Electronic Spectra of Aromatic Hydrocarbons: A Deductive Approach to the Diffuse Interstellar Bands

219 Pages·2015·24.81 MB·English

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The University of Sydney
Electronic Spectra of Aromatic
Hydrocarbons: A Deductive
Approach to the Diffuse Interstellar
Bands
Author: Gerard Dean Supervisor: Prof. Timothy
O’Connor W. Schmidt
A thesis submitted in fulfilment of the requirements
for the degree of Doctor of Philosophy
at
The Faculty of Science
of
The University of Sydney
November 9, 2015
Declaration of Authorship
I, Gerard D. O’Connor, declare that this thesis titled, ’Electronic Spectra of Aromatic
Hydrocarbons: A Deductive Approach to the Diffuse Interstellar Bands’ and the work
presented in it are my own. I confirm that:
This work was done wholly while in candidature for a research degree at this Univer-
(cid:4)
sity.
No part of this thesis has previously been submitted for a degree or any other quali-
(cid:4)
fication at this University or any other institution.
WhereIhaveconsultedthepublishedworkofothers, thisisalwaysclearlyattributed.
(cid:4)
Where I have quoted from the work of others, the source is always given. With the
(cid:4)
exception of such quotations, this thesis is entirely my own work.
Where the thesis is based on work done by myself jointly with others, I have made
(cid:4)
clear exactly what was done by others and what I have contributed myself.
Editorial advice, proof reading and consolation was provided by my research super-
(cid:4)
visor, Prof. Timothy W. Schmidt. No other editorial assistance was obtained in the
preparation of this thesis.
Gerard Dean O’Connor
November 9, 2015
i
“Once you eliminate the impossible, whatever remains, no matter how improbable, must be
the truth”
Sir Arthur Conan Doyle
“Because it is there”
George Mallory
Abstract
Electronic Spectra of Aromatic Hydrocarbons: A Deductive Approach to the
Diffuse Interstellar Bands
by Gerard D. O’Connor
The diffuse interstellar bands are a series of more than 500 interstellar absorption features,
the carriers of which have remained unidentified since 1919.
In order to determine which aromatic chemical species are likely to be carriers of the dif-
fuse interstellar bands, trends observed in the spectroscopic features of polycyclic aromatic
hydrocarbon (PAH) species with chromophores ranging from 6 to 17 carbon atoms are con-
sidered. These trends are explored for PAHs with differing charge states and multiplicities,
for multiple electronic transitions.
Previously unreported electronic transitions of the neutral radicals 1-naphthylmethyl, 2-
napthylmethyl,9-methylanthraceneand1-pyrenylmethylaswellasthe9-methylanthracenium+
and phenalenium+ radical cations and the closed-shell neutral molecule 1H-phenalene were
recorded. The consideration of these experimental spectra in light of theoretical results in a
synergistic understanding of the electronic and spectroscopic properties of these molecules
that experiments or theory in isolation cannot achieve.
The D ← D transitions of small PAH resonance stabilized radicals (RSRs) are shown
1 0
to be unlikely to be responsible for the DIBs, due to their weak transition intensities and
pronounced vibronic structure. The spectroscopic properties of larger PAH RSRs were
empirically extrapolated from experimental and computational trends, showing that these
larger molecules are also unlikely to be responsible for the diffuse interstellar bands. The
vibronic structures of these molecules were assigned. These assignments were guided by
TD-B3LYP methods as well as Franck-Condon and Herzberg-Teller simulations.
As the D ← D transitions of PAH RSRs are weak, techniques were developed to obtain
1 0
the gas-phase spectra of more intense electronic transitions to higher excited-states by
double-resonance spectroscopy. Several strong transitions were observed, which were then
assigned using ab-initio and TD-DFT computational methods (including TD-B3LYP, TD-
CAM-B3LYP and TD-M06). These transitions are also dismissed as potential carriers of
the DIBs due to the extremely broad spectral features they produced.
The spectra of PAH radical cations were observed through the resonant photodissociation
of weakly bound van der Waals complexes. The recorded spectra covered a range from
the mid infrared to the ultraviolet, with one such spectrum showing features of seven
distinct electronic transitions. These electronic transitions were assigned based with the
assistance of X-MCQDPT2 calculations. The vibronic spectra of the D ← D transition
1 0
of the 9-methylanthracene+ radical cation was also assigned, and the intensity pattern was
modelled as a combined Franck-Condon pseudo-Jahn-Teller system.
Molecules based on the phenalenyl motif were examined spectroscopically and theoreti-
cally, giving an overview of the effects of symmetry, multiplicity, charge and heteroatomic
substitution on the spectroscopic properties of aromatic molecules.
As a result of the work presented, several classes of PAH can now be dismissed as possible
carriers of the DIBs. Further avenues of research have been suggested. The role of this
work as part of the ongoing search for the carriers of the DIBs will be discussed.
Acknowledgements
First and foremost, I would like to thank my supervisor, Prof. Timothy Schmidt. The
first time I met Tim, he went above-and-beyond helping me negotiate the bureaucratic
minefield required for me to re-enter university as an Honours student. Most people would
react differently when a bus-driver (in uniform) arrives at their office door with a less-than-
stellar set of academic transcripts. Since then, Tim’s energy and enthusiasm have been
infectious, and his patience and wisdom have been greatly appreciated (particularly in the
last few months). I’d also like to thank my associate supervisor Prof. Scott Kable. He was
a helpful source of advice, but I would mainly like to thank him for his role in bringing
together the people and equipment that made for such a productive, and entertaining,
working environment.
