***********************************************************
OATS-DAUT SEMINAR
***********************************************************
Speaker: Franco CATALDO (Lupi Chemical Institute, Rome, I)

Title: Astrochemical aspects of elemental carbon: chains, clusters and grains

Date: Wednesday, December 3rd, 2008
Time: 12:00
Venue: Villa Bazzoni

***********************************************************
Abstract:
Elemental carbon is ejected in the interstellar medium by late-type carbon rich stars. The rich organic chemistry occurring in the circumstellar envelope of post-AGB (asymptotic giant branch) carbon-rich giant stars is essentially due to the peculiar properties of the carbon vapour. In fact carbon vapour in the circumstellar environment forms quite easily carbon chains of acetylenic nature known as polyynes. The polyynes have been detected by radioastronomy and one of the largest organic molecule till known to exist in such environment is the monocyanopolyyne:
H-C≡C-C≡C-C≡C-C≡C-C≡C-C≡N

Monocyanopolyynes and dicyanopolyynes can be synthesized quite easily in laboratory with a process developed by the undersigned. A process mimicking the process occurring in the circumstellar envelope. In fact, monocyanopolyynes having the general formula can be synthesized together with ordinary polyynes series H-(C≡C)n-H by arcing graphite electrodes in acetonitrile or in presence of ammonia. Dicyanopolyynes N≡C-(C≡C)n-C≡N are produced almost pure by arcing graphite electrodes directly into liquid nitrogen. As previously mentioned, these molecules are present in the envelope of post-AGB carbon-rich giant stars and also in dark molecular clouds. Our work underlines the parallelism existing between the polyynes formed by laser ablation experiments of graphite targets (made by H. W. Kroto group) and those produced from the submerged carbon arc. In both cases the same products are obtained. The products consist of hydrogen-terminated polyynes if water is present, monocyanopolyynes (mixed with hydrogen-terminated polyynes) if the carbon arc is sparked in acetonitrile or ammonia and dicyanopolyynes if the arc is struck in liquid nitrogen. The mechanism of formation of polyynes in the submerged carbon arc involves essentially neutral species; similar species and pathway may occur also in the circumstellar environment where polyynes have been detected by radioastronomy. It is shown that the relative abundances of the polyynes formed in the submerged carbon arc or in a carbon arc in vacuum decrease by a factor between 3 and 5 as the chain length increases by a C2 unit. Exactly the same trend has been observed by radioastronomy both for polyynes and cyanopolyynes in the circumstellar environment around red giants and AGB (asymptotic gian branch) stars. This fact may be a simple coincidence or may suggests that the mechanism of formation of the polyynes in the carbon arc is the same occurring in the surroundings of the carbon-rich stars.
The discovery of the easy formation of polyynes molecules in laboratory conditions may explain why these molecules are so ubiquitous in the space. Polyynes are also just the precursors of more complex molecules of astrochemical interest. For instance, in a hydrogen-rich atmosphere the polyynes are converted into a mixture of polycyclic aromatic hydrocarbon (PAHs) and this has been verified experimentally. In their turn, the PAHs are just the building blocks of graphene sheets and graphene sheets tends to clump together to form carbon dust. The interstellar carbon dust may account of 1-2% of the mass of the interstellar matter. In our laboratory experiments with carbon arc, the polyynes, the PAHs and the carbon dust are formed together. By changing the conditions it is possible to exalt or suppress the formation of one or more chemical species. Additionally, also fullerenes can be produced together with carbon dust from a carbon arc. They are commonly considered as carbon clusters and their formation require special conditions like a rich helium atmosphere which ensure the optimum cooling. In any case, it has been demonstrated that polyynes are the precursors also of fullerene formation.
***********************************************************
contact: Giovanni Vladilo (OATS)