Rotationally adiabatic Saddle point

Fig. 11 Representation of lowest adiabatic potential of singlet (S = 0) and triplet (S = 1) Fe(CO)4 around T Jahn-Teller conical intersection at tetrahedral (7 ) geometry. There are three equivalent two-dimensional troughs in the space spanned by each pair-wise selection of equal L-M-L angles (boxed vs unboxed). The topological connectivity where the troughs intersect is indicated. There are two non-equivalent epikemel distortion directions E[ 2(Td,h) leading to 6 equivalent C2v minima ( ), and 12 equivalent Cs(x) saddle-points respectively. The non-Berry pseudo-rotation barrier is very small ( 5kcal mol ). CASSCF optimised geometrical parameters for singlet and triplet states are shown at the top left...

The situation is quite different in bimolecular reactions with an activation energy (E >0). In particular, the "diatomic" model is certainly a bad approximation for radical-radical rebinding along a double bond in which the maximum of the effective potential (35 IV) lies near the saddle-point of the potential energy surface /141/, In this case no central forces govern the nuclear motion hence, the total angular momentum is not a constant, which means that the reaction cannot be rotationally adiabatic. Therefore, in this situation the statistical theory cannot correctly reproduce the results of the simple collision theory. 

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