Thermoneutrality

Metabolic rate, basal (BM) Metabolic energy transformation calculated from measurements of heat production or oxygen consumption in an organism in a rested, awake, fasting, and thermoneutral state, in W nr. ... 

In general, it was found that this process is strongly endothermic for sulfur diimides, approximately thermoneutral for selenium diimides and strongly exothermic for tellurium diimides, consistent with experimental observations. These differences can be attributed to the expected trend to lower r-bond energies for chalcogen-nitrogen (np-2p) r-bonds along the series S (n = 3), Se (n = 4) and Te (n = 5). 

For a thermoneutral reaction (AEq = 0) the TS is exactly half-way between the reactant and product (as expected), while it becomes earlier and earlier as the reaction becomes more and more exothermic (AEq negative). The activation energy is given as... 

Let us define the activation energy for a (possible hypothetical) thermoneutral reaction as the intrinsic activation energy, AEq. As seen from eq. (15.10), a = 4A q. The TS position and activation energy now become... 

The thermoneutral potential is 2.72 V this means that during charging at voltage below this value ZEBRA cells will be cooled thermodynamically. 

Evidence that p-toluenesulfonyl radicals abstract a chlorine atom from CC14 under certain conditions has also been given50 from available thermodynamic data this reaction should be nearly thermoneutral. 

Thus, the process of hydride ion abstraction from a primary position is approximately thermoneutral, and hence we must conclude that it is an energetically allowed process, although possibly with a relatively small reaction rate. A process competing with primary H abstraction (Reaction 13) is methide ion abstraction (Reaction 11, loss of CH4 from the... 

In addition it is possible that alkenyl ions are formed by loss of H2 from an alkyl ion by an approximately thermoneutral reaction. 

Reactions like (71), even if not precisely thermoneutral, are likely to be much faster than charge transfers involving atomic species alone, such as... 

As it pertains to the solid state photodecarbonylation reaction, the model assumes that most aliphatic ketones have similar excitation energies, that reactions are more likely along the longer-lived triplet excited state, and that each reaction step must be thermoneutral or exothermic to be viable in the solid state. " Using acetone and its decarbonylation intermediates as a reference reaction (dashed lines in Fig. 7.24), we can analyze the energetic requirements to predict the effects of substituents on the stability of the radical intermediates. The a-cleavage reaction of triplet acetone generates an acetyl-methyl radical pair in a process that is 3.5 kcal/mol endothermic and the further loss of CO from acetyl radical is endothermic by 11.0... 

Fig. 4 Free energy reaction coordinate profiles that illustrate a change in the relative kinetic barriers for partitioning of carbocations between nucleophilic addition of solvent and deprotonation resulting from a change in the curvature of the potential energy surface for the nucleophile addition reaction. This would correspond to an increase in the intrinsic barrier for the thermoneutral carbocation-nucleophile addition reaction.

The intrinsic barriers for the reaction of [12+] correspond to intrinsic rate constants of ( mcoh)o = 1 x 108m-1 s-1 and (kp)0 = 450 s-1 (equation 4). This analysis shows that the thermoneutral addition of methanol to [12+] is an intrinsically fast reaction, with a rate constant that is only 50-fold smaller than that for a diffusion-limited reaction.16... 

The partitioning of ferrocenyl-stabilized carbocations [30] between nucleophile addition and deprotonation (Scheme 18) has been studied by Bunton and coworkers. In some cases the rate constants for deprotonation and nucleophile addition are comparable, but in others they favor formation of the nucleophile adduct. However, the alkene product of deprotonation of [30] is always the thermodynamically favored product.120. In other words, the addition of water to [30] gives an alcohol that is thermodynamically less stable than the alkene that forms by deprotonation of [30], but the reaction passes over an activation barrier whose height is equal to, or smaller than, the barrier for deprotonation of [30], These data require that the intrinsic barrier for thermoneutral addition of water to [30] (As) be smaller than the intrinsic barrier for deprotonation of [30] (Ap). It is not known whether the magnitude of (Ap — As) for the reactions of [30] is similar to the values of (Ap - As) = 4-6 kcal mol 1 reported here for the partitioning of a-methyl benzyl carbocations. 

A different picture is observed when a polar radical reacts with a C—H bond of a polar molecule. For example, the reaction of an oxygen atom with the methane C—H bond is characterized by the activation energy of thermoneutral reaction /ic0 54.6 kJ mol-1 and parameter bre= 13.11 (kJ mol-1)172 while the reaction with the methanol C—H bond is characterized by Ed) 50 kJ mol-1 and parameter brc 12.55 (kJ mol-1)172 [30]. For these values of bre, the difference between the activation energies is 4.6 kJ mol-1. The decrease in the activation energy can be explained by the fact that the polar O—H group in the O H C—OH transition state interacts with the O H C polar reaction center. 

An additional polar interaction called multidipole interaction is observed in reactions of peroxyl radicals with polyatomic alcohols [55], A few polar O—H groups interact with the polar reaction center C H O in such systems. A few examples of such interaction are given here [17]. Multidipole interaction sufficiently changes the thermoneutral activation energy of the reaction HOO + alcohol [54] and can be characterized by increment A A E. ... 

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