CNMR model

Some mathematical models describing a CNMR have been developed by various investigators [Harold et al., 1989 Sloot et al., 1990 Sloot et al.. 1992 Veldsink et al., 1992 Harold etal., 1992]. 

Sloot et al. [1990] presented a simplified isothermal CNMR/ORG model which assumes that the two chambers divided by the membrane are well mixed. In practical applications, the model needs to be incorporated into a more complex model which, for example, considers the effect of flow configuration (cocurrent or countercurrent mode). In their model, mass transfer in the direction perpendicular to a flat membrane (i.e., y-direction) is considered for a general instantaneous, reversible reaction... 

It is now possible to satisfactorily calculate CNMR shifts with an average error of only 5 ppm. The results for the above compounds are given in Table 7, and it can be seen that they are remarkably good. It has not as yet been possible to derive a simple physical model to explain the variation in chemical shifts, but it seems likely that such a model will be forthcoming. It will then be possible to understand a variety of effects found in NMR spectroscopy, including the upheld shifts of cyclopropane hydrogens and carbons. 

This reactor, sketched in Figure 24.2g, was developed especially to prevent slip in reactions required to be strictly stoichiometric. Modeling of a CNMR-E has been attempted for both fast irreversible and reversible reactions (Sloot et al., 1990, 1992 Zaspalis et al., 1991 Veldsink et al., 1992), notably the Claus reaction 2H2S -I- SO2 —> (3/8)Sg + 2H2O. This concept can also be used in partial oxidations in organic technology, for example, partial oxidation of ethylene to acetaldehyde (Harold et al., 1992). The stoichiometry of this reaction can be represented as... 

In the case of plasticized poly(butyral-co-vinyl alcohol) [73], use of dipolar rotational spin-echo CNMR in conjunction with C) determinations, has shown that the frequencies but not the amplitudes of cooperative main-chain motions of the polymer in the hard regions, corresponding to solid polymer associated with partially immobilized plasticizer, are influenced by interactions with the soft regions attributed to liquid plasticizer containing mobile polymer. From this result, a schematic representation of the partitioning of the polymer and plasticizer in terms of a two-phase domain model has been proposed. 

In Chapter 3 Howarth et al. describe an extension of the approach employed above to actually derive the conformational descriptions (RIS models) of polymers by comparison of chemical shifts calculated via the y-gauche effect method with their observed CNMR spectra, followed by iterative adjustment of the RIS models until they yield calculated Cs in agreement with observed values. 

Many analytical techniques have been utilized to analyze the SAN microstructure including LALLS [38], CNMR [19,31,39-44], infrared spectroscopy [45-49], ultraviolet spectroscopy [50-52], pyrolysis GC [8,27,53], pyrolysis mass spectroscopy [54,55], fluorescence [20,56], GPC-IR [57,58], and GPC-UV [52]. Since the terminal model allows the calculation of sequence distribution, the calculated and measured sequence distributions can be compared. This comparison generally shows deviation of the measured sequence distribution vs that predicted using the terminal model. Ham [59] was the first to notice the deviation and explained the deviation based upon penultimate effects. Since that time several other researchers have also notic deviation of their data from the terminal model and have applied more elaborate copolymerization models (Scheme 4) to explain the mechanism of SAN copolymerization. The penultimate [60,61] and complex participation models [33,62,63] have both been evaluated and give a better fit to the SAN system than the terminal model. 

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