Robert J. Hanrahan

Measurement of thermal electron dissociative attachment rate constants for halogen gases using a flowing afterglow technique

A flowing afterglow apparatus was constructed and used to measure thermal electron attachment rate constants in the halogen gases fluorine, chlorine, and bromine. The operation of the afterglow system and the mathematical models applied in treating the data were tested by measuring the thermal attachment rate constant for electrons in sulfur hexafluoride. The average value obtained for this rate constant is 4.2±1.1×10−8 cm3 molecule−1 sec−1 when a microwave discharge was used as the electron source and 3.6±1.8×10−8 cm3 molecule−1 sec−1 when a filament was used as the electron source. The average electron temperature was estimated to be approximately 600 °K for the microwave discharge source and approximately 350 °K for the filament source. A charge transfer reaction between sulfur hexafluoride and the ion O2− was also investigated in the present study to further assess the operation of the flowing afterglow apparatus in the microwave discharge source configuration. The average rate constant obtained for t...

Ion–molecule reactions in the systems CF4–CH4 and CF4–C2H6

A mass spectrometric study of ionic reactions in CH4−CF4 and C2H6−CF4 mixtures was undertaken at pressures as high as 0.52 torr. It was found that CF3+ ions react in a hydride transfer process with ethane with a rate constant of 3.3×10−10 cm3 molecule−1⋅s−1. A rate constant of 1.6×10−10 cm3 molecule−1⋅s−1 was determined for the reaction of C2H5+ ions with C2H6. In mixtures of CH4 and CF4, both CH3+ and CH5+ react with CF4, apparently by fluoride ion transfer, for which we calculated rate constants of 8.6×10−10 and 2.3×10−10 cm3 molecule−1⋅s−1, respectively. CF3+ ions do not attack molecular methane. There is good agreement with published rate constants for the disappearance of CH2+, CH3+, and CH4+ ions in pure methane. However we observed a decreasing CH5+ yield above 0.1 torr which may be due either to impurities or to excess energy in CH5+ ions as a result of a relatively high repeller field strength within the ion source in our experiments.

Formation of ground state ozone on pulse radiolysis of oxygen

Abstract The rate of formation of ground-state ozone ( 1 A 1 ) in the fast electron pulse radiolysis of oxygen has been followed using the 253.7 nm Hg resonance line, and a monochromator set for 1.1 nm bandpass. Under these conditions the rate of formation of vibrational ground state O 3 is essentially third order with a rate constant of 2.64 × 10 -34 cm 6 molec -2 s -1 . It is formed direct combination, from collisional quenching of vibrationally excited ozone and from an electronically excited ozone precursor, probably O 3 ( 3 B 2 ). Computer modeling suggests that the latter can decay either to vibrationally excited or vibrational ground-state 1 A 1 ozone. Decrease of the optical density at 253.7 nm indicated that most of the ozone formed disappeared from the system within 0.2–0.5 sec. This process is probably due to diffusion out of the optical path as well a slow collisional decomposition following absorption of the monitoring light by ground state ozone.

Gas phase formation of hydrogen chloride by thermal chlorine-steam reaction

Abstract Electrolysis of HCl solution to form H2 and Cl2 is of interest in applications such as utility load leveling, solar energy storage and commercial hydrogen production. In the latter case, the chlorine generated must be reconverted to HCl for continued use in the process. The gas phase reaction of steam with chlorine to form hydrogen chloride plus oxygen was investigated for this purpose. Flowing systems with 2, 4 and 8 cm3 min−1 of chlorine and 25 cm3 min−1 of steam were studied from 563 to 873 K. The reactant/product mixture was quenched in 50 cm3 of water at 298 K. The yield of HCl was determined by acid/base titration. After removal of HCl and residual chlorine using alkaline KI solution, oxygen was measured by use of a gas burette. The HCl and O2 analyses were in agreement. Using a Cl 2 H 2 O mole ratio of 4 25 , conversion of chlorine was 24.4% at 563 K, 39.7% at 648 K, 53.1% at 773 K and 80.0% at 873 K. Strong photolysis at 563 and 648 K had no observed effect on the product yields. The experimentally determined ΔH value from plots of log(Kp) vs 1 T is in good agreement with the ΔH value determined from standard thermochemical tables, for the reaction between chlorine and steam.

