Negative ions, molecular electron affinity and orbital structure of cata-condensed polycyclic aromatic hydrocarbons

RATIONALE

Polycyclic aromatic hydrocarbons are molecules of ecological, astrochemical significance that find practical applications in organic electronics, photonics and the chemical synthesis of novel materials. The utility of these molecules often implies the occurrence of their ionized forms. Studies in the gas phase of elementary processes of energy-controlled interaction of molecules with low-energy electrons shed light on the mechanisms of transient negative ion formation and evolution.

METHODS

Experiments with the individual compounds representing homologous and/or isomeric series of cata-condensed polyaromatic hydrocarbons were carried out by means of negative ion mass spectrometry in the resonant electron capture mode. Literature data obtained by complementary techniques and theoretical quantum chemical methods (ab initio and density functional theory (DFT)) were invoked to treat the experimental observations.

RESULTS

Most polycyclic aromatic hydrocarbon (PAH) molecules form long-lived molecular negative ions when exposed to free electrons of thermal or epi-thermal energy, and no fragmentation is observed up to ca 5 eV. The lifetimes of such ions with respect to the spontaneous loss of extra-electron vary from tens of microseconds for angular and branched PAH molecules to milliseconds for linear ones, and correlate with the adiabatic electron affinity (EA) of molecules. Detailed analysis of the electronic (orbital) structure of the molecules made it possible to rationalize the relatively low EAs of angular and branched PAH compared with those of linear ones.

CONCLUSIONS

The obtained results contribute to the field of electron-molecule interactions and may be of importance for the better comprehension of the functioning of organic electronics, for the synthesis of relevant novel materials, and the development of efficient analytical methods capable of discriminating structural isomers.