Thermodynamic cycle effect

Because these stability measurements pertain to the gas phase, it is important to consider the effects that solvation might have on the structure-stability relationships. Hydride affinity values based on solution measurements can be derived from thermodynamic cycles that relate hydrocarbon p T, bond dissociation energy and electrochemical potentials. The hydride affinity, AG, for the reaction... 

Energy effects associated with the dissolution of a given substance (which in the following is distinguished with the index " ) can be determined experimentally. They depend on the system s initial and final state, but not on the path taken by the process. Hence, for calculations, the device of thermodynamic cycles is often used, where the true path of the process is replaced by another path (which may even be a path that actually cannot be realized) for which the energy effects of the individual intermediate steps can be determined. 

Since electrostatic effects dominate the thermodynamic cycle as shown in Figure 10-2, major development efforts have focused on the calculation of electrostatic energy for transferring the neutral and charged forms of the ionizable group from water with dielectric constant of about 80 to the protein with a low dielectric constant (see later discussions). This led to the development of continuum based models, where water and protein are described as uniform dielectric media, and enter into the linearized Poisson-Boltzmann (PB) electrostatic equation,... 

When the gas-phase reactions, such as the relative acidities or basicities were compared with their counterparts in solution (in a solvent such as water) it was generally found16,17 that the energetics in the solvent were strongly affected by solvation effects and particularly the solvation of the ionic reactants. Relationships between the gas-phase and solution-phase reactions and the solvation energies of the reactants are generally obtained through thermodynamic cycles. From the cycle,... 

It should also be mentioned here that many of the chemical reactions which have been "explained with the HSAB model (2) occur in polar solvents and many involve the formation of ionic species. Thermodynamic cycles can be constructed for these reactions which show how many different kinds of effects are operative. When one considers that much of the data involve rate constant and equilibrium constant measurements, the explanation of this data becomes even more complex for there are entropy terms as well as enthalpy terms for all the steps in any cycle that is constructed. Even less information is available concerning these steps than we had above for the coordination model yet explanations are offered based solely on one step (4) — the strength of the bonding. 

The natural cycles of the bioelements carbon, oxygen, hydrogen, nitrogen and sulphur) are subjected to various discrimination effects, such as thermodynamic isotope effects during water evaporation and condensation or isotope equilibration between water and CO2. On the other hand, the processes of photosynthesis and secondary plant metabolism are characterised by kinetic isotope effects, caused by defined enzyme-catalysed reactions [46]. 

Specific Heat Release Rate. To utilize many combustion systems most effectively, the maximum power output is to be obtained for the smallest possible size and weight. As a result, the physical size of the combustion chamber as well as all other components should be held to a minimum. This requirement specifies that the specific heat release should be as high as possible. This quantity, usually expressed in energy units per unit volume, unit time, and unit pressure squared, is a measure of the ability to heat the gases used in the thermodynamic cycle. Some idea of the orders of magnitude of prevailing heat releases in combustion equipment can be obtained from the values in Table II. 

An unsuccessful attempt has been made to determine the separate electronic and steric effects of alkyl groups on the acidities of hydrocarbons, acetophenone derivatives, and acetone derivatives CH3COCHR1R2 (at either site) by multivariational analyses of experimental and theoretical acidities for each set.15 A thermodynamic cycle has been used to estimate the aqueous phase p/C, = 22.7 1.0 for the methyl group of acetic acid and p/C, = 3.3 1.0 for the corresponding enol.16 Equilibrium acidities have been determined for several nitroaryl substituted nitroalkanes and cyanomethanes, 2,4,6-TNT, and 9-cyanofluorene17 in acetonitrile the influence of common cation BH+ on the electronic spectra of the anions obtained in the presence of strong guanidine bases (B) has been attributed to formation of two types of ion pair.18... 

Important milestones in the rationalization of enzyme catalysis were the lock-and-key concept (Fischer, 1894), Pauling s postulate (1944) and induced fit (Koshland, 1958). Pauling s postulate claims that enzymes derive their catalytic power from transition-state stabilization the postulate can be derived from transition state theory and the idea of a thermodynamic cycle. The Kurz equation, kaJkunat Ks/Kt, is regarded as the mathematical form of Pauling s postulate and states that transition states in the case of successful catalysis must bind much more tightly to the enzyme than ground states. Consequences of the Kurz equation include the concepts of effective concentration for intramolecular reactions, coopera-tivity of numerous interactions between enzyme side chains and substrate molecules, and diffusional control as the upper bound for an enzymatic rate. 

The effect of the solvent on the standard potential of electrode reactions can be derived from a thermodynamic cycle. In the case of the ion-transfer type of reaction, where the reduced form is the metal deposited on the solid electrode, we have... 

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