Endothermicity values, calculating

One should mention however that our conclusions have been very recently questionned by Axe and Marynick (42) who carried out calculations on the reaction (3) with various basis sets ranging from split valence to double zeta quality, with and without polarization functions on C, O and H atoms. They found a marked increase in the endothermicity value on going from the unpolarized basis sets ( values ranging between 8.7 and 15.2 kcal/mol) to the polarized basis s.ets (with values between 19.5 and 25.2 kcal/mol, i.e. close to our SD-CI values). We have now carried out calculations adding to our original split valence basis set polarization functions on C, O and H. One polarized set includes the two sets of polarization functions ( = 0.920... 

Here is another instance of the extreme model dependency of strain energies in the fluorocarbon series. A strainless C(F)2(C)2 group increment of - 104.9 kcal mol" had earlier been derived ". In principle, one might expect that this increment would be applicable to calculating the strain energy of hexafluorocyclopropane. Applying this value, we concluded that the total strain in the molecule was 80.9 kcal mol" This was even higher than the value calculated by Bernett some years earlier 68.6 kcal mol" We were buoyed by the fact that the endothermicity of equation 18 indicated a strain of... 

Comment The estimated value based on average bond energies is quite close to the hue value calculated using A//y data. In general, Equation (9.4) works best for reactions that are either quite endothermic or quite exothermic, that is, for A//° > 100 kJ or for LH° < -100 kJ. 

The initial kinetic energy of 0 ions produced by dissociative attachment in 02 at an electron energy of 6.9 e.v. may be determined from Equation 4 to be 1.64 e.v. using values of 1.465 e.v. (1) for A(0) and 5.09 e.v. (7) for D(O—O). The residence time for 0 ions calculated from Equation 1 is 6.0 X 10 7 sec. at 10 volts repeller potential. Rate constants for Reaction 6 determined from data at varying Vr are shown in Table I and are seen to increase sharply with increasing repeller potential, as expected for an endothermic process. 

Back strain effects are most important for the homolysis of hydrocarbons (4), a highly endothermic reaction, which does not produce a stable molecule byproduct, as do diazenes (N2) and peresters (CO2). Destabilization of the reactants in reaction 4 back strain is essential in lowering the energy of activation of reaction. The results of this study suggest that only reaction 4 requires the use of A values to obtain a good correlation between reaction temperatures and calculated product radical stabilities. 

Figure 6.4a shows the behavior of an endothermic reaction as a plot of equilibrium conversion against temperature. The plot can be obtained from values of AG° over a range of temperatures and the equilibrium conversion calculated as illustrated in Examples 6.1 and 6.2. If it is assumed that the reactor is operated adiabatically, a heat balance can be carried out to show the change in temperature with reaction conversion. If the mean molar heat capacity of the reactants and products are assumed constant, then for a given starting temperature for the reaction Ttn, the temperature of the reaction mixture will be proportional to the reactor conversion X for adiabatic operation, Figure 6.4a. As the conversion increases, the temperature decreases because of the reaction endotherm. If the reaction could proceed as far as equilibrium, then it would reach the equilibrium temperature TE. Figure 6.4b shows how equilibrium conversion can be increased by dividing the reaction into stages and reheating the reactants...

The heat of formation of 3-membered rings according to Reaction 4 is calculated to be quite endothermic (AH = 23 kcal/mole) due to the strain energy required to reduce to 136.7° from its equilibrium value, 151°, and to reduce the tetrahedral (O-Si-O) angle to 103.3° from 109.5° (46). 

The values of reaction enthalpies in a forward search can be of use in predicting the products of a reaction the more exothermic a reaction, the more it should be preferred. The situation will be different in a retrosynthetic search where retroreactions should be calculated to be endothermic to some degree. This underlines the point previously made (Sect. 2, Fig. 9) that the differences between a forward and a retrosynthetic search do not reside in the way reactions are generated — in both cases in EROS by the formal reaction schemes — but in the way they are evaluated. 

All these reactions are endothermic and, in addition, occur with a loss of entropy. Back unimolecular reactions are exothermic and occur with an increase in entropy. So, the role of such reactions should be negligible due to high activation energy and very fast back reaction. The values of the rate constants for addition reactions CH302 + carbonyl compound, calculated by the IPM method are presented in the following table ... 

In accordance with the increased charge separation in the transition state and products, the gas-phase EA of 22.6 kcal mol-1 for the reaction reduces to just 4.4 kcal mol-1 with inclusion of semiempirical solvation energies in water while the overall reaction, which is very endothermic in the gas phase becomes exothermic by 5.1 kcal mol-1 with solvation.179 The calculated EA is lower than the experimental values for substitution by A-methylaniline in methanol which fall in the range of 6-15 kcalmol-1 (Table 5).42,43 However, in aqueous solution these barriers would be lower than in methanol. 

Exothermic events, such as crystallization processes (or recrystallization processes) are characterized by their enthalpies of crystallization (AHc). This is depicted as the integrated area bounded by the interpolated baseline and the intersections with the curve. The onset is calculated as the intersection between the baseline and a tangent line drawn on the front slope of the curve. Endothermic events, such as the melting transition in Fig. 4.9, are characterized by their enthalpies of fusion (AHj), and are integrated in a similar manner as an exothermic event. The result is expressed as an enthalpy value (AH) with units of J/g and is the physical expression of the crystal lattice energy needed to break down the unit cell forming the crystal. 

For the cyclopentyl cation (20) formation, the reaction is slightly endothermic, while for methylcyclohexyl cation (21), the reaction profile itself is slightly exothermic. However, relative to the most stable 2-heptyl cation, (the 1,5-bridged structure 16) this reaction is also slightly endothermic. Based on calculated AG values at 298K, both reaction profiles are exergonic due to a larger entropy contribution for the product side in each case. 

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