Solar-assisted production of hydrogen and chlorine from hydrochloric acid using hexachloroiridate (III) and (IV)

Abstract The production of H 2 and Cl 2 from HCl solution is important in relation to commercial H 2 production, utility load leveling and solar energy storage. The production of H 2 from electrolysis and Cl 2 from photolysis of a 6M HCl solution containing IrCl 6 3 − /IrCl 6 2 − was investigated for this purpose. Using a 1 m 2 solar concentrator, the photolysis of IrCl 6 2 − was carried out, forming IrCl 6 3 − and converting aqueous Cl − ions to gaseous Cl 2 . The amount of IrCl 6 2 − destroyed under photolysis was established using light absorption at 425 nm; corresponding yields for formation of Cl 2 were inferred by stoichiometry. Electrolysis of the solution was carried out at a constant current density. During electrolysis the H 3 O + is reduced at the cathode to form H 2 , while IrCl 6 3 − formed during photolysis is oxidized back to IrCl 6 2 − at the anode. Electrolysis in the presence of the iridium complex proceeds at a lower voltage than the electrolysis of HCl solution itself. Formation of H 2 was inferred from Faradays laws, as well as from a stoichiometric relationship with the amount of IrCl 6 2 − formed under electrolysis, which was again measured by light absorption at 425 nm. Overall, aqueous HCl/IrCl 6 3 − solutions form IrCl 6 2 − plus H 2 during electrolysis, whereas aqueous HCl/IrCl 6 2 − solutions form IrCl 6 3 − plus Cl 2 during photolysis.

Ion-molecule reactions in trifluoromethyl iodide and pentafluoroethyl iodide

Abstract Ion-molecule reactions in trifluoromethyl iodide and pentafluoroethyl iodide were studied using time-of-flight and ion cyclotron resonance mass spectrometry, respectively, to obtain information about reaction rates and mechanistic pathways. In the CF 3 I system, CF 3 + reacts with parent via F − transfer (exoergic) or via charge exchange (strongly endoergic). The I + ion reacts by I atom abstraction (exoergic) giving I 2 + or by endoergic charge exchange. The parent ion CF 3 I + can decay via collisionally induced dissociation, but also undergoes condensation reactions with neutral CF 3 I yielding CF 3 I 2 + and the dimer ion (CF 3 I) 2 + ; the ether-structure product (CF 3 ) 2 I + was not seen. Many of the same reaction types are seen in the C 2 F 5 I system. The CF + ion undergoes I − and CF 3 − transfer reactions with parent, giving C 2 F 5 + and CF 2 I + , respectively. As in the CF 3 I system, CF 3 + (and also CF 2 I + and C 2 F 5 + ) react with substrate either via energetically favorable F − transfer or via strongly unfavorable charge transfer processes, giving C 2 F 4 I + and C 2 F 4 I + , respectively, with each reactant. Again, I − reacts by energetically favorable I atom abstraction or unfavorable charge transfer. Endoergic charge transfer also occurs between parent and C 2 F 4 + ion. Two condensation products were seen which are analogous to the methyl case — namely C 2 F 5 I 2 + and (C 2 F 5 I) 2 + . In both the CF 3 I and C 2 F 5 I systems, there is strong evidence for participation of ions with as much as 1.5 eV of excitation energy.

An electron impact investigation of pentafluoroethyl iodide

Abstract An electron impact investigation on C2F5I was made using the Retarding Potential Difference technique in a Bendix time-of-flight mass spectrometer. Appearance potentials were measured as follows: CF3+, 13.73 eV; C2F5+, 11.71 eV; and C2F5I+, 10.66 eV. From these results and previously published values it is possible to derive ΔHf0(C2F5I) ⩾ −236.4 kcal mol−1; ΔHf0(C2F5I+) ⩽ +9.4 kcal mol−1, and D(C2F5I) ⩽ 47.0 kcal mol−1. Observation of certain reaction channels in a parallel Ion Cyclotron Double Resonance experiment suggests that ΔHf0(C2F4I+) = +82.0 ± 2 kcal mol−1. From the latter result it is also possible to derive that D(FC2F4I+) = 91.5 kcal mol−1. In principle, D(CF3CF2I) could also be established if ΔHf0(CF2I) were known. If D(FCF2I) is estimated as 120 kcal mol−1, then ΔHf0(CF2I) = −40 kcal mol−1, and D(CF3CF2I) ⋍ 79 kcal mol−1. Uncertainties of ±0.1 eV are assigned to appearance potentials and ±2 kcal mol−1 to heats of formation.

