Activation energy table

In addition to the repulsion, the reaction enthalpy has influence on the activation energy. Table 15.14 illustrates the increments AEh of this influence, as well as the geometrical... 

Carbon dioxide decomposes behind a shock front in accordance with the kinetics expected of a unimolecular reaction in its low-pressure region31-37. The second-order rate coefficients obtained by a number of experimentalists in the temperature range 2500-11000 °K are in reasonable agreement, but there is a considerable spread in the values derived for the Arrhenius activation energy (Table 3). Furthermore, even the highest of these values31 is much smaller than the endothermicity (D = 125.8 kcal.mole-1) of... 

Cycloadditions with 1,3-cyclohexadienes proceeding with very low activation energies (Table IV) bear a close relationship to thermal Diels-Alder reactions (see ref. 5 and references cited therein). Hoffmann and Woodward237 have developed selection rules for thermal and photochemical concerted cycloaddition reactions according to which Diels-Alder reactions can occur in a concerted fashion with singlet ground-state... 

It may be seen from Table I that the activation energy varies from 0 to 11 kcal., and the pre-exponential factor from 0.2 X 10 10 to 5 X 10-10. It may be stated, though tentatively (as information is scarce) that starting from a certain low value the pre-exponential factor drops with the activation energy. Table IV shows that activation energies for oxygen atom reactions vary from 0 to 8000 cal., while the values of pre-exponential factors range within 1.8 X 10 u to 3.4 X 10-11. 

The associated activation energy (Table 1) is 56=bl0kJ, a value significantly lower than the one derived from mechanical measurements (Table 1). In the same way, the activation entropy (Table 2) of 53 d= 10 J K-1 mol-1 corresponding to the centre of the relaxation peak is lower that the value obtained from mechanical measurements. The symmetrical character of the dielectric P peak is also reflected in the constant value of ASa over the temperature range (Table 2). 

The shift of the /> peak from - 70 to - 80 °C is associated with a decrease of the activation energy (Table 1), as well as the activation entropy (Table 3). The latter result indicates that the cooperativity of the processes involved in the remaining mechanical peak is close to the one associated with the... 

Within the sense of the Arrhenius equation, Eq. (14), the isotope effect k /k in 1 can be ascribed to a difference E° — EH = 3.3 Kcal/mol in the activation energies (Table 22) while the preexponential factors are sensibly equal for both isotopic species, AD/AH = 0.95. This last result, AD/AH 1 is unusual for reactions showing large tunnelling effects68, where AD/AH > 1 is usually observed, but is entirely foreseen by the theoretical analysis46. ... 

The experimental data obtained in the hydrocracking of coal, coal oil and phenanthrene were tested with equations (iii) and (iv) as shown in Figures 8 and 9 and the data were found compatible with the model. This compatibility confirmed that the rates of hydrocracking were controlled by surface chemical reactions as were also indicated by arrhenius activation energies (Table VIII). 

As demonstrated by the association rate constants listed in Table 10, association is relatively fast and has low activation energies. Table 10 also tabulates the equilibrium constants for reaction of a variety of nucleophiles with carbenium ions. Most of the equilibrium constants involving trityl carbenium ions were obtained from UV studies, whereas those of me-thoxymethylium carbenium ions with both dimethyl ether and methylal were calculated using dynamic NMR [64]. Values for isobutoxy alkyl derivatives have been estimated from polymerization kinetics. The data presented in Table 10 demonstrate that the equilibrium constants are lower for weaker nucleophiles and more stable carbenium ions. For example, carbenium ions react faster with diethyl ether than with the less nucleo-... 

Mixtures of CI2 with various isotopic hydrogens were irradiated with visible light from a mercury arc. The isotopic content of the residual hydrogen molecules was then determined either mass spectrometrically (stable isotopes) or with a Geiger counter (radioactive species). The rate coefficient ratios with respect to H2 were calculated from these data and their temperature dependence furnished ratios of pre-bxponential factors and differences in Arrhenius activation energies. Table 10 summarizes the results. 

The thermal decompositions of diazirines appear to be homogeneous, uni-molecular processes " . Cyclic diazirines decompose with a slightly smaller activation energy (Table 14) due to ring strain. Product distributions presumably result from intramolecular rearrangement of carbene intermediates, and the fact that the product ratios differ from those obtained on photolysis supports the hypothesis that the carbenes generated photolytically contain considerable excess vibrational energy. 

When passing from metal-carbon to metal-hydrogen bonds, the reactivity of olefin addition sharply increases due to the decrease in the activation energy (Table 9). The activation energy of the addition to the Al—H bond is practically the same for various olefins, and for the same olefin it increases in the order R,AlH[Pg.84]

The comparison by Schroder and Thiel [62] (section 6.3.1) of SE (MNDO and MNDOC) and ab initio geometries and energies conclnded that the SE methods usually overestimate activation energies. Of 21 activation energies (Table IV in Ref. [62], entries I, K, W omitted), MNDO overestimated (compared with best correlated ab initio calcnlations) 19 and nnderestimated 2 the overestimates ranged from... 

The dehydrogenation steps of CH OH are exothermic on both Pt and Ru and have relatively low activation energies (Table 7). Pt and Ru under UHV behave similarly for CH3OH decomposition [88,89], and the calculated results are consistent with the experimental observations. The greatest differences occur in the first (methanol dissociation) and last (CHOads—>COads + Hads, Reaction 4) steps. 

The trend in activation energies (Table I) shows the electrophilic nature of attack by the sulfur atom increasing number of alkyl substituents on the doubly bonded carbons decreases the value of E, while increasing number of halogen substituents on the doubly bonded carbons increases the value of . These variations are correlated with molecular properties such as ionization potentials, excitation energies, and bond... 

The relaxation time, the d.c. and a.c. conductivities, the equilibrium constants of ions and valence defects, and the mobility of valence defects show a temperature dependence with characteristic activation energies (Table III). The mobility of ions is held to be temperature independent (tunnel eflFect). Magnan and Kahane 112), however, determined mobilities at very low temperatures (—80°C. to —120°C.) which show a pronounced temperature dependence (1.4 X lO" to 1.2 X 10" cm. volts" sec." ). 

In the same way a reduction in symmetry can make a forbidden reaction partly allowed, but still one that has a high activation energy. Table 2 shows some rate data for isomerization of substituted Dewar benzenes to the corresponding benzene derivatives. We see that a single chlorine or fluorine substituent increases the rate of isomerization markedly. 

Poor agreement is observed between the experimental and theoretical values [calculated by a combined method, that is, quantum-chemical calculation of the activation energy (Table 37.7 and Table 37.8) and the frequencies of the vibrations of the bonds in the prereaction complex and in the transition state and calculation of the rate constants on the basis of RRKM theory] for certain processes where the effects of electron correlation and the contributions of the excited electronic configurations are not predominant [68-73]. 

Ti, Zr, and Hf) are electrical insulators, as expected for these d° or d electronic configurations. When the d shell is partially filled, semiconducting properties are normally observed with generally low activation energies (table 5). Except for the low-spin d situation, the conduction mechanism is most likely electron hopping between the M sites. For Pt" (5d ), the t2g band is filled and the activation energy is presumably related to excitation into the empty e, band. The fact that the conduction mechanism of the Pt" " " compounds differs from the others is supported by the... 

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