Reactions of excited and ground state H 3+ ions with methyl substituted hydrides

Abstract

The reactions of H 3 + with C 2H 6, CH 3NH 2, CH 3OH, CH 3F, CH 3SH, and CH 3Cl have been studied using ion cyclotron resonance pulse ejection techniques. The product distribution obtained is strongly dependent upon hydrogen pressure due to a large difference in reactivity between excited and ground state H 3 + ions. The H 3 + ions originally formed by the reaction of H 2 + with H 2 are highly excited. At low hydrogen pressures, these excited H 3 + ions react mainly by direct processes: by charge transfer and by a process equivalent to hydride ion abstraction. The product distribution changes as the hydrogen pressure is raised due to rapid deactivation of the H 3 + ions by collisions with H 2 molecules. At intermediate hydrogen pressures, the hydride ion abstraction process disappears and both ground state and partially deactivated H 3 + ions react principally by proton transfer to give a longer-lived protonated intermediate. With the exception of ethane, decomposition of the protonated intermediate occurs via vicinal hydrogen elimination and except for C 2H 6, CH 3SH, and CH 3Cl, CX bond scission to give the methyl cation is observed as well. From the dependence of the relative rate for charge transfer on hydrogen pressure, evidence is obtained indicating that the excited H 3 + ions may not be directly deactivated to the ground state but to some intermediate state(s) still containing a significant amount of internal energy, and that these intermediate H 3 + ions are subsequently only very slowly deactivated to the ground state.