To all the people from the group formerly known as the Laser Spectroscopy Group, I thank
you for all of your comradeship, support, interest and senses of humour. I especially want
to thank Klaas Nauta, Tyler Troy and Nahid Chalyavi for all that they taught me, Olha
Krechkivska for assistance in the latter years of my PhD, and Callan Wilcox and Mitchell
Quinn for their friendship, calming presence and frequent assistance in the lab.
One of the things that I most enjoyed about the last few years has been the opportunity
to interact with the wider molecular spectroscopy community. I especially want to thank
Michael McCarthy and Evan Bieske for letting me loose in their labs, as well as Kyle
Crabtree, Oscar Martinez Jr, Vik Dryza and Julian Sanelli for getting me involved, and
for showing me how everything worked. I’d also like to thank John F. Stanton and Scott
Reid for all that I have learnt from them and for the many fun times. And to all those
whom I have met at conferences, I thank you. The excitement of joining this quirky little
community is what kept the fires burning on the darkest of days.
I would not have been able to continue to pursue this work without the understanding of
my (less-scientific) co-workers at Sydney Buses and FKG Nyngan and I would like to thank
all my co-workers for keeping me honest. I’d also like to thank my parents and family for
all of their encouragement, from my school days onward.
To all of my friends in Sydney, and especially those from the Engineering Revue, I thank
you. Over the past few years you have also become my family. This is particularly true
for my former house mates Paddy and Danya, and my best friends Laura and Dave, who
have all gone above-and-beyond in keeping me somewhat social and at least functionally
sane in recent years.
And to my wonderful wife Natasha, who has completely changed my life, I thank-you for
continuing to love and support me. You make me the person I want to be, without actually
trying.
And to everyone else, Thanks.
v
Contents
Declaration of Authorship i
Abstract iii
Acknowledgements v
Contents vi
List of Figures xii
List of Tables xvi
1 Introduction 1
1.1 Astronomical Chemistry and Spectroscopy . . . . . . . . . . . . . . . . . . 1
1.1.1 The Sun and Other Stars . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 The Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 The Interstellar Medium (ISM) . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1 Electronic Transitions of Interstellar Molecules. . . . . . . . . . . . 8
1.3 The Diffuse Interstellar Bands (DIBs) . . . . . . . . . . . . . . . . . . . . . 9
vi
Contents vii
1.3.1 Properties of DIB Carriers . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.2 Polycyclic Aromatic Hydrocarbon (PAH) Hypothesis . . . . . . . . 12
1.4 Focus of This Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2 Computational Methods 18
2.1 Molecular Electronic Structure Theory . . . . . . . . . . . . . . . . . . . . 19
2.1.1 The Variational principle . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.2 The Adiabatic Born-Oppenheimer Approximation . . . . . . . . . . 20
2.2 Computational Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.1 Ab-initio methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.1.1 Restricted [Open shell] Hartree Fock (RHF/ROHF) - Self
Consistent Field (SCF) Method . . . . . . . . . . . . . . . 23
2.2.1.2 Configuration Interaction (CI) . . . . . . . . . . . . . . . . 24
2.2.1.3 SecondOrderExtendedMulti-ConfigurationQuasi-Degenerate
Perturbation Theory (X-MCQDPT2) . . . . . . . . . . . . 25
2.2.2 Density Functional Theory (DFT) . . . . . . . . . . . . . . . . . . . 26
2.2.2.1 Time-Dependent Density Functional Theory (TD-DFT) . 27
2.3 Calculation of Molecular Properties . . . . . . . . . . . . . . . . . . . . . . 29
2.3.1 Optimised Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.3.2 Electronic Transition Energy and Intensity . . . . . . . . . . . . . . 29
2.3.3 Normal Mode Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.4 Harmonic Frequencies of Vibrational Normal Modes . . . . . . . . . 33
2.3.5 Vibronic Transition Intensities . . . . . . . . . . . . . . . . . . . . . 34
2.3.6 Simulating Intensities in Gaussian09 . . . . . . . . . . . . . . . . 37
2.3.6.1 1-Phenylpropargyl (1-PPr) . . . . . . . . . . . . . . . . . . 38
Contents viii
2.3.6.2 1-Naphthylmethyl (1-NpMe) . . . . . . . . . . . . . . . . . 39
2.3.6.3 2-Naphthylmethyl (2-NpMe) . . . . . . . . . . . . . . . . . 40
2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3 Experimental Techniques and Apparatus 43
3.1 Vacuum Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.1.1 Supersonic Free-Jet Expansion . . . . . . . . . . . . . . . . . . . . . 44
3.1.2 Pulsed Discharge Nozzle (PDN) . . . . . . . . . . . . . . . . . . . . 46
3.1.3 Electron Bombardment/Electrospray . . . . . . . . . . . . . . . . . 48
3.2 Jet-Cooled Action Spectroscopy Techniques . . . . . . . . . . . . . . . . . 48
3.2.1 Resonance-Enhanced Multi-Photon Ionisation (REMPI) Spectroscopy 49
3.2.2 Resonant Photodissociation Spectroscopy and Argon-Tagging . . . 54
3.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4 D ← D Transitions of Benzylic Resonance Stabilised Polycyclic Aro-
1 0
matic Hydrocarbon Radicals 57
4.1 Author’s Note . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.3 Theoretical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.4 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.4.1 Excitation Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.4.1.1 Hole-Burning . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.5 Excitation Energies (TD-DFT and CASPT2) . . . . . . . . . . . . . . . . . 61
4.6 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.6.1 9-Anthracenylmethyl (9-AnMe) Radical Spectrum . . . . . . . . . . 69
4.6.1.1 9-AnMe Spectral Assignments . . . . . . . . . . . . . . . . 69