Computational study of polycyclic aromatic hydrocarbons and their derivatives
EMSL Project ID
3530
Abstract
Polycyclic aromatic hydrocarbons (PAHs) constitute a large class of conjugated pi-electron systems that are key molecular species in many branches of chemistry, such as, interstellar, combustion, environmental, and materials chemistry. These species are detected in meteorites, are strong candidates for the carriers of interstellar infrared emission features and diffuse interstellar visible absorption bands, and are thought to be a major carbon reservoir in the interstellar medium. They are also primary intermediate species that form in combustion processes and are the most ubiquitous environmental contaminants from natural and man-made sources with varied mutagenic and carcinogenic activities. It has been proposed that PAHs are the precursors of flame-produced soot and fullerenes. We have recently shown that time-dependent density functional theory (TDDFT) was a tool particularly well-suited for the interpretation of the electronic absorption spectra of PAHs and their radical ions and the photoelectron spectra of neutral PAHs. Remarkably, for the PAH radical cations of small to intermediate sizes that we studied previously, the excitation energies obtained from TDDFT were typically within a few tenths of an electron volt of the experimental data. Unlike multi-reference methods in which a selected group of orbitals is treated differently from the rest, TDDFT provides an unbiased description of both the pi*<-pi and pi*<-sigma excitation processes. In this study, we propose a computational study of the ground and excited states of PAHs (primarily their radical ions) and their derivatives by TDDFT and related methods: (1) Theoretical interpretation and prediction of vibrational, photoelectron, and electronic absorption spectra of selected important PAH neutral and radical ions and their hydrogen or proton abstracted species by TDDFT. As an initial example, we propose to study benzofluorene radical cation, for which a new set of IR and electronic absorption data in the Ar matrix has been detected recently by the co-authors. (2) Theoretical study of light-induced hydrogen abstraction process of PAHs. Previously, we have found that several common PAH radicals undergo a series of hydrogen abstractions upon irradiation of visible lights and transform to a variety of exotic carbon-rich PAHs or pure carbon compounds. We plan to study the energetics of hydrogen abstraction in an excited state of PAHs by TDDFT or other excited state methods. (3) Accurate theoretical calculations of electronic absorptions of linear carbon chain cations or anions. The series of light-induced hydrogen abstraction of PAHs lead to pure carbon compounds of various forms. Among them are the cations and anions of linear carbon chains, and electronic absorption spectra of the chains including the one that consists of as many as 29 carbon atoms have been measured and a systematic chain-length dependence of the spectra has been obtained experimentally by a team of JS and MV. Here, we propose to use TDDFT and more accurate excited state methods such as CIS(D) or EOM-CCSD to calculate the absorption band positions and explain the chain length dependence.
Project Details
Project type
Exploratory Research
Start Date
2003-05-01
End Date
2004-05-03
Status
Closed
Released Data Link
Team
Principal Investigator
Related Publications
Banisaukas J, J Szczepanski, M Vala, and S Hirata. 2004. "Vibrational and Electronic Absorption Spectroscopy of 2,3-Benzofluorene and its Cation. ." Journal of Physical Chemistry A 108(17):3713 -3722.
Hirata S, C Zhan, E Apra, TL Windus, and DA Dixon. 2003. "A New, Self-Contained Asymptotic Correction Scheme To Exchange-Correlation Potentials For Time-Dependent Density Functional Theory." Journal of Physical Chemistry A 107:10154-10158.
Hirata S, MP Head-Gordon, J Szczepanski, and M Vala. 2003. "Time-dependent density functional study of the electronic excited states of polycyclic aromatic hydrocarbon radical ions." Journal of Physical Chemistry A 107(24):4940-4951.