The Symmetrical Potential Energy Barrier

It is possible to use the 3-D surface with its corresponding 2-D contour diagram and potential energy profile to discuss the general reaction A + BC — AB + C where A, AB, BC and C are all polyatomic molecules. 

An Introduction to Chemical Kinetics. Margaret Robson Wright 2004 John Wiley Sons, Ltd. ISBNs 0-470-09058-8 (hbk) 0-470-09059-6 (pbk) 

In the entrance valley rAB is large and decreases towards the col. At the entrance to the exit valley rBC is large, and decreases towards the col. At the col rAB = rBC, and this holds for all points along the line, PQ, as drawn. On the profile the activated complex lies at a maximum where rAB = rBC- 

Other situations can occur where the activated complex lies in the entrance valley, an early barrier, or in the exit valley, a late barrier. 

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