Isomerization Processes in Ions of the Empirical Formula

Ions of the formula $$ have been generated with different initial energies by ionizing ethylene ( $$ , where the $$ ion is formed with an initial energy of > 11.51 eV), cyclobutane (initial energy of $$ , > 10.84 eV), methylcyclopropane (> 10.15 eV), 1-C₄H₈ (> 9.58 eV), and i-C₄H₈ (> 9.06 eV) with 11.6-11.8 eV photons, and in some cases also with 10 eV photons and with gamma radiation. The structures of the ions have been determined from the structures of the C₄H₈ products formed in charge transfer reaction between the ions and charge acceptors such as dimethylamine and nitric oxide, as well as from the structures of the butanes formed in $$ transfer reactions with methylcyclopentane-d ₁₂ ( $$ ). At low pressures the $$ ions initially formed in ethylene, cyclobutane, and methylcyclopropane isomerize to the thermodynamically most stable configurations, $$ and $$ . The $$ structure predominates in all the experiments. As the pressure is raised, the $$ ion yield diminishes as that of $$ increases, indicating that when the precursor of the $$ ion is collisionally deactivated, it ends up as $$ . At high pressures, $$ ions are intercepted; their yield increases with increasing pressure, indicating that $$ is an intermediate which isomerizes further unless it is collisionally deactivated. The $$ ion formed in methylcyclopropane (initial energy > 10.15 eV) is more easily deactivated than that formed in cyclobutane (initial energy > 10.84 eV). That the isomerization of the $$ ion to lower energy structures such as $$ and $$ requires excess internal energy is demonstrated by the fact that in the photolysis with 10 eV photons, a negligible amount of isomerization is observed, but with 11.6-11.8 eV photons, more than half of the $$ ions isomerize to the $$ structure at a pressure of 2 torr. Isomerization of the low energy $$ ions formed in the photolysis of i-C₄H₈ to other structures is relatively unimportant at 11.6-11.8 eV. Taking the ratio $$ as an indicator of the amount of energy removed by collisions from the intermediate $$ species under conditions where only i- and $$ ions are intercepted, it is shown that the efficiency of energy transfer from the ions to helium, hydrogen, neon, krypton, xenon, nitrogen, and carbon dioxide is related to the polarizability of the added deactivator.