Double dagger, superscript

The position of this equilibrium is related to the free energy required for attainment of the transition state. The double-dagger superscript ( ) is used to specify that the process under consideration involves a transition state or activated complex ... 

We have seen that the addition of HBr to 2-butene is a two-step reaction (Section 3.6). The structure of the transition state for each of the steps is shown below in brackets. Notice that the bonds that break and the bonds that form during the course of the reaction are partially broken and partially formed in the transition state—indicated by dashed lines. Similarly, atoms that either become charged or lose their charge during the course of the reaction are partially charged in the transition state. Transition states are shown in brackets with a double-dagger superscript. 

The activation energies for the reactions of species B with the species AX and AY with different substituents X and Y are the energy changes for the processes B + AX—>[BAX] and B + AY—>[BAY], where the double-dagger superscript denotes the transition state. The difference in activation energies for these two reactions corresponds to the hypothetical process AX — AY [BAX] - [BAY] or AX -I- [BAY] —>AY + [BAX]. This is an isodesmic process, for which we can expect the correlation contribution to the energy change to be small and the Hartree-Fock method to work well. 

Like AG°, AG has both an enthalpy component and an entropy component. Notice that any quantity that refers to the transition state is represented by a double-dagger superscript ( ). 

Unlike the reactants or products, a transition state is unstable and cannot be isolated. It is not an intermediate, because an intermediate is a species that exists for some finite length of time, even if it is very short. An intermediate has at least some stability, but the transition state is a transient on the path from one intermediate to another. The transition state is often symbolized by a superscript double dagger ( ), and the changes in variables such as free energy, enthalpy, and entropy involved in achieving the transition state are symbolized AG, A//, and AS. AG is similar to Ea, and the symbol AG is often used in speaking of the activation energy. 

How can we draw the structure of the unstable transition state The stmcture of the transition state is somewhere in between the structures of the starting material and product. Any bond that is partially broken or formed is drawn with a dashed line. Any atom that gains or loses a charge contains a partial charge in the transition state. Transition states are drawn in brackets, with a superscript double dagger ( ). 

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