Reaction energies, performance

Table 13-2. Computed activation (AEa) and reaction energies (AEr) for the concerted gas-phase cycloaddition of ethylene to Irans-butadiene [kcal/mol]. The HF and DFT calculations were performed with the 6-311+G(d,p) basis set and include zero-point vibrational contributions.

The ratio of the quantum partition functions (Eq. (4-29)) for two different isotopes can be obtained directly through free energy perturbation (FEP) theory by perturbing the mass from the light isotope to the heavy isotope. Consequently, only one simulation of a given isotopic reaction is performed, while the ratio of the partition function, i.e., the KIE, to a different isotopic reaction, is obtained by FEP. This is conceptually and practically an entirely different approach than that used previously [23]. 

Analysis of the kinetic parameters showed that the apparent activation energy for the reaction was reduced from 105 to 57 kj mol-1 (Tab. 3.2). This observation is consistent with the polar mechanism of this reaction implying the development of a dipole in the transition state (Fig. 3.8) even when the reaction was performed in a polar solvent. 

Scola et al. [67] studied the kinetics of the MW cure of a phenylethyl-terminated imide polymer model compound and an oligomer using a variable frequency MW source and found that the activation energy of the MW cures were 68% and 51% of the thermal cure respectively. It should be noted that the reactions were performed in the liquid phase in the absence of solvent. 

Microwave-induced, catalytic gas-phase reactions have primary been pursued by Wan [63, 64], Wan et al. [65] have used pulsed-microwave radiation (millisecond high-energy pulses) to study the reaction of methane in the absence of oxygen. The reaction was performed by use of a series of nickel catalysts. The structure of the products seemed to be function of both the catalyst and the power and frequency of microwave pulses. A Ni/Si02 catalyst has been reported to produce 93% ethyne, whereas under the same irradiation conditions a Ni powder catalyst produced 83% ethene and 8.5 % ethane, but no ethyne. 

Deng et al. (1997) studied the reaction of metallic iron powder (5 g 40 mesh) and vinyl chloride (15.0 mL) under anaerobic conditions at various temperatures. In the experiments, the vials containing the iron and vinyl chloride were placed on a roller drum set at 8 rpm. Separate reactions were performed at 4, 20, 32, and 45 °C. The major degradate produced was ethylene. Degradation followed pseudo-first-order kinetics. The rate of degradation increased as the temperature increased. Based on the estimated activation energy for vinyl chloride reduction of 40 kilojoules/mol, the investigators concluded that the overall rate of reaction was controlled at the surface rather than the solution. 

This chapter assesses the performance of quantum chemical models with regard to the calculation of reaction energies. Several different reaction classes are considered homolytic and heterolytic bond dissociation reactions, hydrogenation reactions, isomerization reactions and a variety of isodesmic reactions. The chapter concludes with a discussion of reaction energies in solution. 

Results from semi-empirical calculations are not as good as those from other models, but somewhat better than might have been anticipated on the basis of previous reaction energy comparisons (note, however, their favorable performance for relative CH bond dissociation energies). 

Step 2 nitrile reduction In this step the two processes are very similar both are Raney nickel-catalyzed nitrile reductions using hydrogen. The reason the enzymatic process has an approximately halved energy is that it is being carried out in the enantiopure form, whereas in the classical resolution process this reaction is performed with a racemic substrate. 

Solution-based polymerization is the most common process used today for SAP manufacture. This process is efficient and generally has a lower capital cost base. The solution process uses a water-based monomer solution to produce a mass of reactant polymerized gel. The polymerization s own reaction energy (exothermic) is used to drive much of the process, helping reduce manufacturing cost. The reactant polymer gel is then chopped, dried, and ground to its final granule size. Any treatment to enhance performance characteristics of the SAP is usually accomplished after the final granule size is created. 

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