Thermoneutral reaction

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... 

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

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. 

The reaction enthalpy is known as a very important factor that determines the reactivity of reactants in free radical abstraction reactions [71]. The IPM method helps to calculate the increment of AEfi that enthalpy determines in the activation energy of the individual reaction. This increment can be estimated within the scope of IPM through the comparison of activation energy Ee of the chosen reaction and activation energy of the thermoneutral reaction Ee0 (see Equation [6.18] in Chapter 6). This increment was calculated for several reactions of different peroxyl radicals with ethers (Table 7.19). 

Aminyl radicals react with C—H bonds of hydrocarbons more slowly than phenoxyl radical when compared to the activation energies of their thermoneutral reactions [33,34,38]. 

The activation energy of the thermoneutral reaction is lower when the methyl group participates in TS formation. There are two ways how the R substituent can have effect on the reaction. First, it is simple electronic effect that makes the TS less stable due to the repulsion... 

The accuracy of the thermochemical data obtained by this technique has been examined in numerous systems. In general, the data compares well, 1 kcal/mol, with that obtained by other spectroscopic and calorimetric methods. The accuracy and reproducibility of the data is dependent on the magnitude and time scale of the heat deposition detected by PAC that is associated with a given chemical process. Highly exothermic reactions are easy to detect, whereas ones that are not are difficult to detect. A thermoneutral reaction is invisible to PAC. Reactions that occur significantly slower than the response time of the transducer are not detected. Reactions that occur either slightly slower or faster than the response time are difficult to resolve accurately. Clearly, the proper choice of the transducer is extremely important in order to resolve accurately a given chemical event. 

In this chapter, we study the variation in the FF during asymmetric stretching and bending in ammonia, internal rotation in H202, and along the intrinsic reaction coordinate (IRC) of three prototypical examples of chemical reactions, viz., (1) a thermoneutral reaction, such as a symmetrical gas-phase SN2 type nucleophilic substitution ... 

The sum of the reactivities of the C and N atoms, [/ +fc L remains more or less conserved when compared to the variations in/N, /, and E. The principle of reactivity conservation, however, seems to be obeyed less well than in the thermoneutral reaction (Reaction 23.3). 

Therefore, for a thermoneutral reaction, the intersection point between the bond order profiles for the bond making and the bond breaking processes coincides with the TS the reactivity of the two reacting atoms also equalizes at the TS, as can be seen from the intersection of their IT profiles. These intersection points of the associated bond orders and condensed FFs he toward the left (right) of the TS for an endothermic (exothermic) reaction, in agreement with the Hammond postulate. 

Fig. 2.6 Potential energy contour diagram for a thermoneutral reaction.

The thermoneutral reactions mostly follow the same behaviour as with the endothermic reactions. If there are late barriers, vibrational energy is most effective, otherwise the translational energy is relatively more important. 

BP, BLYP, EDFl and B3LYP density functional models all lead to significant improvements over both Hartree-Fock and local density models, at least in terms of mean absolute deviations. While most reactions are better described, there are exceptions. Most notable among these is the bond separation reaction for tetrachloromethane. All four models show a highly exothermic reaction in contrast with both G3 and experimental results which show a nearly thermoneutral reaction. Similar, but somewhat smaller, effects are seen for isobutane and trimethylamine. As was the case with Hartree-Fock calculations. 

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