Gibbs barrier

Fig. 3.3 Reaction of nitrosoperoxycarbonate anion with guanine, a Initial reactant complex, b, c, d Intermediates and e Product complex (S-oxoG + NO + COj). Gibbs barrier (positive) and released (negative) energies (kcal/mol) at each step obtained at the B3LYP/AUG-cc-pVDZ level in aqueous media are given near the arrows. The last step is barrierless [65]...

Fig. 3.5 a Reactant complex, b Intermediate complex, and c Product complex consisting of G" with NOj in presence of a water molecule. The water molecule facilitates proton transfer. Gibbs barrier above the arrows) and released below the arrows) energies (kcal/mol) at each step are given. A negative barrier energy implies a barrierless reaction [63]... 

Geometric mean approximation, 29 Germane barrier of internal rotation, 391 Gibbs, free energy, 30 function, 20... 

Table A4.6 gives the internal rotation contributions to the heat capacity, enthalpy and Gibbs free energy as a function of the rotational barrier V. It is convenient to tabulate the contributions in terms of VjRTagainst 1/rf, where f is the partition function for free rotation [see equation (10.141)]. For details of the calculation, see Section 10.7c.

To see how the catalyst accelerates the reaction, we need to look at the potential energy diagram in Fig. 1.2, which compares the non-catalytic and the catalytic reaction. For the non-catalytic reaction, the figure is simply the familiar way to visualize the Arrhenius equation the reaction proceeds when A and B collide with sufficient energy to overcome the activation barrier in Fig. 1.2. The change in Gibbs free energy between the reactants, A -r B, and the product P is AG. 

Fig. 1. Schematic one-dimensional cross section through the Gibbs free energy surface G(R) of a spin-state transition system along the totally symmetric stretching coordinate. The situation for three characteristic temperatures is shown (B = barrier height, ZPE = zero-point energy, 28 = asymmetry parameter, J = electronic coupling parameter, AG° = Gh — GJ...

Chemical models of photosynthesis have been used to investigate two types of reactions photosynthesis and photocatalysis. In photosynthetic processes the standard Gibbs free energy of the reaction is positive, and solar energy is utilized to perform work. In photocatalytic processes the free energy is negative and solar energy is used to overcome the activation barrier. 

The activation energy is now made up of two parts, one part being the Gibbs energy to create a vacancy, —AGv, and the other to surmount the energy barrier, — AGm. The entropy terms can be incorporated into the geometrical factor to give... 

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