On the Origin of Cometary C2 and C3: Hydrogen Atom Migration in Diacetylene?

The photolysis of C10H2 in air-saturated hexane by 253.6 nm photons yields the polyyne C8H2 in approximately 5% of all C10H2 disappearances; perhaps due to the migration of the hydrogen atom on Γ1 (I use the symbol Γn to designate the specific carbon atom number n in the chain; Γ1 is carbon atom 1) to Γ3 in the electronically excited C10H*2 molecule followed by the rupture of the Γ2-Γ3 carbon-carbon bond. C6H2 and C12H2 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 C2 from diacetylene by hydrogen migration from Γ1 to Γ3 followed by the rupture of the Γ2-Γ3 carbon-carbon bond. Hydrogen migration from Γ1 to Γ4 in excited diacetylene followed by the rupture of the Γ3-Γ4 carbon bond might form cometary C3. Neither C2 nor C3 were detected in the current study. Their formation by hydrogen migration is therefore hypothetical but the case for C2 is observationally stronger than for C3. Removal of air from the solution increased the disappearance rate of C10H2 by a factor of almost 103, which implies that the excited molecule is in a triplet state with an estimated lifetime of 160 μs.