Activation energy, negative

Although, in principle, this sum could be negative and give a negative activation energy for the permeability, no examples are known. The permeability increases with increasing temperature. 

The coefficient a includes the solvent viscosity dependence and accounts for the negative activation energy found in meehanochemical degradation. 

Degradation is appreciable only if K > l/tr this occurs whenever the coefficient a M e offsets the dissociation energy term — U0. It is physically unrealistic that an elementary chemical process can have negative activation energy, therefore the permissible maximum for K is controlled by the pre-exponential factor A (i — 1). 

Third-order kinetics, equation (166), have also been obtained330 for the iodination of mesitylene and pentamethylbenzene by iodine monochloride in carbon tetrachloride, the negative activation energies of —4.6 and —1.6 (from measurements at 25.2 and 45.7 °C) obtained being attributed to a mildly exothermic preformation of ArHICl complexes (c/. molecular bromination, p. 123) which subsequently react with two further molecules of iodine monochloride to give the products, viz. equilibria (167) and (168)... 

Reaction rates almost always increase with temperature the rare ones that do not have a negative activation energy will be dealt with later. The expression of the temperature dependence is always given for the rate constant, rather than the rate. For now, only elementary reactions will be considered, with composite reactions and other more complicated situations deferred to Section 7.5. Two forms are commonly used to express the rate constant as a function of temperature. The first is the familiar Arrhenius equation,... 

As the temperature increased, the concentration of CHj- radicals increased up to about 700 "C, and the concentration of Cj products correspondingly decreased. This phenomenon was observed even in the absence of NO, and it is partially explained by the fact that the residence time decreased from 0.055 s at 90 °C to 0.036 s at 600 "C. Of greater significance is the fact that the activation energy for CHj-radical coupling determined from this experiment was - 90 cal/mol. Both positive and negative activation energies have been reported for this reaction in the literature [15]. 

On the other hand, the low temperature dependance of the rate constants with activation energies around 5 kcal/mole indicates a diffusion limited reaction rate which could refer to diffusion of oxygene into the fibers of the board, i.e. into the fiberwalls. The corresponding negative activation energy for the groundwood based hardboard and the effect of fire retardants there upon are difficult to understand. 

The second intermediate s identity has been debated since the mid-1980s. In 1984, Liu and Tomioka suggested that it was a carbene-alkenc complex (CAC).17 Similar complexes had been previously postulated to rationalize the negative activation energies observed in certain carbene-alkene addition reactions.11,30 A second intermediate is not limited to the CAC, however. In fact any other intermediate, in addition to the carbene, will satisfy the kinetic observations i.e., that a correlation of addn/rearr vs. [alkene] is curved, whereas the double reciprocal plot is linear.31 Proposed second intermediates include the CAC,17 an excited carbene,31 a diazo compound,23 or an excited diazirine.22,26 We will consider the last three proposals collectively below as rearrangements in the excited state (RIES). 

The reactions shown in Table 19 exhibit a very small overall energy of activation, and in some cases [e.g. the reaction of 2,4-dinitroanisole with butylamine in benzene (Figure 11), and with cyclohexylamine in cyclohexane and in benzene (not shown)144] negative activation energies are observed. Since the equilibrium association constant, K1,... 

Mozurkewich M, and Benson, S. W., Negative activation energies and curved Arrhenius plots. 1. Theory of reactions over potential wells, J. Phys. Chem. 88, 6429 (1984). 

Occasionally, a negative activation energy is reported. For example, an value of -13.0 kJ mol was obtained for tRNA binding to the E-site of poly(U)pro-grammed ribosomes containing unlabeled tRNA at its P-site. In such circumstances, one must be especially wary, because there are several reasons why an erroneously negative E value is obtained ... 

One might expect that this is an elementary reaction because the reaction agrees with the stoichiometry (after multiplying aU coefficients in the preceding expression by two). However, one peculiarity of this reaction is that the rate actually decreases with increasing temperature or has a negative activation energy, an unreasonable situation for an elementary process. 

The reaction rate constant increases with increasing temperature (except for rare gas-phase reactions with negative activation energies), and may either... 

Chain reactions ieading to negative activation energy... 

Experimentally, the reaction is found to be third order with reaction law of d / df=kii5pS[0] [02]. From the reaction law, it seems that the reaction is an elementary reaction. However, itwas found (Bamford and Tipper, 1972, p. 169) that kii5 = T exp(-0.187 + 1000/T) for 293 Arrhenian behavior of reactions. A reaction mechanism that involves chain reactions that can explain the apparent negative activation energy is as follows. Suppose the above reaction is accomplished by the following two elementary reactions ... 

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