Experimental and theoretical investigations of the photochemistry of styrene and the creation and characterisation of shaped femtosecond ultraviolet laser pulses

This thesis is composed of three projects that are linked by the theme of light-molecule interactions. These are covered separately in Chapters 2, 3, and 4. In Chapter 1 some background to the thesis is described so that the various links between the science in the different chapters, and the motivation for the project as a whole are explained. Elements of photochemistry, both experimental and theoretical, are described in this chapter and some material about the most important experimental tools used in this work, ultrafast lasers, is covered, as well as the methodology of time-resolved spectroscopies, and timeresolved photoelectron spectroscopy in particular; the field of laser control of chemistry is also briefly reviewed. Chapter 2 is an account of the design and use of a UV pulse shaper and characterisation setup. This is an applied optics experiment, whose application is to the control of photochemical reactions with specifically shaped ultrafast laser light. Several demonstrations of the pulse shaping capacity of this new experiment are presented. In Chapter 3, calculation of the excited electronic states of the molecule styrene is described. This project is a computational study of the electronic spectroscopy and ionisation of the styrene molecule. In Chapter 4, the direct observation of internal conversion in styrene using time-resolved photoelectron spectroscopy is reported. This is an experimental laser spectroscopy project in which some of the results from the computations in the theory project, Chapter 3, will be used to analyse the experimental spectra. Chapter 5 summarises the conclusions drawn from Chapters 2, 3 and 4 and provides an outlook for future research based on the work in this thesis. Throughout this thesis, but more particularly in Chapters 1 and 2, there is quite a large volume of literature review and background material. This content reflects the personal perspective from which the thesis was approached. Much of the field of ultrafast optics and spectroscopy was entirely new to the writer at the outset of the PhD programme, and most of the review-based writing about these topics found here was originally written early on in the PhD project, as a means of helping to bridge the gap between work on optical experiment design and an undergraduate training in chemistry

Isomer-specific product detection of gas-phase xylyl radical rearrangement and decomposition using VUV synchrotron photoionization

Xylyl radicals are intermediates in combustion processes since their parent molecules, xylenes, are present as fuel additives. In this study we report on the photoelectron spectra of the three isomeric xylyl radicals and the subsequent decomposition reactions of the o-xylyl radical, generated in a tubular reactor and probed by mass selected threshold photoelectron spectroscopy and VUV synchrotron radiation. Franck-Condon simulations are applied to augment the assignment of elusive species. Below 1000 K, o-xylyl radicals decompose by hydrogen atom loss to form closed-shell o-xylylene, which equilibrates with benzocyclobutene. At higher temperatures relevant to combustion engines, o-xylylene generates styrene in a multistep rearrangement, whereas the p-xylylene isomer is thermally stable, a key point of difference in the combustion of these two isomeric fuels. Another striking result is that all three xylyl isomers can generate p-xylylene upon decomposition. In addition to C8H8 isomers, phenylacetylene and traces of benzocyclobutadiene are observed and identified as further reaction products of o-xylylene, while there is also some preliminary evidence for benzene and benzyne formation. The experimental results reported here are complemented by a comprehensive theoretical C8H8 potential energy surface, which together with the spectroscopic assignments can explain the complex high-temperature chemistry of o-xylyl radicals

Conformations And Relative Stabilities Of The Cis And Trans Isomers In A Series Of Isolated N-phenylamides

The gas-phase conformations of a series of isolated N-phenylamides have been determined from vibrationally resolved electronic spectra obtained by resonant two-photon ionization in a supersonic jet expansion. Both the cis and trans isomers of formanilide were identified, with the cis isomer in 6.5% abundance. The spectral features displayed by this isomer are consistent with a nonplanar geometry which undergoes a large change in the phenyl torsional angle following electronic excitation. The more abundant trans isomer of formanilide adopts a planar structure and is stabilized by 2.5 kcal/mol with respect to the cis isomer. In the excited electronic state the relative stabilities of the two isomers are reversed. Acetanilide, in contrast, is found exclusively as the trans isomer, also having a planar structure. N-Methyl substitution causes a reversal of the relative isomer stabilities found in formanilide and leads to an isomer distribution consisting of approximately 90% E and 10% Z in N-methylformanilide. These experimental observations are compared to previous condensed phase structural determinations as well as to the relative energies and structures predicted from ab initio Hartree-Fock geometry optimizations

Electronic Spectroscopy of Jet-Cooled Benzylidenecyclobutane, a Sterically Hindered Styrene

