Graham B. I. Scott

ION-MOLECULE REACTIONS RELEVANT TO TITAN'S IONOSPHERE

Abstract Twenty four new ion-molecule reactions are presented for inclusion in the modeling of the ionosphere of Saturns satellite Titan. Sixteen reactions were re-examined to reduce uncertainties in the previous literature results. In this study we have examined the reactions of N + and N 2 + with CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , HCN, CH 2 CHCN and HC 3 N; the reaction of N + with CH 3 CN; the reactions of C 3 H 5 + with CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , H 2 , HCN, HC 3 N and CH 2 CHCN; the reactions of C 2 N 2 + with C 2 H 2 ; C 2 H 2 + and C 2 N 2 ; C 2 H 4 with C 2 H 3 + , C 2 H 4 , CHCCNH + , and HC 5 N + ; HCNH + with C 2 H 6 ; C 3 H 6 + with C 3 H 6 ; HCN with C 2 H 6 + , C 3 H 6 + , cC 3 H 6 + ,C 2 N 2 + and NO + ; N 2 with C 2 H 2 + and C 2 H 5 + ; C 2 H 4 + and HC 3 N. The ions selected for this study were derived either from nitrogen, appropriate hydrocarbons or nitrites. The reactant neutrals were selected on the basis of their known presence in Titans atmosphere. The reaction products are consistent with the expected increase in ion size through ion molecule reaction processing. Data are also presented for the reactions of 23 ions with molecular nitrogen. Almost all of these ions are unreactive with N 2 .

Gas-phase reactions of some positive ions with atomic and molecular nitrogen

The reactions of the cations CN+, HCN+, HCNH+, HC3N+, HC3NH+, H3+, H2O+, H3O+, N2+, CO+, HCO+, O2+, CO2+, HCO2+, and C2H2+ with atomic and molecular nitrogen have been characterized using a selected ion flow tube (SIFT) operating at room temperature. Rate coefficient and branching ratio data are reported for all ion–neutral reactions studied. Constraints arising from spin conservation considerations are found to be unimportant in cation-N atom processes.

Gas phase reactions of some positive ions with atomic and molecular hydrogen at 300 K

The reactions of CO+, CO2+, SO2+, NO2+, CS2+, CN+, C2N2+, and C2H3+ with H atoms and H2 molecules have been studied in a selected ion flow tube operated at (300±5) K. The H atom reactions proceed variously by the processes of atom exchange and charge transfer (when allowed), none proceed at the Langevin rate, and the rates of several of them appear to be influenced by the spin states of the product species. Most of the H2 reactions proceed by H atom abstraction and at a large fraction of the Langevin rate.The reactions of CO+, CO2+, SO2+, NO2+, CS2+, CN+, C2N2+, and C2H3+ with H atoms and H2 molecules have been studied in a selected ion flow tube operated at (300±5) K. The H atom reactions proceed variously by the processes of atom exchange and charge transfer (when allowed), none proceed at the Langevin rate, and the rates of several of them appear to be influenced by the spin states of the product species. Most of the H2 reactions proceed by H atom abstraction and at a large fraction of the Langevin rate.

Ab initio calculations related to the formation of propynal and propadienone in interstellar clouds

Abstract The proposal that propynal, CHCCHO, observed in interstellar clouds is formed by the ion molecule reacion of C 2 H 3 + and CO has been investigated in a G2 study. The calculations show that propadienone CH 2 CCO could be formed by this reaction but not propynal. At the G2 level of theory, propadienone is calculated to be lower in energy than propynal.

The reaction H3+ + N: a laboratory measurement

Abstract We report a study of the ion-atom reaction H 3 + +N using a selected ion flow tube (SIFT) operating at room temperature. The product channel is NH 2 + +H and the rate coefficient is k =(4.5±1.8) × 10 −10 cm 3 s[ su −1]. This reaction is relevant to the synthesis of ammonia in interstellar clouds.

A selected ion flow tube study of the reactions of small CmHn+ ions with O atoms

The reactions of thirteen CmHn+ ions (m⩽6) with atomic and molecular oxygen and nitric oxide have been measured using a selected ion flow tube operating at room temperature. Rate coefficients and product distributions are reported for each reaction. Most of the hydrocarbon ions studied exhibit relatively rapid reactions with O atoms and proceed at substantial fractions of the collision rate. All O atom reactions showed multiple product channels and the formation of a –C–O bond, either in the ion product or the neutral product of reaction.

ION-MOLECULE ASSOCIATION OF H3O+ AND C2H2 : INTERSTELLAR CH3CHO

The C2H2· H3O+ product of the ion–molecule association reaction between H3O+ and C2H2 is found to consist of a ca. 50 : 50 mixture of two isomeric ions. These two isomeric ions are identified in a selected ion flow tube, by their different proton transfer behaviour with the neutral reagents C2H5Br, 4-fluorotoluene, CH3OH and benzene, as protonated vinyl alcohol, CH2CHOH2+ and either protonated acetaldehyde, CH3CHOH+ or the electrostatic complex H3O+· C2H2. These conclusions are supported by Gaussian G2 level calculations based on ab initio molecular orbital theory, which are applied to calculate the proton affinities of CH3CHO, CH2CHOH, oxirane and acyclic CH2OCH2. Reaction rate coefficients and product ratios are also reported for the reactions of specific C2H5O+ isomers, viz: CH3CHOH+, CH3OCH2+ and [graphic omitted] with CH3OH, 4-fluorotoluene and C6H6. The implications of the current results to the interstellar synthesis of CH3CHO are discussed briefly.

The association reaction C2H3+ + CO and interstellar propynal

Abstract Two different C 3 H 3 O + ions were generated in a selected-ion flow tube operating at room temperature. C-2 protonated propadienone, H 2 CCHCO + , was produced by the association reaction between C 2 H 3 + and CO. Protonated propynal, HCCCHOH + , was produced by proton transfer to propynal, HCCCHO. The ions were identified by their proton transfer reactions with C 2 H 5 NH 2 , CH 3 NH 2 , C 4 H 5 N, NH 3 , ( n -C 4 H 9 ) 2 O, (C 2 H 5 ) 2 CO, C 6 H 10 O, C 6 H 6 and C 2 H 5 I. Rate coefficients and branching ratios are reported for these reactions. The implications of this work for the synthesis of HCCCHO in interstellar clouds are discussed.

SIFDT study of the SO+2/H2 H-atom abstraction reaction

Abstract The exothermic H-atom abstraction reaction of SO+2 with H2 has been studied in a selected ion flow drift tube (SIFDT) over a range of center-of-mass energies from thermal (300 K) to about 0.12 eV. The measured rate coefficient at 300 K is 4.2 × 10−12 cm3 s−1 which is very much less than the Langevin capture rate. The increase in rate coefficient with ion kinetic energy gives a linear Arrhenius-type plot with a slope that indicates a barrier of ∼5 kJ mol−1 exists on the potential surface. The H2SO+2 potential surface is also explored in an ab initio investigation using the G2 procedure. An (SO+2.H2)∗ transition state between reactants and products is identified, corresponding to the barrier found from experiments.