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

Abstract

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.

Steady State Kinetic Theory Model Calculations

“…Under favorable circumstances reactive collision energy Information can be obtained from hot atom mixture experiments [13,19,20,[32][33][34][35][36][37]. Both inert and reactive additives may induce changes in the secondary product distributions that accompany energetic '*F substitution reactions.…”

“…However, recent evidence has shown that Information concerning the energy ränge characteristics of nonthermal reactions is difficuh, if not impossible, to obtain from conventional moderator experiments [14,16,17]. In the present study the influence of Ar additive upon the dynamical and energetic features of the nonthermal + Hj reaction [19][20][21][22][23] has been elucidated with the aid of the steady State kinetic theory of hot atom reactions [21,24,25]. This work represents the first adaptation of the steady State theory for modeling a mukicomponent reaction system.…”

“…The %"{t), and K (f) terms foUow directly from the Boltzmann equation [21,26] together with analytical representations for the quasiclassical trajectory reactive cross sections [20,26].…”

“…Theoretical reactive cross sections obtained from quasiclassical trajectory caiculations [20] have been utilized in simulations of the nuclear recoil '*F + H^ process [21][22][23]. The computational procedure developed at this laboratory, which has been described in detail elsewhere [21,26], has utilized a local equilibrium Solution of the time dependent Bohzmann equation.…”

“…(5) designate energy restoring collisions. A Single quantum State model is sufficiently detailed for present purposes since reactive cross sections for the nonthermal ''F + Hj process are insensitive to the H2 rotational quantum number over the ränge of states populated at 300°K [20,29,30].…”