Robert A. Morris

Competition between electron detachment and monomer evaporation in the thermal destruction of hydrated electron clusters

We have examined the competition between electron detachment and monomer evaporation in the thermal destruction (dissociation plus detachment) of hydrated electron clusters by monitoring the products in a selected ion flow tube apparatus as (H2O)−n clusters, 14≤n≤24, were heated over the temperature range 100 to 150 K. The destruction of the smaller clusters is dominated by electron detachment, and the detachment occurs over the narrow temperature range 120–145 K. The larger clusters initially undergo sequential evaporation of neutral monomer units, forming smaller and smaller ionic clusters. As the temperature increases, the electron detachment channel begins to compete with monomer evaporation, and the smaller ions eventually decay by electron detachment. Second‐order rate constants and activation energies were obtained for the thermal destruction of clusters 14≤n≤17 and 23≤n≤24. The activation energies for the destruction of the larger clusters, n≥17, are nearly constant at ∼0.34 eV, which is close to ...

Rate constants for the reactions of N+ and N2+ with O2 as a function of temperature (300–1800 K)

Abstract We have measured the rate constants for the reactions of N + and N 2 + with O 2 from room temperature to 1600 and 1800 K, respectively. In the N + reaction the rate constants increase with increasing temperature to 1000 K and level out at the collisional value above that. For the N 2 + reactions the rate constant decreases with temperature to 1000 K and increases above that value. Internal temperature dependences are derived by combining the data with previous drift tube measurements. The internal energy dependence of the N + reaction appears to be a complicated function of vibrational, rotational, and electronic energy. In the N 2 + reaction, rotational and translational energy behave similarly, and O 2 vibrations are found to increase the rate constant dramatically. If all O 2 vibrations are involved, a factor of six enhancement is found. Alternatively, O 2 ( v = 2) allows exothermic production of O 2 + ( a 4 Π u ). If the enhancement is due to this channel, then a factor of 20 enhancement is derived.

Rate coefficients for the endothermic reactions C+(^2P)+H2(D2)→CH^+(CD^+)+H(D) as functions of temperature from 400–1300 K

We have measured the bimolecular rate coefficients for the reactions of C+(2P) with H2 and D2 as functions of temperature from 400 to 1300 K using a high temperature flowing afterglow apparatus. The temperature dependences of these rate coefficients are accurately fit by the Arrhenius equation, with activation energies equal within experimental uncertainty to the reaction endothermicities. Internal energy dependences have been deduced by combining the present data with previous drift tube and ion beam measurements. We found that reactant rotational energy and translational energy are equally effective in surmounting the energy barrier to reaction, and that vibrational excitation of the neutral reactant to the v=1 state enhances the rate coefficients by a factor of ∼1000 for the reaction with H2 and by ∼6000 for the reaction with D2 at temperatures of 800 and 500 K, respectively. This vibrational enhancement is larger than the enhancement that would be produced if the same amount of energy were put into tr...

Negative ion chemistry of SF4

A selected ion flow tube was used to conduct an extensive study of negative ion–molecule reactions of SF4 and SF−4. Rate constants and product ion branching fractions were measured for 56 reactions. The reactions bracket both the electron affinity of SF4 (1.5±0.2 eV or 34.6±4.6 kcalu2009mol−1) and the fluoride affinity of SF3 (1.84±0.16 eV or 42.4±3.2 kcalu2009mol−1). These results may be combined to give the neutral bond energy D(SF3–F)=3.74±0.34 eV or 86.2±7.8 kcalu2009mol−1, independent of other thermochemical data except for the accurately known electron affinity of F. The heat of formation of SF−4 is derived from the electron affinity of SF4: ΔfH(SF−4)=−9.2±0.3 eV or −212.9±7.5 kcalu2009mol−1. Lower limits to EA(SF2) and EA(SF3) are deduced from observation of SF−2(35%) and SF−3(65%) ion products of the reaction S−+SF4. Rapid fluoride transfer from both SF−2 and SF−3 to SF4 places upper limits on the electron affinities of SF2 and SF3. The combined results are 0.2 eV≤EA(SF2)≤1.6 eV and 2.0 eV≤EA(SF3)≤3.0 eV. We revi...

Observation of thermal electron detachment from cyclo-C4F8− in FALP experiments

Abstract : The methodology for use of a flowing afterglow-Langmuir probe apparatus to measure thermal electron detachment rate coefficients is described. We determined the thermal detachment rate coefficient (1010 + or - 300/s) for cyclo-C4F(-)8 ions and the rate coefficient (1.6 + or - 0.5 x 10(exp -8)cu cm/s) for electron attachment of cyclo-C4F8 at 375 K. The sole ionic product of attachment is cyclo-C4F(-)8. The equilibrium constant for the attachment/ detachment reaction yields a free energy for attachment at 375 K of -0.63 +or - 0.02 eV, from which we estimate the electron affinity (O K value) Of Cyclo-C4F8 to be about 0.63 eV. Electron attachment, Electron detachment, Cyclo perfluorobutane

Rate constants for the reactions of CO3−(H2O)n=0–5 + SO2: Implications for CIMS detection of SO2

The rate constants for the CO3−(H2O)n + SO2 reactions were measured for n=0–5 using a variable temperature-selected ion flow tube. Temperature dependences were measured for n=0–3 over the following temperature ranges: n=0, 158–550 K; n=1, 158–270 K; n=2, 158–193 K; n=3, 158–193 K. The n=4, 5 rate constants were measured exclusively at 158 K. Good agreement is found with previous measurements of the n=0 and 1 rate constants. Expressions are estimated for use in chemical ionization mass spectrometric detection of SO2 in the troposphere.

