Barrier height potential energy surfaces

Fig. 13.11. A schematic drawing of the potential energy surfaces for the photochemical reactions of stilbene. Approximate branching ratios and quantum yields for the important processes are indicated. In this figure, the ground- and excited-state barrier heights are drawn to scale representing the best available values, as are the relative energies of the ground states of Z- and E -stilbene 4a,4b-dihydrophenanthrene (DHP). [Reproduced from R. J. Sension, S. T. Repinec, A. Z. Szarka, and R. M. Hochstrasser, J. Chem. Phys. 98 6291 (1993) by permission of the American Institute of Physics.]...

Figure 9. Potential energy surface for cellobiose at 400 K. The trajectory of conformational changes during a portion of the simulation are shown on the left. Energy contours in the vicinity of minima 1-3 are shown on the right. Barrier heights 5.3 Kcal/mol between minima 1 and 2,1.3 Kcal/mol between 2 and 3. (MM2(85) functions).

A for step (a), B for step (d). The electronic activation energies AVg aloi the reaction path (these are not the effective activation energies) are 15.5 kcal/mole for (a), but only 5.1 kcal/mole for (d). The barrier heights along the —45° sections through the saddle point (AV ) in the potential energy surfaces show the same trend ... 

Note that the conventional TST expression is simply the special case of VTST where evaluation is done exclusively for s = 0. As such, the VTST rate constant will always be less than or equal to the conventional TST rate constant (equal in the event that s = 0 minimizes Eq. (15.35)). Put differently, when very accurate potential energy surfaces are available, the conventional TST rate constant is typically an overestimate of the exact classical rate constant. (Note that it is possible, however, for a compensating or even offsetting error to arise from overestimation of the barrier height if the potential energy surface is not very accurate.)... 

Figure 5. Energetics for CH2CHO dissociation, including qualitative picture of the reaction coordinate for CH3 + CO production on ground-state potential-energy surface. See text for discussion of barrier heights.

Fig. 2. The potential energy surface for the electron motion from a donor to an acceptor in a condensed medium. U,t(r) and Ut, (r) are the potentials of the cores of the donor and the acceptor, the rest of the potentials are created by the molecules of the medium r is the electron coordinate, and R is the distance between the donor and the acceptor. The broken horizontal line corresponds to the under barrier electron motion from the donor to the acceptor. I is the height of the barrier for tunneling.

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