John W. Root

Hydrogen abstraction reactions by atomic fluorine. V. Time‐independent nonthermal rate constants for the 18F+H2 and 18F+D2 reactions

The general time‐independent collision theory formulation for the bimolecular rate constant has been adapted for the description of hot atom systems. Two types of hot atom energy distribution functions have been considered in an application to the 18F+H2 reaction system: (i) a δ‐function distribution, and (ii) a steady‐state Maxwellian distribution characterized by a hot atom temperature TA. From the time‐independent solution of the Boltzmann equation together with microscopic reactive cross sections determined from quasiclassical trajectory computations, nonthermal 18F+D2 processes. The results showed little sensitivity to the assumed shape of the hot atom energy distribution or to the magnitude of the barrier height along the reaction coordinate. The intermolecular kinetic isotope effect κH2/κD2 provided a sensitive probe of the average energy of hot reaction, suggesting an average 18F laboratory kinetic energy of 50±10 eV for the 18F+H2 process under nuclear recoil conditions.

Hydrogen abstraction reactions by atomic fluorine. III. Temperature dependence of the intermolecular kinetic isotope effect for the thermal F+H2 reaction

The intermolecular kinetic isotope effect for the title reaction has been investigated using a moderated nuclear recoil technique. The following Arrhenius temperature dependence was obtained: kH2/kD2 = (1.04±0.06)exp[(382±35)/RT]. This result is in quantitative agreement with an independent study based upon a discharge flow reactor method. The advantages and limitations of the nuclear recoil procedure are considered, and relevant literature results are critically reviewed.

Chemistry of Nuclear Recoil 18F Atoms. V. Mechanism and Systematics in CH3CF3

Nuclear recoil 18F atoms undergo hot F‐for‐F and F‐for‐H atomic substitution and hot F‐for‐CH3 and F‐for‐CF3 alkyl replacement reactions in CH3CF3. The primary absolute yields corresponding to these processes are 3.56 ± 0.07, 8.22 ± 0.09, 5.79 ± 0.31, and 8.5 ± 2.5 % (estimated value), respectively. The total primary hot yield for organic products is 26.1 ± 2.5 %, and that for all hot reactions including F‐to‐HF and F‐to‐F2 abstraction is 83 ± 3 %. There is no evidence in favor of hot F‐for‐2F or F‐for‐2H double substitution reactions in CH3CF3. Recoil 18F exhibits approximately a sixfold systematics preference for alkyl replacement reactions at the carbon—carbon bond in CH3CF3 relative to the average of substitution reactivities at carbon—fluorine and carbon—hydrogen bonds. The per‐bond preference for primary substitution reactions at carbon—hydrogen relative to carbon—fluorine bonds is 2.30 ± 0.06. The sums of primary hot yields for organic products are comparable for recoil 18F in CH3CF3 vs recoil 3H i...

Chemistry of Nuclear Recoil 18F Atoms. VI. Approximate Energetics and Molecular Dynamics in CH3CF3

Energetics and molecular dynamics results are reported from an extensive set of high energy recoil 18F experiments with CH3CF3. Based upon thermochemical evidence alone, substantial fractions of the primary hot F‐for‐H, F‐for‐CH3, and F‐for‐CF3 reaction products are indicated to involve minimum excitation energies of 7.9 ± 0.2, 9.3 ± 0.1, and 3.5 ± 0.2 eV, respectively. The primary F‐for‐F reaction products in CH3CF3 do not exhibit unimolecular decomposition via a carbon—carbon bond scission mode in apparent violation of RRKM theoretical predictions. The primary F‐for‐H products decompose both via β elimination of HF and via carbon—carbon bond scission in apparent accord with theory. More than one kind of microscopic dynamics is involved in the primary hot F‐for‐H and F‐for‐CH3 processes in CH3CF3 and in the primary hot F‐for‐F process in CF4. Direct, concerted, and collusive dynamics are required for the higher energy reaction modes for these processes.

