Arrhenius activation parameters

From the temperatures for the one- and ten-hour half lives, calculated using equation 6, Arrhenius activation parameters can be calculated for each initiator and compared to the experimental values. This comparison is made for some of the entries of reactions 1-4 in Table V. At least five entries were chosen for each reaction, spanning a wide range of reactivity, using common entries as much as possible for the four reactions. 

In order to find out whether captodative substitution of a methyl radical can lead to persistency, the rate of disappearance by bimolecular selfreaction was measured for typical sterically unhindered captodative radicals (Korth et al., 1983). The t-butoxy(cyano)methyl radical, t-butylthio(cyano)-methyl radical and methoxy(methoxycarbonyl)methyl radical have rate constants for bimolecular self-reactions between 1.0 x 10 and 1.5 X 10 1 mol s Mn the temperature range —60 to - -60°C. The dilTusion-controlled nature of these dimerizations is supported by the Arrhenius activation parameters. Thus, it has to be concluded that there is no kinetic stabilization for captodative-substituted methyl radicals. On the other hand, if captodative-substituted radicals are encountered which are kinetically stabilized (persistent) or which exist in equilibrium with their dimers, then other influences than the captodative substitution pattern alone must be added to account for this phenomenon. 

Formation of A-acyl-A-alkoxynitrenium ions has been demonstrated by solvent kinetic isotope studies, Arrhenius activation parameters and substituent effects as well as product studies. 

Table 12 gives Arrhenius activation parameters and rate constants for thermal decomposition of A,A-dimethoxybenzamides 194a-d. AS values are low when compared to homolysis of diacyl peroxides and peroxides, for which values are typically around 8-11 cal K moG. This has been attributed to significant ji stabilization and restricted... 

Head-to-tail dimerization of 1,1-diphenylsilene (19a), produced by laser flash photolysis of 1,1-diphenylsilacyclobutane (17a), yields the 1,3-disilacyclobutane 2761,62 with a rate constant fcdim = (1-3 0.3) x 1010 M 1 s 1 in hexane solution at 25 °C (equation 17)46. This value is within a factor of two of the diffusional rate constant in hexane at this temperature, indicating that dimerization of this silene is faster than reaction with even the most potent of nucleophilic trapping reagents (see Table 3). More recently, the temperature dependence of the rate constant for dimerization of 19a has been studied63. The results of these experiments are shown in Figure 1, and lead to Arrhenius activation parameters of a = -4 2 Id moD1 and log(A/M 1 s"1) = 9.2 0.4. 

The Brook silene 28, produced photochemically as shown in equation 18, dimerizes to yield a mixture of the 1,2-disilacyclobutane 29 and the acyclic ene-dimer 3064, the common mode of dimerization for the large majority of l,l-bis(trialkylsilyl)silenes that have been studied to date12. Conlin and coworkers determined the absolute rate constant for dimerization of 28 in cyclohexane solution, k, tm = 1.3 x 107 M 1 s 1 at 23 °C65. Arrhenius activation parameters for the reaction were determined over the 0-60 °C temperature range. The values obtained, a = 0.9 0.4 kJmol 1 and log(A/M 1 s 1) = 7 1, are consistent with the stepwise mechanism for head-to-head dimerization originally proposed by Baines and Brook (equation 19)64, provided that the rate of reversion of the... 

It is now of importance to know the relations between the Eyring and the Arrhenius activation parameters and the potential barrier Fmax. One has 2> ... 

Arrhenius activation parameters for cleavage reactions of aryl halide radical anions"... 

Very recently, the gas phase pyrolysis (155-200°) of methyl azide at low conversions (< 1%) was studied . Nitrogen was the major non-condensable gas, in addition to small amounts of hydrogen (6% at lowest initial pressure of azide to less than 1% at the highest pressure) and methane (<2%). Ethane, ethylene, ammonia and hydrazoic acid were not detected, although the ethane, ethylene and hydrazoic acid determinations were subject to some uncertainty. Two solid white products were also obtained which were not characterized. The results showed that the thermolysis of methyl azide is of first-order, homogeneous and free from chains, in agreement with previous work . The Arrhenius activation parameters (see Table 1)... 

Table 1. Arrhenius activation parameters for gas phase pyrolysis of alkyl...

The important point that arises from the Rodger-Sceats reduction is that the dynamics can take place on an effective potential P defined by Eq. (2.19). The origin of the potential can be traced back to the requirement that at long times the system must achieve a thermal distribution which is consistent with a Boltzmann distribution on the full potential and the use of P simply ensures that the partition functions of the system will be given correctly. This will be very important in applications to chemical reactions, because the partition function plays an important role in determining Arrhenius activation parameters. The dependence of P on the reaction coordinate simply accounts for this effect. For example, the reactant and transition state configurations are defined by the minima and maxima of P at qi and gj, and the Boltzmann factor for activation is... 

TEMPERATURE EFFECTS. The Arrhenius activation parameters viz., Ep and Ep for the processes of diffusion and permeation have been... 

Observed Arrhenius activation parameters for allylic alcohol isomerizations are also consistent with a unimolecular rate-limiting step. In aqueous ethanol and aqueous dioxane, differences in reactivity due to structural changes in the allylic system are reflected primarily in the energy of activation. Values of entropy of activation are generally near Harris... 

Our group was able to obtain hydrogen atom transfer rate constants (Table 1) with typical organic solvents and their Arrhenius Activation parameters (Table... 

All calculated data are summarized in Tables 1 and 2. Comparison of the Arrhenius activation parameters evaluated according to the method proposed in this work with those obtained by "conventional" procedure (see Table 1), shows their good coincidence. This suggests that the proposed method allows performing analysis of the temperature responses of linear voltammograms for irreversible electrode processes it seems to be less tedious in comparison with the "conventional" procedure. 

Daneshfar, R. Klassen, J. S. Arrhenius activation parameters far the loss of neutral nucleobases from deprotonated oUgonucleotide anions in the gas phase. J. Am. Soc. Mass Spectrom. 2004, 15, 55-64. 

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