Reaction free energy barrier

One of the main advantages of the MD over the static quantum chemical approaches is that it can be utilized to directly determine the reaction free energy barriers, as it explicitly includes entropic effects. An estimation of the free energy via a normal (static) DFT approach requires frequency calculations that are relatively expensive for large molecular systems. Such an approach assumes in addition the harmonic (normal mode) approximation, which breaks down for processes where weak intermolecular forces dominate.10... 

Figures 10.8(a) and (b) also indicate that the TS position A along the solvent coordinate shifts with reaction asymmetry. This is due to the fact that the addition of the ZPE to an asymmetric (cf. Fig. 10.8(b)) shifts the maximum of G away from the maximum of at AE = 0, in the direction consistent with the Hammond postulate [15], e.g. later for endothermic reactions AGrxn>0- This indicates that the ZPE contribution at A to the free energy barrier AGl in the solvent coordinate will increase with increasing reaction asymmetry, a crucial qualitative characteristic [3] to which we will return. Also visible in Fig. 10.8(b) is the isotope dependence of the shift AEi- and the associated increase in ZPE at AEi- with increasing reaction asymmetry the latter leads to a KIE reduction, since the ZPE contribution at A will become more and more similar to that of the reactant. Here one can see that the variation in ZPE along the reaction coordinate and its isotopic difference plays a significant role in the reaction free energy barrier variation, and hence the KIE as well.

Here AGIlo.o is the 0-0 reaction free energy barrier Eq. (10.33) and /AAG, ... 

Reaction free energy barrier Agi Free energy function... 

Figure A3.8.1 A schematic diagram of the PMF along the reaction coordinate for an isomerizing solute in the gas phase (frill curve) and in solution (broken curve). Note the modification of the barrier height, the well positions, and the reaction free energy due to the interaction with the solvent.

Free energies, barriers and reactivity patterns in oxidation-reduction reactions. N. Sutin. Acc. Chem. Res., 1968,1, 225-231 (39). 

Previous SECM studies have suggested that a Butler-Volmer type approximation could be used for the ITIES at low driving forces [83], For a system where an ET reaction occurs between an aqueous electron donor and an oxidant in the organic phase, the free energy barrier is given by... 

With the characterized mechanism, the next key question is the origin of its catalytic power. A prerequisite for this investigation is to reliably compute free energy barriers for both enzyme and solution reactions. By employing on-the-fly Born-Oppenheimer molecular dynamics simulations with the ab initio QM/MM approach and the umbrella sampling method, we have determined free energy profiles for the methyl-transfer reaction catalyzed by the histone lysine methyltransferase SET7/9... 

Table 14-1. Calculated reaction free energies and barrier heights (kcal/mol) for uncatalyzed model and catalyzed transesterification reactions in solution and in the hairpin ribozyme3...

The reaction of eh with H2 is very slow ( 105 M-1s-1), due to a positive free energy barrier. Nitrogen has negative electron affinity it is unreactive toward eh. 02 has a high electron affinity, and it reacts with eh with a diffusion-controlled rate (see Table 6.6). The immediate product of the reaction, 02- or its acidic form HOz, reacts further with itself, giving H202and Or... 

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