Distribution of Reaction Products. VI. Hot-Atom Reactions, T + HR

Abstract

A three-atom model has been employed in a first study of the dynamics of the reactions of hot tritium with hydrogen-containing organic molecules, e.g., T + CH4. After exploring many extended-London–Eyring–Polanyi–Sato (LEPS) potential-energy hypersurfaces of the type introduced in Part II, a surface was obtained which was in qualitative accord with experiment in that it predicted predominantly abstraction at the low end of the hot-atom range of energies (taken to be 2 eV). Abstraction (ABS) consists in T + HR → TH + R; displacement (DIS): T + HR → TR + H; fragmentation (FRAG): T + HR → T + H + R. The model was employed in a computer study of the 3-D classical dynamics of abstraction, displacement, and fragmentation in the prototype reaction T + HR and in isotopic variants D + HR, T + DR, and T + HR′ (masses H = 1, D = 2, T = 3, R = 15 and R′ = 31 amu). The quantities calculated were the total reactive cross section as a function of collision energy (2–18 eV), the partial reactive cross section as a function of the initial THR angle α, and the partial reactive cross section as a function of the initial impact parameter b. In addition, product vibrational, rotational, and translational energy distributions, and product angular distributions, were computed. The principal findings were (i) that the abstraction and displacement both constituted direct (as opposed to complex) and concerted (in contrast to sequential) reactions. The outcome of a particular reactive encounter depended on a delicate balance between strong repulsive forces, and, consequently, was no easier to predict for these hot-atom reactions than for thermal ones. (ii) Displacement was favored at intermediate collision energy (4–6 eV) because of the moderating effect that attraction from the heavy R group produced in the speed of T. (iii) At high energies (≳7 eV) a new, stripping, reaction path opened up which made abstraction again dominate displacement; consequently, over all, the mean collision energy for abstraction exceeded that for displacement. This is in accord with recent experiments. (iv) In general, translational energy in the products accounted for the largest part of the collision energy, with a fairly broad energy distribution. (v) At 2–4-eV collision energy the peak of the angular distribution for the molecular product was sideways following abstraction, backwards following displacement; higher collision energy shifted both peaks (especially abstraction) in the forward direction. (vi) Fragmentation accounted for only a few percent of the total reaction at collision energies 25% in excess of that required for formation of T + H + R, but at higher energies (≳7 eV) was comparable in importance to abstraction. (vii) At 2–4-eV collision energy the cross section for abstraction decreased when T was replaced by a mass equivalent to D, H by D, or R by R′. The cross section for displacement also decreased when T was replaced by D, or H by D (providing further evidence of concerted reaction), and increased when R was replaced by R′.