Structures of gas phase C₅H₈ radical cations: A collisional ionization study

Collisional ionization (charge stripping) and charge exchange ionization spectrometry were utilized to determine structures of fourteen cyclic and acyclic C₅H₈ radical cations, including ionized 1,2‐ 1,3‐, 1,4‐ and 2,3‐pentadienes (‐PD), isoprene, 1‐ and 3‐methylcyclobutenes (1‐ and 3‐MCB), 3‐methyl‐1,2‐butadiene (3‐M‐1,2‐BD), methenecyclobutane (MECB), cyclopentene, 3‐methyl‐1‐butyne (3‐MB), 1‐ and 2‐pentynes and vinylcyclopropane (VCP). The pressure of the charge exchange reagent gas in the ion source was adjusted to generate ions of different energy contents. The structures of the C₅H₈ ions are energy dependent, and their isomerization reactions can be monitored as a function of the amount of internal energy deposited by charge exchange. 1,3‐PD, isoprene and cyclopentene radical cations are identified as stable ion structures. 1‐MCB, 3‐M‐1,2‐BD and 3‐MB radical cations isomerize to isoprene ions, whereas ionized VCP, 3‐MCB, 1,2‐PD, 2,3‐PD, 1,4‐PD, 1‐pentyne and 2‐pentyne ultimately isomerize to the [1,3‐PD]⁺˙. Thermodynamic arguments are invoked to corroborate these isomerization reactions. The critical energies of the isomerizations are also estimated.