The electronic spectrum of the styrene derivative, benzylidenecyclobutane, seeded in a supersonic jet expansion has been recorded using resonantly enhanced two-photon ionization spectroscopy. The main vibronic features in the spectrum are associated with a low frequency progression assigned to the torsional motion of the phenyl ring. Analysis of the observed torsional levels reveals an excited state potential energy surface characteristic of a planar equilibrium geometry which undergoes large amplitude motion and a ground state surface having a minimum at a torsional angle of 25° between the phenyl and vinyl groups. Ab initio calculations of the ground state torsional potential surface predict a minimum in the range of 28°-26°, depending on the size of the basis set. In these structures the cyclobutane ring adopts a puckering angle between 17° and 19°. Deuterated isotopomers have also been synthesized and their corresponding photoionization spectra analyzed to reveal the mixing between the torsion and other low frequency modes such as cyclobutane ring puckering. The extent of this mixing is found to be sensitive to the sites of deuteration on the molecule. © 1996 American Institute of Physics

Electronic Spectroscopy of Jet-Cooled Benzylidenecyclobutane, a Sterically Hindered Styrene

The electronic spectrum of the styrene derivative, benzylidenecyclobutane, seeded in a supersonic jet expansion has been recorded using resonantly enhanced two-photon ionization spectroscopy. The main vibronic features in the spectrum are associated with a low frequency progression assigned to the torsional motion of the phenyl ring. Analysis of the observed torsional levels reveals an excited state potential energy surface characteristic of a planar equilibrium geometry which undergoes large amplitude motion and a ground state surface having a minimum at a torsional angle of 25° between the phenyl and vinyl groups. Ab initio calculations of the ground state torsional potential surface predict a minimum in the range of 28°-26°, depending on the size of the basis set. In these structures the cyclobutane ring adopts a puckering angle between 17° and 19°. Deuterated isotopomers have also been synthesized and their corresponding photoionization spectra analyzed to reveal the mixing between the torsion and other low frequency modes such as cyclobutane ring puckering. The extent of this mixing is found to be sensitive to the sites of deuteration on the molecule. © 1996 American Institute of Physics

Excited-state non-radiative decay in stilbenoid compounds:an ab initio quantum-chemistry study on size and substituent effects

In the framework of optoelectronic luminescent materials, non-radiative decay mechanisms are relevant to interpret efficiency losses. These radiationless processes are herein studied theoretically for a series of stilbenoid derivatives, including distyrylbenzene (DSB) and cyano-substituted distyrylbenzene (DCS) molecules in vacuo. Given the difficulties of excited-state reaction path determinations, a simplified computational strategy is defined based on the exploration of the potential energy surfaces (PES) along the elongation, twisting, and pyramidalization of the vinyl bonds. For such exploration, density functional theory (DFT), time-dependent (TD)DFT, and complete-active-space self-consistent field/complete-active-space second-order perturbation theory (CASSCF/CASPT2) are combined. The strategy is firstly benchmarked for ethene, styrene, and stilbene; next it is applied to DSB and representative DCS molecules. Two energy descriptors are derived from the approximated PES, the Franck-Condon energy and the energy gap at the elongated, twisted, and pyramidalized structures. These energy descriptors correlate fairly well with the non-radiative decay rates, which validates our computational strategy. Ultimately, this strategy may be applied to predict the luminescence behavior in related compounds

The Investigation of the Chlorine Initiated Oxidation of 2-Phenylethanol and Stability of Superalkali Lithium Substituted Silyls.

This thesis investigates the combustion potential of 2-phenylethanol and the superalkali properties of small lithium substituted silicon compounds. All combustion experiments were performed at the Advanced Light Source of Lawrence Berkeley National Laboratory at the Chemical Dynamics Beamline 9.0.2. The chlorine initiated oxidation of 2PE was investigated at 298 and 550 K using a multiplex photoionization mass spectrometer, coupled with the tunable vacuum ultraviolet radiation. Reaction products were identified using kinetic time traces and photoionization spectra. Additionally, the stability of small superalkali silicon-lithium compounds has also been investigated. All structures and energetics were calculated using the CBS-QB3 composite method. The first chapter of this thesis discusses atmospheric pollution, engine technology, biofuels and other alternative energy sources. The ALS, the experimental apparatus and their components are explained throughout Chapter 2. Chapter 3 explains the theory behind the computational methodology, as well as how to analyze the results obtained from the experimental apparatus. Chapter 4 evaluates the chlorine initiated oxidation of 2-phenylethanol. Chapter 5 investigates the superalkali properties of small silicon-lithium compounds

