On the Origin of Cometary C₂and C₃: Hydrogen Atom Migration in Diacetylene?

Abstract

The photolysis of C 10 H 2 in air-saturated hexane by 253.6 nm photons yields the polyyne C 8 H 2 in approximately 5% of all C 10 H 2 disappearances; perhaps due to the migration of the hydrogen atom on À1 (I use the symbol Àn to designate the specific carbon atom number n in the chain; À1 is carbon atom 1) to À3 in the electronically excited C 10 H Ã 2 molecule followed by the rupture of the À2-À3 carbon-carbon bond. C 6 H 2 and C 12 H 2 were not seen to form. This new result strengthens the hypothesis that hydrogen migration along carbon chains of photon-excited polyynes followed by the rupture of one carbon bond could be very common among these compounds. It is suggested here that diacetylene forms photochemically from acetylene in the cometary coma followed by the swift photochemical formation of C 2 from diacetylene by hydrogen migration from À1 to À3 followed by the rupture of the À2-À3 carboncarbon bond. Hydrogen migration from À1 to À4 in excited diacetylene followed by the rupture of the À3-À4 carbon bond might form cometary C 3 . Neither C 2 nor C 3 were detected in the current study. Their formation by hydrogen migration is therefore hypothetical but the case for C 2 is observationally stronger than for C 3 . Removal of air from the solution increased the disappearance rate of C 10 H 2 by a factor of almost 10 3 , which implies that the excited molecule is in a triplet state with an estimated lifetime of 160 s.

UV photolysis of polyynes at λ=254 nm and at λ>222 nm

“…Therefore, both lamps generate a photon flux which is able only to excite to the triplet state the polyynes in solution, in line with the suggestion already made by Heymann (2008). From the excited state the polyynes undergo a fragmentation reaction probably with the hydrogen shift to the 2,3 bond as proposed by Heymann (2008). Figure 5 provides evidence for the fact that both the photons at 254 nm from the monochromatic light source or the photons from the continuous emission from 222 nm are able to photolize simultaneously all polyynes in the mixture.…”

“…In fact, a k value for all the polyynes considered of the order of 10 x3 s x1 has been found. More precisely, the averaged data for polyyne species taken from Table 1 Heymann (2008) in the photolysis of C 10 H 2 in the absence of air and our results under argon: k Heymann =6.37r10 x2 s x1 in comparison to an average value found in the present work (Table 1)…”

“…In fact, the interference of oxygen from air in the polyyne photolysis was observed with a slowdown of the decomposition process (Cataldo 2004c(Cataldo , 2006b). However, Heymann (2008) has made a quantitative measurement of the oxygen effect on the photolysis of C 10 H 2 . He found that the presence of oxygen inhibits the C 10 H 2 photolysis by three orders of magnitude in the pseudofirst-order kinetic rate constant.…”

“…Concerning the kinetic rate constant, the polyyne photolysis can be described by first-order kinetics, but as stated by Heymann (2008) : 'Processes were either first order or pseudo first order but second order processing could not be decisively ruled out '.…”

“…Photostability may also derive from the fact that relatively labile molecules are incorporated into solid carbonaceous particles ; in such a case they may survive their journey into and through the interstellar medium followed by their incorporation into cometary bodies as organic molecules (Kwok 2004). In addition, the photochemistry of polyynes also has implications in the formation of simple organic compounds in cometary coma (Heymann 2008).…”