Gas phase radiation chemistry of trifluoromethyl iodide

Abstract The gamma-radiolysis of gaseous CF3I was studied at 25 Torr (3.33kPa) and 24°C, both pure and with added HI. The major radiolytic products and their corresponding G values in the pure system are I2, 0.50; CF4, 0.55; and C2F6, 0.11. With added HI as the scavenger, the additional products CF3H, CF2IH and H2 were observed. In general, the results can be interpreted in terms of known ion fragmentation and ion-molecule chemistry of CF3I, investigated in parallel with the present work, as well as neutral fragmentation process and radical reactions observed during photolysis. However, a comparison of the present work with result from other laboratories shows that CF3I can break down under different radiolysis conditions to give C2F6 plus I2 (Shah, et al.(7), CF4 plus CF2I2 (McAlpine and Sutcliffe(8,9), or CF4+I2+(CF2) n (present work). Some suggestions are presented concerning the factors which control branching of the reaction pathways during radiolysis of this compound.

Radiolytic processes in mixtures of cyclobutane and perfluorocyclobutane in the gas phase

Abstract The gas phase radiolysis of mixtures of cyclobutane and perfluorocyclobutane has been studied for systems containing 0–100 per cent of the fluorocarbon. The decomposition of cyclobutane to hydrocarbon products is sensitized by the presence of the fluorocarbon, but the hydrogen yield is directly proportional to the fraction of the energy deposited in the cyclobutane. It is suggested that energy is transferred from excited perfluorocyclobutane to cyclobutane molecules, which subsequently decompose, ultimately forming hydrocarbon products including ethane, ethylene, propane, butenes and others. It appears that unsaturates formed in the radiolysis scavenge free H· atoms, leaving only the unscavengable H2 yield for detection. It is suggested that electron capture by perfluorocyclobutane, followed by a neutralization reaction with the parent positive ion, leads to efficient rupture of the CF bond and production of HF, as in other fluorocarbon-hydrocarbon mixtures. However, the concomitant interference with the production of H· atoms, seen in other systems, does not lead to a significant lowering of the H2 yield, which arises largely from other sources in cyclobutane radiolysis. Twenty-eight products have been identified in all, and yields over the entire mixture range have been recorded for twenty-four of them. The majority of the products contain only carbon and hydrogen. Four perfluorocarbon compounds were found, as well as three mixed species containing both fluorocarbon and hydrocarbon moieties. Such products are probably formed by combination of radicals from both parent compounds.

The radiation chemistry of methyl iodide in the gas phase

Abstract The 60 Co-gamma radiolysis of pure methyl iodide vapor at 300 torr (40 kN m −2 ) and 25°C has been studied. Major products found and their respective G-values are: I 2 , 0·16; HI, 0·13; H 2 , 0·55; CH 2 I 2 , 1·4; CH 4 , 2·9; C 2 H 2 , 0·12; C 2 H 4 , 0·065. Addition of I 2 as a radical scavenger reduced the initial G value for methane production to 1·4. The mechanism for production of CH 2 I 2 and molecular CH 4 was assumed to involve hot methyl radicals. Iodine scavenging did not alter G (H 2 ), G (CH 2 I 2 ), G (C 2 H 2 ) or G (C 2 H 4 ), indicating that molecular processes may account for as much as 70 per cent of all observed product formation. Ethane was found to be a minor product; the initial 100 eV yield of about 0·07 was reduced to 0·0055 by scavengers added initially or produced in the radiolysis. Use of HI as a radical scavenger resulted in a low initial G (C 2 H 2 ) which increased with dose to its value in pure CH 3 I. The C 2 H 4 yield was nearly twice that observed in the pure system. The rate of methane production showed an initial maximum of about 25 molecules/100 eV, and the G-value decreased to about 2·6 with a dose of 1 × 10 20 eV. A chain mechanism is proposed to account for the high CH 4 yields in the presence of excess HI, and some of the G (CH 4 ) observed in pure CH 3 I.