The reaction C3H3+ + acetylene and the structural isomers of C5H5+

The authors study here the isomers of C{sub 5}H{sub 5}{sup +} and possible reactions between C{sub 3}H{sub 3}{sup +} and acetylene that lead to the formation of C{sub 5}H{sub 5}{sup +} using quantum chemical means. They find very many stable C{sub 5}H{sub 5}{sup +} isomers, the most stable being the vinylcyclopropenylium cation. A pyramidal form is calculated to be quite stable as are distorted planar cyclopentadienyl cations. Nonplanar distortions in these five-membered rings, however, do not appear stable. With an eye toward identification of these species in sooting flames and in Fourier transform ion cyclotron resonance experiments, they report the calculated vibrational and electronic spectra of these species. They also study reactions of C{sub 3}H{sub 3}{sup +} with acetylene, attempting to model the nucleation step in the formation of soot. They find that the cyclic form of C{sub 3}H{sub 3}{sup +} does not react with acetylene without sizeable barrier, whereas the linear propargyl cation reacts with acetylene without barrier to form many different C{sub 5}H{sub 5+} tautomers, some of which are quite stable. They discuss these findings in conjunction with the results of experiments on these systems.