Mechanism Construction and the Sources of Data

Critical reviews of reaction rate data are constantly appearing in the literature and are an important source for mechanism construction. Some examples of recent reviews are given below. 

The approach with the partitioning of the system into a QM and a classical molecular mechanical (MM) part, thus usually termed hybrid QM/MM procedure, provides a reasonable reduction of the computational effort by restricting the time-consuming QM calculation of forces to the most relevant part of the liquid system. The main error sources in this approach are a too small choice of the QM region, an inadequate level of theory for the QM calculation, the choice of suitable potentials for the MM part of the system, and smooth transitions of particles between QM and MM region. In conventional QM/MM procedures, the whole system is first evaluated at MM level and then corrected by the QM data. This means that classical potential functions (with all their problems and difficulty of construction) are needed for all components of the system. A recently developed methodology can reduce the need for such potentials to the solvent only, as will be outlined below. 

In the Laue method a polychromatic beam of x-rays impinges on a stationary crystal. A given reflecting plane extracts from the beam the particular wavelength which allows for constructive interference or reflection to occur. Hence, the data collection time is set only by the exposure time, without any mechanical time overhead. With an unfocussed SR beam, exposure times become of the order of a few seconds for a complete or near complete data set, in favorable cases. With the optimization of the source type and beam line optics, exposure times as short as 10 10 seconds for one Laue pattern have been achieved. 

As illustrated in Fig. 1, there are essentially four methods for obtaining thermochemical data for the species in our reaction mechanism. The first choice is to find the needed data in databases or in the literature in general. This includes both published experimental data and published quantum chemical calculations, which can also be a reliable source of thermochemical data. If no information on a substance is available in the literature, one should consider whether it can be treated by group additivity methods. If a well-constructed group additivity method is available for the class of molecules of interest, the results, which can be obtained with minimal effort, will be comparable in accuracy to those from the best quantum chemistry calculations. If group additivity is not applicable to the molecules of interest, then we may want to carry out quantum chemistry calculations for them, as discussed in detail in an earlier chapter. In some cases, the effort required to carry out the quantum chemical calculations may not be warranted, and we may want to make coarser, empirical estimates of thermochemical properties. 

Since C/eff includes contributions from higher-order potentials, it will, in general, depend on T and p. Such pair potentials have been constructed essentially from two sources either from quantum-mechanical calculations of the energy of a pair of water molecules at some selected configurations or by assuming some model potential function, the parameters of which are determined by fitting the results to some experimental data. This is currently an active and changing field of research. We shall describe very briefly the essential features of such a potential function in section 7.4. 

Therefore, the radical source and NOx offgasing rates indicated by the characterization data for the first series of experiments for this chamber is probably as low as one can obtain for reactors constructed of FEP Teflon film, which is generally believed to be the most inert material that is practical for use as chamber walls. Although die radical source and NOx offgasing rates for the second series of experiments is higher (see also Figure 3), they are still about an order of magnitude lower than observed for the UCR and UNC chambers previously used for mechanism evaluation. 

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