Three-dimensional geometry, transition

Both these methods require equilibrium constants for the microscopic rate determining step, and a detailed mechanism for the reaction. The approaches can be illustrated by base and acid-catalyzed carbonyl hydration. For the base-catalyzed process, the most general mechanism is written as general base catalysis by hydroxide in the case of a relatively unreactive carbonyl compound, the proton transfer is probably complete at the transition state so that the reaction is in effect a simple addition of hydroxide. By MMT this is treated as a two-dimensional reaction proton transfer and C-0 bond formation, and requires two intrinsic barriers, for proton transfer and for C-0 bond formation. By NBT this is a three-dimensional reaction proton transfer, C-0 bond formation, and geometry change at carbon, and all three are taken as having no barrier. 

The behavior of CA is linked to the geometry of the lattice, though the difference between running a simulation on a lattice of one geometry and a different geometry may be computational speed, rather than effectiveness. There has been some work on CA of dimensionality greater than two, but the behavior of three-dimensional CA is difficult to visualize because of the need for semitransparency in the display of the cells. The problem is, understandably, even more severe in four dimensions. If we concentrate on rectangular lattices, the factors that determine the way that the system evolves are the permissible states for the cells and the transition rules between those states. 

Fig. 3. Fe(CO)4 cross section of the Jahn-Teller surface around a tetrahedral geometry (Td), which has a triply degenerate singlet electronic state. The surface is a two-dimensional cross section through the three-dimensional Jahn-Teller surface. There are four equivalent C2v minima connected via four equivalent Cs transition structures. The CASSCF CFeC angles are given to the left. Further C2v minima and Cs transition structures exist in the remaining orthogonal coordinate.

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