Isomerisation rates

The hexane isomers are primary products of n-hexane cracking, so that the partial pressures determing the deactivation effect of coke on these reactions are related to the n-hexane conversion. Hence, the effect of coke on the isomerisation rate of n-hexane is rigorously quantified, provided that the alkane isomers are not significantly involved in further reactions, however. 

Fig. 9.5 Transformation of m-xylene over HMORIO and NaHMORlO samples. Disproportionation / Isomerisation rate ratio (D/I) versus m-xylene conversion (X%) on fresh and on deactivated catalysts HMORIO (0), 14NaHMOR ( ), 28NaHMOR (A), 45NaHMOR (+), 63NaHMOR (o)...

O Total rates of reaction for linear C3—Ce alkanes and neopentane A hydrogenolysis rates only A isomerisation rates only broken line, results for ethane. 

Relative isomerisation rates rj/r naturally vary with the structure of the reactant butene, depending on the variance of the initial condition from that of equilibrium between the butene isomers (Figure 7.3) and on the stabilities a the adsorbed intermediates. Values of r,7rft at room temperature over Pt/Al203 were estimated as ° ... 

The reactions of linear or branched alkenes containing more than four carbon atoms reveal no types of reaction not already met, but their lower volatility permits their study as liquids or in solution as well as in the vapour phase. Thus for example the relative isomerisation rate r,7r/, of liquid 1-pentene over Pd/C at 290 K was independent of the conditions of agitation, showing there were no mass-transport effects attributable to the hydrocarbons. The presence of solvents was also without effect. ... 

That isomerisation is intramolecular has been demonstrated by observing C and W satellite features in the n.m.r. spectra of (60) as a function of temperature, and by noting the lack of dependence of isomerisation rates... 

For tropospheric ozone, the isomerisation of the RO radical, followed by the association with O2, can be important if it is followed by the isomerization of the 02R hOH radical, leading to a complex hydroperoxide bearing an aldehyde or epoxy function. The role played in atmosphere by the isomerisation of the RO2 radical has been analysed by Carter et al [3a]. They estimated the ratios of 1,5 H shift alkylperoxy isomerisation rate to rate of major competing processes under ambient conditions. Using a NO concentration of 0.1 ppm, they obtained the following values ... 

In Table 1 the experimental results for the alkanes investigated are summarised together with the branching ratio a and b (as defined in Fig. 1) and the ratios DNO2 / DOH and DNO2 / ARH (equivalent to the NOCON factor) extrapolated to lower tropospheric conditions. The NOCON factors have been determined neglecting the pressure dependence of the decomposition/isomerisation rate constants. C to C6-alkanes have been investigated within LACTOZ I. 

The isomerisation between syn and anti rotamers of a large number of M(NR)(CHR XOR )2 (M = Mo (H W) complexes has been studied. Rate constants and acdvaticHi energies for this process are dependent on the nature of R, R, R2 and the solvent. The result of bulk polymerisations with 2,3-b/s(trifluoromethyl)norbom Kliene suggests a relationship between alkylidene rotamer isomerisation rates and the polymer cis/trans content.33 The molybctenum complex, Mo OC(CF3)2Me 2 N(2,6-P r 2C6H3) (CHCMe2Ph), catalyses the cross-metathesis of aryl- and alkyl-substituted terminal alkenes to unsymmetrical alkenes widi trans selectivities of >95%.335 The molybdenum alkylidene... 

As it has been just aforementioned, for fast azobenzene-based artificial muscle-like actuators to be achieved, it is essential that the azo-chromophore used returns to its thermodynamically stable trans form in the dark as fast as possible. In this way, azobenzenes that undergo their thermal isomerisation through the rotational mechanism, of which push-pull azoderivatives are the most well-known example, will be valuable candidates for this aim since they are endowed with high thermal isomerisation rates at room temperature. In fact, the two only polysiloxane-based photoactive artificial muscle-like actuators exhibiting low relaxation times published so far are based on this strategy. Indeed, these systems contain the well-known push-pull azo-dyes 4-amino- (Camacho Lopez et al. 2004) and A-N, W-dimethylamino-4 -nitroazobenzene (Harvey and Terentjev 2007) as photo-active molecules, which are doped into host elastomeric networks but not covalently bonded to the polymeric structure decreasing thereby the stability of the final photo-actuator. 

However, before moving on to LSCE materials, we will discuss briefly the isomerisation of azophenols in both isotropic and liquid-crystalline environments. Specifically, 4-(5-hexenyloxy)-4 -hydroxyazobenzene (AZO-OH) has been chosen as a representative example since it will be further used as a comonomer in LSCEs. The cis isomer of AZO-OH exhibits relaxation times of 311 and405 ms in ethanol and acetonitrile at 298 K, respectively (Fig. 18.8b). Otherwise, its isomerisation rate is more than 1000-fold slower when it is dissolved in toluene at the same temperature (t = 23 min). In both ethanol and acetonitrile, the thermal back reaction of AZO-OH proceeds through a solvent-assisted tautomerisation to yield a hydrazone-like intermediate. Subsequently, a rotation around the N-N bond of the hydrazone-like intermediate can occur regenerating thereby the thermodynamically stable trans isomer. Such an intermediate cannot be promoted when the azo dye is dissolved in toluene. 

The response time of the artificial muscle-like actuator is mostly determined by the thermal isomerisation rate of the azo dye used. Specifically, the introduction of both alkoxynitro-substituted azobenzenes and azophenols yield fast-responding... 

Figure 8.5 Isomerisation rates of cyclopropane at 765 K, as a function of its pressure. The circles represent experimental data from several authors, and the curves represent the results of calculations with the theories indicated in the plot.

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