Activation energies of isomerization

It has been already mentioned in passing that indications exist in the literature showing that the 3C isomerization can take place by formation of at least two different 3C complexes, having different activation energies of isomerization, different particle size effects, different responses to alloying, etc. (157, 195-198). The suggestions presented above offer a choice of different complexes for further speculations. However, a definitive description of isomerization mechanisms under different conditions (H2 pressure, temperature, etc.) and with different catalysts (pure metals, alloys, etc.) is not yet possible. 

The key to the synthesis of stable disilenes has been the protection of the Si=Si bond by sterically encumbering substituents. Other disilenes are made by reduction of R2SiCl2 with Li, Na,118 or Li naphthalenide. The l,2-di(l-adamantyl)dimesitylsi-lene analogue has cis and trans isomers for which activation energies of isomerization are 120 kJ mol-1.119... 

Table V collects values for the activation energies of isomerization and protonation as deduced by De Gauw and van Santen [138] from kinetic measurements. A comparison of the turnover frequency per proton (TOF) and / iso is made in Table V. One notes that the large differences in measured overall TOFs of different zeolites disappear for the elementary rate constant fciso. This implies that the difference in apparent acidity of the zeolite is due mainly to the difference in adsorption isotherms of the different zeolites. One notes the small variation in activation and protonation energy values, which implies a slight dependence of protonation on the micropore channel size and dimension.

Recently, unsymmetrical bismuthonium salts bearing four different aryl ligands have been prepared, and the chirality at bismuth has been investigated by H-NMR [990M5668]. The activation energy of isomerization at the bismuth center is strongly dependent on the nucleophilicity of the counter anions as well as the polarity of the solvents employed. This is rationalized by a pseudorotation mechanism via pentacoordinate species at the transition state. 

A kinetic scheme and a potential energy curve picture ia the ground state and the first excited state have been developed to explain photochemical trans—cis isomerization (80). Further iavestigations have concluded that the activation energy of photoisomerization amounts to about 20 kj / mol (4.8 kcal/mol) or less, and the potential barrier of the reaction back to the most stable trans-isomer is about 50—60 kJ/mol (3). 

In conclusion, the steady-state kinetics of mannitol phosphorylation catalyzed by II can be explained within the model shown in Fig. 8 which was based upon different types of experiments. Does this mean that the mechanisms of the R. sphaeroides II " and the E. coli II are different Probably not. First of all, kinetically the two models are only different in that the 11 " model is an extreme case of the II model. The reorientation of the binding site upon phosphorylation of the enzyme is infinitely fast and complete in the former model, whereas competition between the rate of reorientation of the site and the rate of substrate binding to the site gives rise to the two pathways in the latter model. The experimental set-up may not have been adequate to detect the second pathway in case of II " . The important differences between the two models are at the level of the molecular mechanisms. In the II " model, the orientation of the binding site is directly linked to the state of phosphorylation of the enzyme, whereas in the II" model, the state of phosphorylation of the enzyme modulates the activation energy of the isomerization of the binding site between the two sides of the membrane. Steady-state kinetics by itself can never exclusively discriminate between these different models at the molecular level since a condition may be proposed where these different models show similar kinetics. The II model is based upon many different types of data discussed in this chapter and the steady-state kinetics is shown to be merely consistent with the model. Therefore, the II model is more likely to be representative for the mechanisms of E-IIs. 

A recent study of Murov and co-workers<106) indicates that the activation energies for isomerization are not the controlling factors. Thus the fluorescence of naphthalene is quenched (5 x 108M-1sec-1) by cis-trans-1,3-cyclooctadiene with isomerization to form cis-cis-1,3-cyclooctadiene. However, the compound bicyclo[4,2.0]oct-7-ene is not formed despite the low activation energy for this process ... 

The fact that quadricyclene is isomerized and dienes are not could be a result of (a) factors that govern how the vibrational energy is partitioned and (b) the large difference in activation energies for isomerization. 

The IPM parameters for hydrogen transfer atom in alkoxyl radicals are presented in Table 6.12. Isomerization proceeds via the formation of a six-membered activated complex, and the activation energy for the thermally neutral isomerization of alkoxyl radicals is equal to 53.4 kJ mol-1. These parameters were used for the calculation of the activation energies for isomerization of several alkoxyl radicals via Eqns. (6.7, 6.8, 6.12) (see Table 6.14). The activation energies for the bimolecular reaction of hydrogen atom (H-atom) abstraction by the alkoxyl radical and intramolecular isomerization are virtually the same. 

Fig. 6.13. Activation energies for isomerization of primary amine molecular ions to distonic isomers with the heats of formation of the precursor M ions normahzed to zero. [46]...

Steric factors are often responsible for skeletal isomerization in ion-radical states. The simple example in Scheme 6.31 illustrates the effect of steric congestion on activation energy of this kind of isomerization and depicts the transition of 2,2,3,3-tetramethylmethylenecyclopropane into 1,1,2,2-tetramethyltrimethylenemethane cation-radical. The rearrangement is brought about by one-electron oxidation of the substrate and represents an entirely barrierless process. Interestingly, methylenecy-clopropane (bearing no methyl groups) is protected from such a spontaneous collapse by a barrier of 7.4 k J mol l (Bally et al. 2005). 

---