Theory and applications of time dependent density functional theory

The diffuse interstellar bands are spectral features in the visible region that are observed from the interstellar medium. Various hypotheses have been made to account for the atomic and/or molecular species responsible for these visible absorptions, however no definitive assignment has been made as of yet. Polycyclic aromatic hydrocarbons are believed to be ubiquitous throughout the interstellar medium and are very plausible candidates for diffuse interstellar band carriers. Theoretical absorption spectra can also provide valuable information in the identification diffuse interstellar band carriers, particularly in predicting trends and characterizing transitions. The calculation of theoretical electronic absorption spectra for large molecules is possible with time dependent density functional theory. This theory is an excellent compromise between accuracy and cost of the calculation, and is used throughout all of the studies presented here.; In Chapter 2, the absorption spectra of a series of peroxy radicals is studied. These species are not directly related to the diffuse interstellar bands. Peroxy radicals are important in tropospheric and combustion chemistry, however the interesting aspect of these species that is addressed in this Chapter, is the existence of a visible absorption for the phenyl peroxy radical, that does not exist for several smaller alkyl peroxy radicals. Time dependent density functional theory and orbital mixing analyses are successful in determining the source of the visible absorption.; Chapters 3--6 deal directly with polycyclic aromatic hydrocarbons, and in some cases with their relation to the diffuse interstellar bands. The electronic absorption spectra of various classes of polycyclic aromatic hydrocarbons are analyzed. Chapter 3 focuses on closed-shell cations, a type of polycyclic aromatic hydrocarbon that had not previously been studied as possible diffuse interstellar band carriers. These first calculated results indicate that the spectra of closed-shell cations resemble those of open-shell cations, and should be investigated experimentally. Chapter 4 presents a series of oligorylenes in neutral, radical cation and radical anion charge states. Time dependent density functional theory results compare well with experimental absorption spectra. Furthermore, the trend in oscillator strength in the series is explained by transition moment orientation and growth. Chapter 5 explores a series of medium to large pericondensed radical cations. These species are stable and exhibit visible electronic absorptions. However, from the knowledge gained in Chapter 4, it appears that the pericondensed radical cations do not have desirable trends in oscillator strength in terms of diffuse interstellar band carrier candidates. Chapter 6 is the last Chapter focused on polycyclic aromatic hydrocarbons. This study investigates a series dibenzopolyacenes and identifies the origin of a low-lying electronic absorption that is observed in the infrared, that is not present in the polyacenes. Orbital mixing between polyacene and 'dibenzo' substituents produce the unusual infrared region electronic feature.; The final Chapter (Chapter 7) addresses shortcomings of time dependent density functional theory that are important in charge-transfer states. It is determined that the local nature of present-day density functional theory exchange-correlation potentials makes the accurate description of charge-transfer states impossible.