Structure and Dynamics by Experiment and Theory: Concerted Applications of Gas Electron Diffraction and Computational Chemistry

To plan and prepare the strongest research proposals for time-resolved gas electron diffraction (TRGED) experiments, the author has launched and overseen the development of two new research programmes in the Wann Electron Diffraction Group. A time-averaged gas electron diffraction (GED) programme has seen the technique re-established in the UK following the relocation, recommission, and modernisation of a 1960s gas electron diffractometer. Two case studies – a) 4-(dimethylamino)benzonitrile, and b) tinII bis(trifluoroacetate), ditinII μ-oxy-bis-μ-trifluoroacetate, and tinIV tetrakis(trifluoroacetate) – highlight the range of chemical samples that are accessible to study using the upgraded gas electron diffractometer. A computational chemistry programme has seen trajectory surface-hopping dynamics (TSHD) introduced to the Wann Electron Diffraction Group, delivering a paradigm shift in the ability of the research group to plan and interpret TRGED experiments. Parallel Python code has been developed to simulate TRGED data and benchmarked with up to 64 CPU cores as part of this programme. High performance is achieved in the strong and weak parallel scaling regimes. The interplay between the two programmes is illustrated in three case studies: the photolysis of 1,2-diiodotetrafluoroethane, the photofission of the disulfide bond in 1,2-dithiane, and the photoisomerisation of E-cinnamonitrile. The photolysis of 1,2-diiodotetrafluoroethane is found to take place on the triplet excited-state manifold, and statistical analysis has revealed that secondary dissociation of I• from the primary photolysis product is more likely following primary photolysis of the antiperiplanar (as opposed to the synperiplanar) isomer of 1,2-diiodotetrafluoroethane. A transient bridged intermediate has been characterised for the first time at the intersection of the D1 and D0 states; the intermediate may appear in less than 100 fs post-photolysis. The photofission of the disulfide bond in 1,2-dithiane is found to trigger a classically-intuitive “Molecular Clackers” mechanism that couples the S1 and S0 states, challenging contemporary understanding of the origin of the photostability of 1,2-dithiane. The “Molecular Clackers” mechanism drives periodic collisions between the termini of a transient thiyl biradical that can result in S0 ← S1 internal conversion and the permanent recoupling of the termini, repairing the broken disulfide bond on the picosecond timescale. The photoisomerisation of E-cinnamonitrile is revealed to be wavelength-dependent, and several key S0 ← S1 internal conversion pathways have been characterised for the first time

Jet-Cooled Spectroscopic Characterization of Anethole (methoxy-4-(prop-1-enyl)benzene), a Natual Styrene Derivative

The molecular structure of anethole (methoxy-4-(prop-l-enyl)benzene), was investigated using Jet-Cooled UV spectroscopy. A laser-induced fluorescence spectrum was obtained of the S₁-S₀ transition. Two 0⁰₀ transitions were observed in the LIF spectrum, separated by 69 cm⁻¹, and were assigned to the syn and anti conformers of anethole. Single vibronic level fluorescence spectra were obtained for both of the origin transitions. Bands of the S|VLF spectrum of each conformer were assigned by comparison with theoretical calculations at the DFT/B3LYP, 6-311G++(d,p) level of theory, as well as experimental information from similar molecules. In order to assign the LIF spectrum further, SVLF spectra were obtained of many of the LIF transitions, and assigned where possible. SVLF transitions of particular importance are the 63⁰₂ and the 39⁰₁ which appear reliably in almost all SVLF spectra, but at slightly different frequencies for each conformer, allowing the assignment of SVLF spectra to a specific conformation of anethole. LIF and SVLF data indicated the possibility of water-anethole van der Waals clusters, which were confirmed by adding water to the jet. Additionally, we performed potential energy scans of the vinyl and methoxy rotations of anethole, and fit these scans in order to determine parameters for anharmonicity and the barrier to rotation. By comparing to experimental fits in the literature, we determined that MP2 calculations predicted the barrier to rotation best, but HF calculations did a better job of predicting the anharmonicity. Opportunities for future work include the modeling of the potential using experimental data, further investigation of anethole-water clusters, finishing the assignment of the LIF and SVLF, and possibly investigating the spectroscopy of cis-anethole