Multiple reaction paths, single-product

EXPLORING MULTIPLE REACTION PATHS TO A SINGLE PRODUCT CHANNEL... 

Figure 12 (from the chapter Exploring Multiple Reaction Paths to a Single Product Channel ). Two-dimensional cut through the potential surface for fragmentation of the transition state [OH CH3 ] complex as a function of the CF bond length and the FCO angle. All other coordinates are optimized at each point of this PES. Pathway 1 is the direct dissociation, while pathway 2 leads to the hydrogen-bonded [CH3OH F ] structure. The letter symbols correspond to configurations shown in Fig. 11. Reprinted from [63] with permission from the American Association for the Advancement of Science.

Figure 16 (from the chapter Exploring Multiple Reaction Paths to a Single Product Channel ). Projections onto 2D surfaces of trajectories (in green) of CH3O — H2 -H HCO. The left column is a projection onto the surface of Fig. 15. The right column is a projection onto the surface of Fig. 14. The black contour represents the saddle point energy for the H+ H2CO H2 + HCO reaction. Blue contours are lower in energy red contours are higher. Reprinted with permission from [67]. Copyright 2001 American Chemical Society.

Exploring Multiple Reaction Paths to a Single Product Channel... 

Carpenter and Borden made two important conclusions that raise significant concerns about the traditional physical organic notions of reaction mechanisms. First, nonstatistical dynamics can occur even when intermediates exist in relatively deep potential energy wells, not just on flat caldera-like surfaces. Second, multiple products can be formed from crossing a single TS. The steepest descent path from a TS can only link to a single product, but reactions can follow nonsteepest descent paths that reach different products. 

For a simple bistable reaction potential, it is clear that maximum curvamre along the reaction pathway will occur near the extrema—the minima and the barrier top. The path endpoints are typically chosen to sit in the reactant and product minima, and in such a case the maximum error will result from the path straddling the barrier top as in Figure 8. Of course, this is the error made in a single segment of the pathway. For a general potential the pathway will consist of multiple segments and may have many barriers. 

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