S. E. Barlow

The tandem flowing afterglow-shift-drift

Abstract The design of a new tandem flowing afterglow-SIFT-drift instrument which provides high sensitivity, resolution, and chemical versatility is described. The performance of the instrument is evaluated in terms of (a) the intensities and variety of ions which can be generated, mass-selected, and injected; (b) the efficiency of the dual annulus SIFT injector as a venturi inlet; and (c) the reliability of the kinetic data.

Studies of the reaction of O+2 with deuterated methanes

In the gas phase O+2 reacts with methane at 300 K to produce a hydrogen atom and the CH3O+2 ion. The structure of this ion has recently been determined to be H2COOH+, methylene hydroperoxide ion. The reaction rate coefficients and product distributions have now been measured at 300 K for the CHnD4−n isotopes. The reaction shows both inter‐ and intramolecular isotope effects, e.g., CH2D2 reacts more slowly than methane and more rapidly than CD4, but loses hydrogen or deuterium with equal probability. The ion readily transfers HO+ to alkenes, CS2, and many other neutral molecules. The reaction with CS2 has been used to investigate the isotopic distribution within mixed isotope product ions. In addition, the reaction rate coefficients for both CH4 and CD4 have been measured as functions of temperature between 20 and 500 K; in both cases a clear minimum is observed in the reaction rate coefficient near room temperature. A mechanism for the reaction is proposed which allows us to model the temperature dependen...

Reactions of C+ He+ and Ne+ with vibrationally excited H2 AND D2

Abstract Several orders of magnitude enhancements in the rate constants for three slow ion-molecule reactions with H 2 have been observed when the H 2 is vibrationally excited. The atomic cations C + , He + , and Ne + were all observed to react near their respective Langevin rates with vibrationally excited H 2 .

Reactions of O−+N2O at 300 K: the totally labeled experiments

The tandem flowing afterglow‐selected ion flow tube was employed to study the isotopically labeled reactions of O−+N2O→NO−+NO at 300 K. In the reactions of 16O−+15N14N16O, 16O−+14N15N16O and 18O−+14N14N16O, both of the possible NO− products are formed with equal probability, indicating that equilibration is achieved within the reaction complex before dissociation. In the totally labeled reactions of 18O−+15N14N16O and 18O−+14N15N16O all possible NO− products are observed which strongly supports the formation of both trigonal and linear N2O−2 intermediates along the reaction path. The reaction mechanism is discussed and these results are compared with those of other workers.

Exchange reactions of 18O− with some di- and triatomic molecules

Abstract Rate coefficients have been measured for the reactions of 18O−1 with H2O, CO2, O2, CO, NO and SO2 using the tandem flowing afterglow selected ion flow tube. For H2O and CO2, isotope exchange occurs at the statistical limit, which suggests the formation of a long-lived complex. In contrast, the exchange reaction with O2 occurs with only 17% efficiency. Both associative detachment and isotope exchange are observed in the reactions of 18O− with CO, NO and SO2. These data are discussed together with previous results to obtain a comprehensive picture of the reaction mechanism.

Vibrational relaxation of NO+(υ) by O2(1Δg) molecules

Abstract The rate constant for NO + (υ) vibrational relaxation by metastable O 2 ( 1 Δ g ) molecules is found to be (3±2)×10 −10 cm 3 s −1 at room temperature, in contrast to the lack of vibrational quenching of NO + (υ) by ground-state O 2 ( 3 Σ) molecules, for which the quenching rate constant −12 cm 3 s −1 . This suggests that NO + ( 1 Σ)+O 2 ( 1 Δ) follows a much more attractive potential curve than NO + ( 1 Σ)+O 2 ( 3 Σ).

An energy-resolved study of the fragmentation of some 2-substituted 2-propoxide ions and the rates of proton abstraction from acetone

Abstract The collision-induced dissociation reactions of R(CH 3 ) 2 CO − ions (R  CF 3 , C 6 H 5 , CH 2 CH, have been studied as a function of collision energy over the range 5–100 eV (laboratory scale). Three fragmentation reactions are observed In agreement with earlier IR multi-photon photodissociation results, reaction 2 is observed to be the dominant low-energy fragmentation route, with reaction 1 increasing in importance with increasing collision energy. Reaction 3 is significant only for R  CH 2 CH. These results, along with those obtained earlier for R  H and C 6 H 5 CH 2 , are discussed in relation to a step-wise mechanism of fragmentation which involves initial formation of the ion/dipole complexes [R − ⋯ CH 3 COCH 3 ] and [CH − 3 ⋯ RCOCH 3 ]. The complex [R − ⋯ CH 3 COCH 3 ] is the same as that formed initially upon reaction of R − with CH 3 COCH 3 , and the details of the potential energy surface revealed by the CID studies are compared with the details revealed from kinetic studies of the rates of proton abstraction from CH 3 COCH 3 by the various R − ions.

Gas-phase association reactions of hydroxide, methoxide, ethoxide, and their deuterated analogs

Abstract The selected ion flow tube technique is used to study 14 termolecular association reactions of hydroxide–water (OH − + H 2 O and OD − + D 2 O), methoxide–water (CH 3 O − + H 2 O, CH 3 O − + D 2 O, CD 3 O − + H 2 O, and CD 3 O − + D 2 O), methoxide–methanol (CH 3 O − + CH 3 OH, CH 3 O − + CH 3 OD, CD 3 O − + CD 3 OH, and CD 3 O − + CD 3 OD), and ethoxide–ethanol (CH 3 CH 2 O − + CH 3 CH 2 OH, CH 3 CD 2 O − + CH 3 CD 2 OH, CD 3 CH 2 O − + CD 3 CH 2 OH, and CD 3 CD 2 O − + CD 3 CD 2 OD) where the cluster bond dissociation energies are similar (∼24–30 kcal/mol). The apparent second-order rate coefficients for association (k IIapp ) are measured as a function of helium pressure over the range of 0.25–1.2 Torr at 300 K. The derived termolecular rate coefficients and complex lifetimes are generally larger for systems with more degrees of freedom in the intermediate ion–molecule complex. In all reactions with alkoxides, association rates are significantly enhanced by deuteration of the alkyl groups (by factors of 1.4–3.0) whereas deuteration of the bridging hydrogen does not affect the association rates. Potential energy surfaces for association (single versus double well) are discussed based on the pressure dependence of the association rates.