Transition State Geometric Structure in the Adiabatic PT Picture

5 Transition State Geometric Structure in the Adiabatic PT Picture 

We have emphasized, throughout, the quite different perspectives of the standard PT and the adiabatic PT pictures, for the reaction coordinate and the relevant barriers, as well as for the rate constant and KIEs. While we have argued for the validity of the adiabatic PT picture, it is useful to pause here and add an important remark. One of the most widespread and important uses of KIEs is in making inferences, via the standard PT picture, about the geometrical structure of the TS of the PT reaction [14, 15, 45]. Indeed, images of the entire reaction path are generated in this fashion. This obviously involves a classical perception of the coordinates, and it is important to ask whether such assessments can legitimately be made when the coordinates of the acid-base PT system are treated classically. 

We have addressed this issue in Ref. [3b], where, for a model PT system in solution treated in the adiabatic PT fashion, we have generated a certain reaction path in the following fashion. At each value of the solvent coordinate AE, we have calculated the quantum averaged values q and Q of the proton and H-bond coor- 

11 [3b] and is compared with a reaction path generated from a bond energy-bond order (BEBO) model [49], which is often used in the standard picture of PT and which completely ignores the solvent. It is seen that the two curves are quite close despite distinct differences between the two methods, most notably that in one case the proton and H-bond coordinates are treated fully quantum, while in the other case they are treated completely classically. This comparison demonstrates that one, in a certain sense, can retain the picture of a path in terms of quantum averaged coordinates and the connection between transition state structure and reaction asymmetry [3, 4[. 

---