Selected ion flow drift tube (SIFDT) studies of reactive ion-neutral encounters (1998)
AuthorsFairley, David Andrewshow all
Data is presented for a series of experiments performed using a selected ion flow drift tube (SIFDT) apparatus operated at room temperature (300 ± 5 K) and at helium pressures typically between 0.3 - 0.5 Torr. Several of the experiments described utilised the apparatus as a conventional selected ion flow tube (SIFT), i.e. the drift capability was only employed in some of the experiments. Three termolecular ion-molecule association reactions proposed to play a potential role in the chemistry of the interstellar medium were investigated in an effort to elucidate the isomeric structures formed and thereby infer the structures likely to be formed in the analogous radiative association reactions at interstellar pressures. The termolecular association of H₃O⁺ with C₂H₂ is shown to produce a mixture of C₂H₅O⁺ isomers and efforts to identify the individual isomers are described. The termolecular association of H₃O⁺ with C₂H₄ produces a C₂H₇O⁺ isomer that exhibits reactivity with the neutrals 2-fluorotoluene and acrylonitlile identical to that observed from protonated ethanol, C₂H₅OH₂⁺. The termolecular association of CH₂NH₂⁺ with HCOOH is shown to produce an electrostatic adduct which undergoes facile ligand switching reactions with NH3 and the amines, CH₃NH₂ and C₂H₅NH₂. The radiative association of CH₂NH₂⁺ with HCOOH is therefore not considered to be a plausible synthetic route to protonated glycine in the interstellar medium. Dissociative charge transfer from Kr⁺, CO⁺ or CO₂⁺ to chlorobenzene is shown to produce the cyclic phenylium cation exclusively. Electron impact on halobenzenes and consecutive ion-molecule reactions of acetylene are known to produce a mixture of C₆H₅⁺ isomers. It is shown that the more reactive of these isomers has the phenylium structure. Low energy dissociative charge transfer to cycloheptatriene or benzyl bromide is used to selectively form the tropylium and benzyl cations respectively. The experiments lead to an estimate of the barrier height for the isomerisation benzyl ⇌ tropylium of 2.7 ± 0.9 eV. Reaction with benzene is used as a diagnostic for distinguishing the isomeric C7H7⁺ product(s) formed in a series of ion-molecule reactions. The allyl, CH₂CHCH₂⁺, and 2-propenyl, CH₃CCH₂⁺, cations are observed as distinct isomeric species in the SIFT. Reaction with methanol is used to distinguish between the two isomers. The isomeric ratio of allyl:2 propenyl formed via protonation of allene or propyne by a protonated base, BH+, is shown to be dependent on the proton affinity of the base, B. The experiments yield an estimate of the barrier height for the rearrangement allyl ⟶ 2-propenyl of 110 ± 30 kJ mor-¹, which is in excellent accord with ab initio calculations performed at the G₂(MP₂) level of theory. The exothermic H-atom abstraction reaction of SO₂⁺ with H₂ has been studied in a SIFDT apparatus over a range of centre-of-mass energies from thermal (300 K) to ~ 0.12 eV. The observed increase in rate coefficient with ion kinetic energy gives a linear pseudo-Arrhenius plot with a slope that indicates a barrier of ~ 5 kJ mor-¹ exists on the potential energy surface. The H₂SO₂⁺ potential surface was also explored in an ab initio investigation using the G2(MP2) procedure. An (SO₂ +•H₂)* transition state between reactants and products is identified, corresponding to the barrier found from experiment. The competition between charge transfer and association was investigated in a SlFDT study of the reactions of NO⁺ with the ketones: acetone, butanone and 3- pentanone. Association is the sole process observed in all three reactions at 300 K (the 3- pentanone reaction possesses a minor channel to the slightly endothermic charge transfer product). As the ion-neutral centre-of-mass energy is increased the rate coefficient for termolecular association decreases markedly, as a result of the decreasing lifetime of the intermediate (NO⁺•ketone) complex with increasing temperature. Bimolecular charge transfer can compete with association once sufficient energy is available to overcome the endothermicity and at higher energies dissociative charge transfer channels open up. The association reactions of CH₃ + with CH₃CN and C₂N₂ were also investigated in the SlFDT. The CH₃ +/CH₃CN reaction exhibits competitive binary and ternary channels. The bimolecular channels to C₂H₅⁺ + HCN and HCNH + C₂H₄ both involve extensive intramolecular rearrangement of the intermediate (CH₃⁺•CH₃CN)* collision complex and the rate coefficients for both channels exhibit a marked (negative) dependence on the ion-neutral centre-of-mass energy.