Significance of limiting low pressure product yields from energetic atomic exchange reactions

Abstract The limiting low pressure yield behavior has been characterized for gas phase substitution reactions of energetic 3 H and 18 F atoms with aliphatic and alicyclic substances. The limiting yields correlate well with the respective critical energies for secondary unimolecular decomposition. The interpretation of this behavior leads to quantitative descriptions for the hydrogen-for-hydrogen and F-for-F primary product internal excitation energy distributions together with approximate descriptions of total energy disposal for the threshold hydrogen-for-hydrogen processes.

Chemistry of nuclear recoil 18F atoms. IX. High‐pressure investigation of 1,1,1‐trifluoroethane

Nuclear recoil 18F reactions in CH3CF3 have been investigated throughout the effective pressure range 0.3–170 atm. The principal reaction channel is F‐to‐HF abstraction for which the combined yield from thermal and energetic processes in the presence of 5 mol% H2S additive is 84.4%±0.1%. Organic‐product‐forming channels include F‐for‐F, F‐for‐2F and F‐for‐H atomic substitution and F‐for‐CH3 and F‐for‐CF3 alkyl replacement with respective primary absolute yields of 4.21%±0.10%, 0.26%±0.03%, 5.75%±0.14%, 1.04%±0.03%, and 1.33%±0.04%. With the exception of the F‐for‐2F channel substantial portions of the organic primary products contain sufficient internal excitation to induce secondary decomposition. At low pressures the average fractional decompositions following single substitution (F‐for‐X) and alkyl replacement (F‐for‐R) reactions are 0.80±0.03 and 0.27±0.04. Alkyl replacement products are fully stabilized through collisional deactivation at pressures below ∼13 atm. At 170 atm only 0.60±0.05 of the single‐substitution products have undergone collisional stabilization, representing 0.50±0.04 of the species capable of decomposition. Experiments with CH3CF3/C3F6 mixtures demonstrated average reaction energy differences for F‐to‐HF and organic‐product‐forming processes in CH3CF3 vs olefinic addition in C3F6.

Chemistry of nuclear recoil 18F atoms. VIII. Mechanisms and yields of caging reactions in liquid phase 1,1‐difluoroethane and 1,1,1‐trifluoroethane

New procedures are reported for the specification of caging yields in nuclear recoil chemistry experiments. All five hot 18F substitution channels in CH3CF3 and CH3CHF2 exhibit caging at large density. The respective total caged yields at 195 °K are 4.0%±0.6% and 5.6%±0.6%, and the total yields of stabilized substitution products are 8.9%±0.4% and 8.6%±0.6%. The simplest plausible caging mechanism involves primary Franck–Rabinowitsch radical recombination of 18F atoms with aliphatic radicals. Density‐variation results cannot be used for the qualitative detection of caging reactions unless excitation‐stabilization complications have been shown to be unimportant.

New cross section functions for hot 18F atom collisions with H2, HD and D2

Abstract New analytic reaction cross-section functions are reported for hot 18 F atoms reacting with H 2 , HD and D 2 . A realistic model is proposed for the total non-reactive cross section in these systems throughout the center-of-mass collision energy range 0–100 eV.

Proton stopping powers: Binary encounter calculations based on accurate speed distributions for target electrons

Abstract Atomic and molecular electronic stopping powers for medium energy protons (≈ 10 keV-10 MeV) have been calculated using the binary-encounter approximation in conjunction with (1) either an energy or maximum impact parameter cut-off based on minimum excitation energies; and (2) ab initio electronic speed distributions. The maximum impact parameter approach yields good agreement with experiment for inert gases and closed-shell polyatomic molecules comprised of first-row atoms.

Isotope effect for the thermal F + H2 reaction: An Arrhenius kinetics experiment based upon nuclear recoil techniques

Abstract Modified nuclear hot atom techniques have been used to characterize the kinetic isotope effect for the reaction of atomic fluorine with molecular hydrogen under conditions of thermal initiation. The following Arrhenius temperature dependence was obtained for the isotopic rate constant ratio: k H 2 / k D 2 = (1.11 ± 0.05) exp [(356 ± 26)/ RT ] temperature range 303–475°K.