Transition state energy diagram

Figure Bl.1.3. State energy diagram for a typical organic molecule. Solid arrows show radiative transitions A absorption, F fluorescence, P phosphorescence. Dotted arrows non-radiative transitions.

The Ir(III) trication is a 5d6 center and the electronic properties of its poly-imine complexes share several features with those of other well-known octahedral complexes of Fe(II) [14], Ru(II) [3], Os(II) [4], and Re(I) [5], whose metal centers are 3d6, 4d6, 5d6, and 5d6, respectively. Figure 3 depicts useful orbital and state energy diagrams for electronic transitions taking place in polyimine complexes of such d6 metal centers. 

Unfortunately, the computational studies differ in quantitative detail regarding the importance of the mechanisms that involve either Glu 165 or His 95 as the acid-base catalysts to catalyze interconversion of the tautomeric enediolate intermediates. Friesner and coworkers concluded that the transition state for proton abstraction from DHAP is the highest point on the energy diagram after formation of the enediolate anion intermediate, the calculations predict that the barrier for the criss-cross mechanism catalyzed by Glu 165 is - 3 kcal moU lower than that for classical mechanism involving catalysis of tautomerization of the enediolate intermediates by His 95, so the criss-cross mechanism is predicted to be the favored mechanism. In contrast, Cui and Karplus concluded that transition state energies for tautomerization of the enediolate anion intermediates via an enediol intermediate are isoenergetic for both the classical and criss-cross mechanisms. 

An uncommonly high content of endo isomer has been recorded by Brown on solvolysis of 2-tert-butyl-2-exo-norbornyl-p-nitrobenzoate (exoiendo product ratio is 95 5) The Goering-Schewene diagram (Fig. 6) allows to recognize a higher stability of the ground state of the endo isomer (by 1.9 kcal/mole) and a small difference in transition state energies (1.7 kcal/mole). 

In the first step, the it bond of the alkene is protonated, generating a carbocation intermediate. In the second step, this intermediate is attacked by a bromide ion. Figure 9.1 shows an energy diagram for this two-step process. The observed r ioselectivity for this process can be attributed to the first step of the mechanism (proton transfer), which is the rate-determining step because it exhibits a higher transition state energy than the second step of the mechanism. 

Figure 3-7 Compiete potential-energy diagram for the formation of CH3CI from methane and chlorine. Propagation step 1 has the higher transition-state energy and is therefore slower. The AH° of the overall reaction CH4 + CI2 CH3CI + HCI amounts to -25 kcal mol" (-105 kJ mol" ), the sum of the AH° values of the two propagation steps.

Figure A3.12.10. Schematic diagram of the one-dimensional reaction coordinate and the energy levels perpendicular to it in the region of the transition state. As the molecule s energy is increased, the number of states perpendicular to the reaction coordinate increases, thereby increasing the rate of reaction. (Adapted from [4].)...

A potential energy diagram for nng inversion m cyclohexane is shown m Figure 3 18 In the first step the chair conformation is converted to a skew boat which then proceeds to the inverted chair m the second step The skew boat conformation is an inter mediate in the process of ring inversion Unlike a transition state an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile... 

With the potential energies shown on a common scale we see that the transition state for formation of (CH3)3C is the highest energy point on the diagram A reaction can proceed no faster than its slowest step which is referred to as the rate determining step In the reaction of tert butyl alcohol with hydrogen chloride formation of the... 

Sketch a potential energy diagram for the reaction of 1 heptanol with hydrogen bromide paying careful attention to the positioning and structures of the intermediates and transition states... 

Section 4 9 The potential energy diagrams for separate elementary steps can be merged into a diagram for the overall process The diagram for the reac tion of a secondary or tertiary alcohol with a hydrogen halide is charac terized by two intermediates and three transition states The reaction is classified as a ummolecular nucleophilic substitution, abbreviated as SnI... 

Potential energy diagram (Section 4 8) Plot of potential en ergy versus some arbitrary measure of the degree to which a reaction has proceeded (the reaction coordinate) The point of maximum potential energy is the transition state Primary alkyl group (Section 2 13) Structural unit of the type RCH2— in which the point of attachment is to a pnmary carbon... 

A more complete analysis of interacting molecules would examine all of the involved MOs in a similar wty. A correlation diagram would be constructed to determine which reactant orbital is transformed into wfiich product orbital. Reactions which permit smooth transformation of the reactant orbitals to product orbitals without intervention of high-energy transition states or intermediates can be identified in this way. If no such transformation is possible, a much higher activation energy is likely since the absence of a smooth transformation implies that bonds must be broken before they can be reformed. This treatment is more complete than the frontier orbital treatment because it focuses attention not only on the reactants but also on the products. We will describe this method of analysis in more detail in Chapter 11. The qualitative approach that has been described here is a useful and simple wty to apply MO theory to reactivity problems, and we will employ it in subsequent chapters to problems in reactivity that are best described in MO terms. I... 

Such diagrams make clear the difference between an intermediate and a transition state. An intermediate lies in a depression on the potential energy curve. Thus, it will have a finite lifetime. The actual lifetime will depend on the depth of the depression. A shallow depression implies a low activation energy for the subsequent step, and therefore a short lifetime. The deeper the depression, the longer is the lifetime of the intermediate. The situation at a transition state is quite different. It has only fleeting existence and represents an energy maximum on the reaction path. 

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