Transition structure geometries

Fig. 5.35 Geometry predicted by CASSCF ab initio calculations of the two possible transition structure geometries for the Diels-Alder reaction between ethene and butadiene. (Figure adapted from Houk KN, J Gonzalez and Y Li 1995. Pericyclic Reaction Transition States Passions and Punctilios 1935-1995. Accounts of Chemical Research 28 81-90.)...

It is also important to always examine the transition structure geometry to make sure that it is the reaction transition and not the transition in the middle of a ring flip or some other unintended process. If it is not clear from the geometry that the transition structure is correct, displaying an animation of the transition vibrational mode should clarify this. If still unclear, a reaction coordinate can be computed. 

An optimization of the transition structure geometry (yields the SCF energy). 

Table 1. Transition structure geometries, activation energies, and reaction energies for transition structures of electrocyclic ring opening of cyclobutene... 

Figure 6-22. Transitions structure geometry for reduction of adamantanone (1 X=H) with AlHj.

Figure 6-24. Transitions structure geometry for syn and anti reduction of 2 with AIH3 (3-2 tCi ).

Table 3 Transition Structure Geometries, Activation Energies, and Reaction Energies of the Concerted Diels-Alder Reaction of 1,3-Butadiene and Ethylene...

Fig. 10.39. Comparison of transition structure geometry for anionic oxy-Cope (right) rearrangement with Cope rearrangement (left) showing atom separation distances and Mulliken charges (B3LYP/6-31IG ). Reproduced from Helv. Chim. Acta, 84, 124 (2001), by permission of Wiley-VCH.

It should be noted that there exists relative insensitivity of radical addition transition structure geometries to computational methods [20]. As a result, the UHF/3-21G level of calculations proves sufficient for determining the qualitative features of transition state structures and relative activation energies for regioisomeric transition structures [20-21]. 

Even though the low-level calculations are able to predict the transition-structure geometries quite well, electron correlation has to be considered for the accurate prediction... 

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