David A. Fairley

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.

A flowing afterglow selected ion flow tube (FA/SIFT) comparison of SIFT injector flanges and H3+ + N revisited

Abstract The performances of two different but interchangeable Venturi injectors (an annulus and a hole injector) have been compared in a new flowing afterglow source-selected ion flow tube (FA/SIFT) instrument built at the University of Canterbury. The tests applied compared the relative “pumping efficiencies” of the two injectors; their ion transmission using (O 2 + ); the relative ease of injecting cluster ions subject to collision induced dissociation (H 3 O · + H 2 O); and the extent of isomerization of ions sensitive to structural changes during the injection process (C 3 H 5 + ). The annulus injector was clearly superior to the hole injector in pumping efficiency. Thereafter the improvement in performance was only marginal. The greater difficulty of construction and maintenance of the annulus injector needs to be balanced against the slightly less versatile hole injector. It was necessary to direct a significant fraction of the total helium buffer gas flow through an outer, noncritical orifice to maintain satisfactory performance in the hole injector when injecting ions susceptible to collision induced dissociation. Finally, the new instrument was used to reexamine the reaction of H 3 + and N atoms, which was found to be a nonreactive system, k −11 cm 3 s −1 .

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.

COMPETITIVE ASSOCIATION AND CHARGE TRANSFER IN THE REACTIONS OF NO+ WITH SOME KETONES : A SELECTED ION FLOW DRIFT TUBE STUDY

The rate coefficients and product ion branching ratios have been determined for reactions between NO+ and three ketones, acetone, 2-butanone, and 3-pentanone, as a function of NO+/reactant ketone centre-of-mass energy, Er, and NO+/helium carrier gas atom centre-of-mass energy, Ec, in a flowing afterglow selected ion flow drift tube apparatus. In these experiments, the helium carrier gas was maintained at a temperature of 300 K. At zero drift field, association was the dominant channel occurring at close to the collision rate forming NO·+ ketone adduct ions. At higher drift fields (Er, Ec < 1 eV), charge transfer and dissociative charge transfer channels became the major channels forming fragment ions of the ketones. The decrease in the association rate coefficient with increasing Ec exhibits an inverse power law dependence k3 ∝ Ec−n where n ∼ 2.5 for all three ketones. This dependence is much larger than predicted by simple theory and may be indicative of low energy vibrations contributing to the total energy pool in the (NO· ketone)+∗ excited complex. Keywords: Ion/molecule association; SIFT; Drift tube; Charge transfer

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.

Flow tube and theoretical study of proton transfer reactions of C3H5+ ions

Abstract The allyl, CH 2 CHCH 2 + , and 2-propenyl, CH 3 CCH 2 + , ions have been observed as distinct isomeric species in a flowing afterglow/selected ion flow drift tube (FA/SIFDT). Reaction with methanol is used as a diagnostic for distinguishing 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. Proton transfer from H 3 O + to allene produces the 2-propenyl cation only, whereas proton transfer from SO 2 H + to allene generates a mixture of allyl and 2-propenyl cations, enabling us to estimate the barrier height for the rearrangement allyl → 2-propenyl as 110 ± 30 kJ mol −1 . This is in accord with ab initio calculations performed at the G2(MP2) level of theory. The C 3 H 5 + product of the reaction between C 2 H 4 ·+ and C 2 H 4 was identified as the 2-propenyl cation. Rate coefficients are also reported for reactions of the allyl and 2-propenyl cations with several neutrals.