Activation parameters for isomerization

The observed ratios of rate constants for the rearrangement of c/s-A-Co(bzac)3, 4 immediately rules out mechanisms (a), (d), and (e) as primary paths. (Fig. 7.8) The very similar activation parameters for isomerization and inversion of cis- and imns-Co(bzac)3... 

Table 10J. Activation Parameters for Isomerization of [RuCl2(CO)2L2] Complexes...

Table 16 Activation parameters for isomerization or racemization of )S-diketone complexes of aluminium and of gallium...

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. 

The thermodynamic activation parameters for the enzyme-catalysed reaction are very different from those for the uncatalysed process (Albers et at., 1990). For the isomerization of succinyl-alanyl-leucyl-prolylphenylalanyl-p-nitroanilide catalysed by recombinant human FK binding protein AH = 5.85 kcal mol"1 and AS = -44. e.u. This compares with figures of 18.9 kcal mol 1 and — l.lbe.u. for the uncatalysed reaction of the same substrate. Probably a different step is rate determining in the enzyme-catalysed reaction. 

Table 17 Activation parameters for meso-dl isomerization of 2,3-disubstituted 2,3-diphenylsuccinonitriles. ...

The data in Table XXXV show that common features for these ammonia and amine complexes are very fast isomerization between the cis and trans isomers of the diaqua species and the fact that the trans diaqua isomers are generally more stable than the cis isomers. In the ammine system the activation parameters for k2 and k 2 are consistent with an isomerization process at cobalt(III), but it is at present not clear how this occurs. It need not be a simple cis-trans isomerization occurring at one of the Co(III) centers, but might involve the participation of both metal centers. The isomerization reaction may proceed via intramolecular proton transfer between a water ligand and one of the two hydroxo bridges with simultaneous bridge cleavage and formation... 

Activation parameters for the high pressure gas-phase approach of 1,2-d2-cyclopropanes to cis, trans equilibrium (equation 1) have been reported as log A, a(kcal mol"1) of 16.0,64.2 and 16.4,65.176,77. From pressure-dependent measurements of rate constants and calculations based on RRKM theory, the threshold energy E for the cis, trans isomerization has been estimated to be 61.1 kcal mol"1 and 61.3 kcal mol"11 16 1, s. 

The determination that the cis and trans isomers of 1,2-d2-cyclopropane could be inter-converted thermally prompted experimental work on similar reactions exhibited by other substituted cyclopropanes to define activation parameters for representative instances of the isomerization and to begin to discriminate among alternative mechanistic suggestions. The 1,2-dimethyl- and 1-ethyl-2-methylcyclopropanes, for examples, were shown to approach cis, trans equilibrium with activation parameters log A, Ed (kcal mol"1) of 15.25, 59.4 and 15.08, 58.91 140. 

The hexasila-Dewar benzene 13 is thermally stable at —150 °C, but it gradually reverted to the hexasilaprismane 1243. The half-life is 11/2 = 0.52 min at 0 °C in 3-methylpentane. The activation parameters for the isomerization of 13 to 12 are a = 13.7 kcalmol-1, A= 13.2 kcalmol-1 and A= — 17.8 cal K-1 mol-1. The small Ea value is consistent with the high reactivity of Si=Si double bonds. Most probably, the small HOMO-LUMO gap of 13 makes it possible that the Si=Si double bonds undergo a formally symmetry forbidden [2 + 2] thermal reaction. Hexasila-Dewar benzene is a key... 

Activation Parameters for a-f Isomerization and Carbonyl Scrambling in [M(CO)3L(ij4-dieneJ] Complexes... 

Table 6. Activation parameters for planar nitrogen inversion from NMR and isomerization data )...

Catalytic rate constants and activation parameters for the trans-cis isomerization of trans-... 

The rates and activation parameters for the thermal cis tram isomerization of A,A -distearoylindigo have been determined in both isotropic and liquid-crystalline solvents [727]. 

Similar rate decelerations in liquid-crystalline solvents have been observed for the thermal cis trans isomerization of a bulky tetrasubstituted ethene in cholesteric phases [728], On the other hand, the activation parameters for the thermal cis trans isomerization of less-dipolar substituted azobenzenes show no dependence on the solvent order. This indicates that the cis isomers and their corresponding activated complexes present a similar steric appearance to the solvent environment [729]. This result is more consistent with an isomerization mechanism which proceeds by inversion rather than by rotation cf. Eq. (5-40) in Section 5.3.2 and [527-529, 561]. The latter reaction represents a nice example of the use of liquid-crystalline solvents as mechanistic probes [729]. 

The activation parameters for decomposition of several thiazole carboxylic acids have been reported and they are tabulated in Table 51. The lower enthalpy of activation for thiazole-2-carboxylic acid (V) compared to its 5-isomer (VI) may be explained on the basis of stability of the developing anionic site in the activated complex. In the 2-isomer the anionic site is developed between the two electron withdrawing atoms, nitrogen and sulfur. In contrast, only the nitrogen atom is adjacent to the developing anionic site in the 5-isomer. Decarboxylation of 2-thiazo-lylacetic acid apparently gives VII as the initial product which isomerizes to... 

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