Barrier height computational

Finally, Keil and Ahlrichs,249 reported PNO-CI and CEPA calculations on several Sn2 reactions, including that giving CH5-. The barrier height computed was 236 kJ mol-1, and the inclusion of correlation decreased the barrier by ca. 30 kJ mol-1. The larger drop found in ref. 245 was probably an artefact of the IEPA method. The errors in these very extensive calculations were believed to be only 10—30 kJ mol-1, which is indicative of the accuracy now attainable. 

Figure 9.6 The barrier height computed as a function of the 0---0 distance forthe isolated ion (dashed line) and forthe ion in the crystal (solid line).

FIGURE 12 Water dimer tunneling paths. The numbers below transition states are barrier heights computed from the symmetry-adapted perturbation theory (SAPT)-5s potential. 

We have seen that the cooperative region, which represents a nominal dynamical unit of liquid, is of rather modest size, resulting in observable fluctuation effects. Xia and Wolynes [45] computed the relaxation barrier distribution. The configurational entropy must fluctuate, with the variance given by the usual expression [77] 5Sc) ) = Cp barrier height for a particular region is directly related to the local density of states, and hence to... 

Comparison of Barrier Heights (BH) in kcal mol for the Proton Transfer in Malonaldehyde Computed by Different Quantum Chemical Methods... 

One of the simplest chemical reactions involving a barrier, H2 + H —> [H—H—H] —> II + H2, has been investigated in some detail in a number of publications. The theoretical description of this hydrogen abstraction sequence turns out to be quite involved for post-Hartree-Fock methods and is anything but a trivial task for density functional theory approaches. Table 13-7 shows results reported by Johnson et al., 1994, and Csonka and Johnson, 1998, for computed classical barrier heights (without consideration of zero-point vibrational corrections or tunneling effects) obtained with various methods. The CCSD(T) result of 9.9 kcal/mol is probably very accurate and serves as a reference (the experimental barrier, which of course includes zero-point energy contributions, amounts to 9.7 kcal/mol). 

Table 13-7. Computed classical barrier heights AE [kcal/mol] for the reaction H2 + H — [ II II 111 —> H2 + H (6-311++G(,3pd) basis set) data compiled from Johnson et al., 1994, and Csonka and Johnson, 1998.

In computer simulations we had chosen the following parameters of the potential h 1, a = 2. With such a choice the coordinates of minima equal Xmin = 1, the barrier height in the absence of driving is A = 1, the critical amplitude Ac is around 1.5, and we have chosen A = 2 to be far enough from Ac. In order to obtain the correlation function K t + x, t] we solved the FPE (2.6) numerically, using the Crank-Nicholson scheme. 

The barrier height for chloride anation of diaquated cisplatin has been determined experimentally to be 16.6 kcal mol-1 [60, 61], in good agreement with current computational results of 18.2 kcal mor1. The reaction is endothermic by ca 4.5 kcal mol-1. 

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