Location of Energy Barriers. IV. Effect of Rotation and Mass on the Dynamics of Reactions A + BC

Abstract

In Paper I of the present series a study was made of the effect of barrier location on the dynamics of thermoneutral reaction $\mathrm{A}+\mathrm{BC}\to \mathrm{AB}+\mathrm{C}$ , for which the atomic masses were m_A=m_B=m_C. Two contrasting potential‐energy hypersurfaces were used; on ``surface I'' the crest of the 7 kcal mole<sup>−1</sup> energy barrier was slightly displaced $\text{(∼ 0.3 Å)}$ into the entry valley of the collinear energy surface, whereas on ``surface II'' the crest of the barrier was displaced by the same amount into the exit valley. Precisely the same potential‐energy hypersurfaces have been used in the present work as were used in Paper I. The present work was undertaken to examine the effect (a) of the inclusion of a small but significant amount of rotational energy in the reagents, and (b) of a change in reagent masses from the extreme case $\mathrm{L}+\mathrm{HH}\mathrm{\hspace{0.17em}(}\mathrm{L}\text{≡ 1\hspace{0.17em}}\mathrm{amu},\mathrm{H}\text{≡ 80\hspace{0.17em}}\mathrm{amu})$ to the opposite extreme $\mathrm{H}+\mathrm{HL}$ . [These mass combinations were identified as extreme cases, the former giving rise to the ``light‐atom anomaly'' and the latter to a maximum of ``mixed energy release,'' in earlier work, see J. Chem. Phys. 44, 1168 (1966)]. The qualitative generalizations introduced in Paper I are found to remain valid despite the introduction of variables (a) and (b), above. Of these generalizations the most important is that reagent translational energy favors reaction on surface I, whereas reagent vibration is the most favorable to reaction on surface II. If barrier location on the ``diagnostic'' collinear potential‐energy surface is to be used as an approximate quantitative guide to reaction dynamics, then the diagnostic surface should be the scaled collinear surface appropriate to the particular mass combination.