGHLY STABILIZED: THE IMPORTANCE
OF “TWO-FOLD” (DOUBLE) HYPERCONJUGATION...........................................5
2.1 ABSTRACT.......................................................................................................6
2.2 INTRODUCTION.............................................................................................7
2.3 METHODS......................................................................................................17
2.4 ELECTRON DELOCALIZATION IN D AND D
2D 4H
CYCLOOCTATETRAENE..................................................................................20
2.5 THE INVERSION BARRIER OF CYCLOOCTATETRAENE.....................22
2.6 ANTIAROMATICITY IN D CYCLOOCTATETRAENE.........................24
4H
2.7 1,3-BUTADIENE ROTATION.......................................................................26
2.8 OTHER MOLECULES WITH “TWO-FOLD” (DOUBLE)
HYPERCONJUGATION......................................................................................32
2.9 CONCLUSIONS..............................................................................................36
2.10 REFERENCES..............................................................................................39
vii
3 IS CYCLOBUTADIENE REALLY DESTABILIZED BY ANTIAROMAITICITY?
AN ALTERNATIVE EXPLANATION BASED ON SIMPLE STRAIN
CONSIDERATIONS...................................................................................................47
3.1 ABSTRACT.....................................................................................................48
3.2 INTRODUCTION...........................................................................................48
3.3 EVIDENCE FOR HIGH RING STRAIN IN THE SIGMA-FRAMEWORK
OF CBD.................................................................................................................52
3.4 BLOCK-LOCALIZED WAVEFUNCTION ANALYSES.............................55
3.5 DISCUSSIONS................................................................................................56
3.6 CONCLUSIONS..............................................................................................58
3.7 REFERENCES................................................................................................59
4 WHY IS THE STRAIN ENERGY OF CYCLOPROPANE SO LOW?.....................61
4.1 ABSTRACT.....................................................................................................62
4.2 INTRODUCTION...........................................................................................62
4.3 METHODS......................................................................................................70
4.4 THE MISSING STRAIN IN CYCLOPROPANE...........................................71
4.5 GEMINAL DELOCALIZATION AND SUBSTITUENT EFFECTS............75
4.6 NUCLEUS INDEPENDENT CHEMICAL SHIFTS......................................79
4.7 CONCLUSIONS..............................................................................................82
4.8 REFERENCES................................................................................................83
5 THE EFFECT OF PERFLUORINATION ON THE AROMATICITY OF BENZENE
AND HETEROCYCLIC SIX MEMBERED RINGS.................................................86
5.1 ABSTRACT.....................................................................................................87
viii