Energy Distribution Among Products of Exothermic Reactions. II. Repulsive, Mixed, and Attractive Energy Release

A series of two-dimensional classical kinematic computer calculations have been made on the hypothetical exothermic exchange reaction A+BC→AB+C, −ΔH=48.5 kcal mole−1. Product energy distribution (vibration, rotation, and translation; Evib, Erot, Etrans) was obtained as a function of initial position, impact parameter, and kinetic energy (αi, b, and Ekini). All eight different combinations of light (L=1 amu) and heavy (H=80 amu) masses were examined. Eight different potential-energy hypersurfaces were explored. All were obtained from an empirical extension of the London—Eyring—Polanyi—Sato (L.E.P.S.) method. The hypersurfaces were categorized in terms of the percentage attraction, A⊥, and percentage repulsion R⊥, read off the collinear three-dimensional surface. This categorization was shown to be helpful in arriving at a qualitative understanding of the product energy distribution to be expected from all the mass combinations reacting on these extended L.E.P.S. hypersurfaces. However, it was also shown that the A⊥, R⊥ categorization could only be of (approximate) quantitative value in predicting product energy distribution for the case that the atomic masses satisfy the inequality mA≪mB, mC. For all other mass combinations three types of energy release must be distinguished; attractive energy release (A approaching BC, at normal BC separation), mixed energy release (A still approaching BC, while BC extends), and repulsive energy release (AB at normal separation retreating from C). These three types of energy release, symbolized A, M, and R were defined. The value of the concept was tested by obtaining % A, M, and R for each mass combination on several energy surfaces of markedly different characteristics, from part of a single collinear trajectory, and then plotting A+M against the mean vibrational excitation, % 〈Evib〉, obtained from a representative group of trajectories on the full hypersurface. For all mass combinations on all the surfaces examined it was found that % (A+M)∼% 〈Evib〉. There was a tendency on surfaces with appreciable repulsive character (significant % A⊥ for the reaction A+BC to give less vibration if A=L than if A=K. This light-atom anomaly became very marked as A⊥ became large. These phenomena could be understood if it was recognized that the mixed energy release had two components; late attractive MA and early repulsive MR, whose proportions on a particular energy surface were characteristic of that surface and were linked to the prop