How to guess a transition state structure (continued)

· Can optimize geometry with a particular symmetry enforced - the TS may have a higher symmetry than the reactants or products.

· For example, the TS for the S_N2 reaction Cl^– +CH₃Cl ® CH₃Cl +Cl^– has D_3h symmetry (the two C-Cl distances will be the same, and the H atoms all lie in a plane):

· There is no single technique which can be guaranteed to give a good TS model for all reactions.

· Finding a good model may be a matter of trial and error. A bad model will not optimize to the correct TS.

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