Growth of unsaturated, cyclic, and polycyclic aromatic hydrocarbons: Reactions under the conditions of the interstellar medium

Hydrocarbons, in particular polycyclic aromatic hydrocarbons (PAHs), have been long discussed to be carriers of interstellar infrared (IR) emission and ultraviolet (UV) absorption features. Yet, their origin in dense phases of the interstellar medium (ISM), such as molecular clouds, remains unclear. In this work, growth mechanisms based on ion-molecule reactions between cationic PAHs/hydrocarbons and methyne (CH) were investigated. The reaction type and the precursor were derived and selected from known chemical and physical properties of the ISM. These chemical reactions were characterised by calculating branching ratios (based on cross sections) and capture rate coefficients, minimum reaction paths, reaction enthalpies, thermal equilibrium constants, and microcanonic isomerisation and radiative deactivation rate coefficients. In order to cope with the variety of reaction parameters, a hierarchic workflow scheme was set up. First, the reaction potential energy surface was sampled by molecular dynamics simulations. Then, minimum energy paths of the most probable reaction channels were investigated. Finally, molecular and kinetic properties of stationary points were calculated. The quantum chemical level of theory was increased at each step from DFTB (tight-binding density-functional), to DFT, and finally to post-Hartree-Fock methods. Results on CH based hydrocarbon growth showed the transition from non-cyclic hydrocarbons to cyclic and aromatic structures and from cyclic to polycyclic aromatic hydrocarbons. Additionally, the reactive collisions between hydrocarbons and CH were found to produce sufficient energy for isomerisation and fragmentation processes even at ultra low temperatures. In all, the results indicate that methyne might be a proper precursor for the formation of large interstellar PAHs.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Technische Universität Dresden

Year

2008

Author dc:creator

• Barthel, Robert

Contributors dc:contributor

• Seifert, Gotthard
• Schreiber, Michael
• Joblin, Christine

• astrochemical reactions
• ion-molecule reactions
• methyne
• methylidyne
• unsaturated hydrocarbons
• PAHs
• minimum energy path
• molecular dynamics
• transition states
• rate coefficients
• reaction cross sections
• reaction enthalpies
• nudged elastic band method,
• astrochemische Reaktionen
• Ion-Molekül-Reaktionen
• ungesättigte Kohlenwasserstoffe
• Minimumenergiepfad
• Molekulardynamik
• Übergangszustände
• Ratenkoeffizient
• Reaktionsquerschnitt
• Reaktionsenthalpie
• Nudged-Elastic-Band-Methode
• DFT. DFTB
• Post-Har