Observed activation energy

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). 

In the amorphous tram- and the side vinyl polybutadienes, the first-order reaction rate constants (Table III) give high initial yields (G0) for olefin disappearance when the initial concentration is inserted in the rate equation kD = n(CJCD), where k = rate constant, C = initial concentration, and CD = concentration after dose D. The activation energies for the disappearance of both these olefinic species range from 3.4 to 4.0 keal. per mole, not very different from the activation energy observed for cis disappearance. 

Schwab and co-workers (5-7) found a parallel between the electron concentration of different phases of certain alloys and the activation energies observed for the decomposition of formic acid into H2 and CO2, with these alloys as catalysts. Suhrmann and Sachtler (8,9,58) found a relation between the work function of gold and platinum and the energy of activation necessary for the decomposition of nitrous oxide on these metals. C. Wagner (10) found a relation between the electrical conductivity of semiconducting oxide catalysts and their activity in the decomposition of N2O. 

Kinetics and activation energies observed in the hydrogenation of buta-1 3-diene over various metals... 

As the ground state of oxygen is triplet the above reaction may also be interpreted as the recombination. This is in correspondence with the high value of the frequency factor as well as with almost zero value of the activation energy observed for this reaction. The activation energy of the backward process is considerably higher. 

Early quantum mechanical calculations treating chemisorption employed a valence bond-type method and a few atoms to simulate the surface. Sherman and Eyring (54) used a 4-atom model to simulate H2 dissociation on charcoal. The calculations were unable to duplicate the low activation energy observed experimentally for this reaction. Later calculations by Sherman and Eyring (55) showed that H2 dissociation on Ni2 has a low activation energy, suggesting that quantum mechanics is a useful tool for such studies. 

The activation energy observed experimentally is the effective activation energy and is a function of applied voltage V and trap density Hb. Several interesting features predicted by this equation are given below. 

Further evidence for pore transport is presented by Yoshida and Roberts [62] in terms of the temperature dependence of iontophoretic flux for solutes of differing size. They showed that the iontophoretic flux for sodium (MW = 23) and cyclosporin (MW = 1203) were relatively temperature insensitive (Fig. 3). The activation energies for iontophoretic transport are similar to activation energies observed for differences of solutes in aqueous solution and indicate that the iontophoretic transport of both solutes is through the pores [62]. 

The first explanation was suggested by Voorhies (ref. 2), and while the high activation energy observed would suggest that any explanation based solely on diffusion is incorrect, it cannot be entirely dismissed if the high activation is the result of surface changes on the catalyst ... 

It is quite possible that the decrease of activation energy observed by Winter is coimected in all the cases considered through the influence of diffusion hindering at higher rates of exchange which was absent in the work of Dzisjak et al. (5) due to the forced circulation of the reaction mixture. 

The activation energies for the backbone motion of P(4HB) and the 3HB and 4HB units in the P(3HB-co-4HB)s, derived from the DEM model analysis, are found to be similar and in the range 42-47 kJ/mol [79]. This range is typical of amorphous polymers at temperatures above Tg, but they are greater than typical ones for polymers in solution, possibly due to the increased apparent viscosity exerted by the amorphous matrix on the moving backbone segment [79]. The activation energy observed for the backbone motion of P(3HB) in chloroform solution is 17 kJ/mol [72]. 

The large positive activation energies observed for temperature induced unfolding of proteins is attributed to the unfolding step... 

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