Chemistry of atmospheric ions reacting with fully fluorinated compounds

Abstract Reactions of the atmospheric ions O+, O2+, O−, O2−, NO+, H3O−, CO3−, and NO3− with five fully fluorinated compounds were studied using the selected-ion flow tube (SIFT) technique at 300 K. Reaction rate constants and product branching fractions were measured for the following selected perfluoro compounds: CF4, C2F6, c-C4F8, n-C6F14, and SF6. The ion O+ reacted at the collisional rate with all five compounds, with the oxygen always being a part of the neutral products. The O2+ ion reacted with c-C4F8 at unit efficiency and with n-C6F14 at 39% efficiency, with no oxygen incorporation in the ionic products in either case. The reactions of O− with c-C4F8 and n-C6F14 proceeded at or near unit efficiency, featuring ten and eight product channels, respectively, including a component from reactive electron detachment in the c-C4F8 case. In both cases, many of the product channels appeared to be paired, that is, two channels differing only in the location of the negative charge. The reaction of O2 with c-C4F8 proceeded with 37% efficiency via non-dissociative charge transfer. The ions NO+, H3O+, CO3−, and NO3− were unreactive with all five perfluorinated compounds under investigation.

The formation and destruction of H3O

We report the first measurements of rate constants for formation and reaction of the hydrated‐hydride ion H3O−. We studied the Kleingeld–Nibbering reaction [Int. J. Mass Spectrom. Ion Phys. 49, 311 (1983)], namely, dehydrogenation of formaldehyde by hydroxide to form hydrated‐hydride ion and carbon monoxide. The OD−+H2CO reaction is about 35% efficient at 298 K, with OD−/OH− exchange occurring in about half the reactions. H3O− was observed to undergo thermal dissociation in a helium carrier gas at room temperature with a rate constant of 1.6×10−12 cm3u2009s−1. We also studied a new reaction in which H3O− is formed: The association of OH− with H2 in a He carrier gas at low temperatures. The rate coefficient for this ternary reaction is 1×10−30 cm6u2009s−1 at 88 K. Rate coefficients and product branching fractions were determined for H3O− reactions with 19 neutral species at low temperatures (88–194 K) in an H2 carrier. The results of ion‐beam studies, negative‐ion photoelectron spectroscopy, and ion‐molecule react...

Temperature dependencies of the reactions of CO−3(H2O)0,1 and O−3 with NO and NO2

We have measured temperature dependencies of the rate constants for CO−3 and O−3 reacting with NO and NO2. In addition, the temperature dependence of the CO−3(H2O) reaction with NO was determined, and a 196 K rate constant was measured for the reaction of CO−3(H2O) with NO2. The reactions with NO all proceed by O− transfer to produce NO−2. The temperature dependencies of the rate constants for the reactions of CO−3 and O−3 with NO are represented as 1.5×10−7*T−1.64 and 4.4×10−7*T−2.15 cm3u2009s−1, respectively, and agree very well with previous measurements. The rate constant for the reaction of CO−3(H2O) with NO is 4.1×10−5*T−2.72 cm3u2009s−1. Previous measurements of the rate constants for CO−3, CO−3(H2O), and O−3 reacting with NO2 appear to be in error; our measured rate constants for the first two reactions are represented as 2.6×10−5*T−2.38 and 9.1×10−9*T−0.79 cm3u2009s−1, respectively. The rate constant for the reaction CO−3(H2O) with NO2 is 7.9×10−11 cm3u2009s−1 at 196 K. The reactions of CO−3 and CO−3(H2O) with N...

Reactions of Fe− with acids: gas-phase acidity and bond energy of FeH

Abstract Kinetics and products for the gas-phase reactions of Fe− with the acids CH3C(O)CH2C(O)CH3, HCO2H, CH3CO2H, CH3CH2CO2H, and H2S have been determined using a selected-ion flow drift tube. Electron detachment is the sole reaction channel for reaction with CH3C(O)CH2C(O)CH3, CH3CO2H, and CH3CH2CO2H, and a dominant reaction channel for reaction with HCO2H and H2S. Proton transfer from HCO2H to Fe− occurs and was studied as a function of increasing kinetic energy. These reactions are used to determine δH° acid,298 (FeH) = 345.2 ± 4.4 kcal mol−1, which determines the homolytic bond energy D°298(Fe−H) = 35.1 ± 4.4 kcal mol−1. The electron detachment reactions and reactions leading to ion products in reactions with HCO2 H and H2S are discussed in view of the available thermochemistry. A mechanism involving initial proton transfer within the collision complex is suggested for all reactions.