Location of energy barriers. VI. The dynamics of endothermic reactions,ab + c

A systematic 3D classical-trajectory study has been made of the effect of reagent energy distribution and mass combination on the dynamics of endothermic reaction. The potential-energy hypersurface was endothermic by 35.7 kcal mole^?1. Reagent translation, T′, and vibration, V′, was varied in the range up to T′ + V′ = 90 kcal mole^?. Among the principal findings for the equal-mass case were the following. (i) There is an orders-of-magnitude increase in reaction cross section when energy is transferred from T′ to V′, until an optimal distribution over T′ and V′ is achieved at V′ ?, T′. There is a qualitative difference between the forms of the reactive cross section dependences S(T′) V′ and S(V′)_T′. (ii) The greater efficacy of V′ than T′ in promoting endothermic reaction is marked even at total reagent energies ≈ 3 x the endothermic barrier height. (iii) The total reactive cross section for endothermic reaction increases to large values (≈ gas kinetic) as V′ increased to several times the barrier height. (iv) Reagent energy in excess of that required to cross the endothermic barrier is subject to the same “adiabatic” transformation into product energy as was previously observed for exothermic reactions: ΔT′ → ΔT + ΔR (where R is enhanced product rotational energy), and ΔV′ → ΔV. This is explained in terms of induced repulsive energy release in the former case, and induced attractive energy release in the latter. (v) The molecular product is backwards or sideways scattered. Enhanced V′ at constant T′ diminishes product repulsion, giving rise to more-forward scattering. (vi) These findings remain qualitatively unchanged when extreme mass combinations H H + L and L H + H (L = 1 amu, H = 80 amu) are substituted for the equal-mass combination, L L + L. Effects ascribable to a “late” barrier are accentuated for HH + L. and diminished for L H + H. (vii) The total reactive cross section at a given reagent energy increases in the sequence H H + L, L L + L, L H + H, for simple kinematic reasons. (viii) The dynamics on this endothermic potential-energy surface resemble, in all the points listed above, the dynamics on a thermoneutral surface which has its barrier crest in the coordinate of separation (surface II of part I of this series).