Ionic Reactions in Gaseous Cyclobutane

Ionic reactions in gaseous cyclobutane have been examined at pressures up to 1 torr in a specially designed ion source fitted to a time-of-flight mass spectrometer. It was observed that C4H8+ and C4H7+ are unreactive with cyclobutane, although both of these ions appear to be produced by reactions of other fragment ions. These conclusions were confirmed by experiments in a tandem mass spectrometer in which the reactions of individual ions of the cyclobutane system were identified at impacting ion kinetic energies as low as 0.3 eV. It was shown that ethylene and acetylene ions charge transfer to cyclobutane with high efficiency while the remaining fragment ions react principally by a hydride-transfer mechanism. Isotopic techniques were employed to aid in establishing the reaction mechanisms. The interactions of cyclobutane ions with various molecules either employed as additives or formed as decomposition products in radiolysis experiments have also been examined by mass-spectrometric techniques. It was observed that the addition of small quantities of NO, (CH3)3N, C2H4, or cis-2-C4H8, for example, leads to the production of the corresponding parent ion–additive complex at higher pressures. The H2S, on the other hand, was observed to intercept C4H7+ ions. Rate constants for charge-transfer reactions of parent cyclobutane ions with a series of molecules have also been determined. Data from these and related experiments are consistent with rearrangement of cyclobutane parent ions to linear butene structures prior to reaction. The implications of these findings for the radiolysis of cyclobutane are discussed.