Orientation ratio

Although A-alkylation is one of the most important and most studied reactions of pyrazoles, its quantitative (orientation ratios) and qualitative (mechanism) aspects are still unclear. All the classical results are reported in (67HC(22)l, B-76MI40402) and can be summarized as follows. 

Problems related to the use of a guest dye can be reduced if the polymer contains a fluorescent chemical group. Gohil and Salem [70] took advantage of such intrinsic fluorescence to characterize the in-plane distribution of orientation in biaxially drawn PET films. In these experiments, the chain-intrinsic fluorescent label is due to the formation of dimers by two terephthalic moieties, exclusively within the noncrystalline regions. A comparison between sequential and simultaneous drawing along the MD and TD directions was undertaken for a fixed MD draw ratio of 3.5 and various TD draw ratios. The orientational order was characterized by two "orientation ratios" Rmd and RTD such that... 

Figure 12 Orientation ratios RMD (open symbols) and RTD (filled symbols) as a function of the TD draw ratio of PET films drawn for a sequential (circles) and simultaneous (triangles) process. Reproduced with permission from Gohil and Salem [70]. Reprinted with permission of John Wiley Sons, Inc.

These results with regard to heme disorder differ from those observed in solution by NMR spectroscopy, where one heme orientation is at least highly favored (Section 11,E,2). The difference in conditions (high salt, PEG) may be involved in the fact that different ratios of the two heme orientations are observed in the crystalline state than in solntion. The difference in heme orientation ratios of 1.5 1 and 10 1, for example, represents a AG difference of only 4.9 kJ/mol. 

Table 11 shows very little variation in the orientation of the addition of trihalogenomethyl radicals to vinyl fluoride, but for vinylidene fluoride there is a 4.5-fold increase in the orientation ratio (kjkj across the table. This change in orientation does not follow polarity and suggests that steric effects may be having some effect. The rates relative to ethylene for the three fluorine-containing radicals are very similar. 

We have seen that the overall rates of addition appear to show evidence of steric effects and we have now found that the trihalomethyl radicals also show some evidence of a steric effect. Table 12 compares the relative rate and orientation of the addition to olefins of perfluoroalkyl radicals. There is a dramatic difference between Tables 10 and 11 on the one hand, where the main feature is the similarity of the rates and orientation ratios, and Table 12. In... 

Table 12 there is a marked increase in the orientation ratio as the attacking radical becomes more bulky. With vinylidene fluoride the orientation ratio for perfluoro-t-butyl radicals is too large to measure. Even with perfluoro-isopropyl the orientation ratio (kjk is 103. In the other tables the orientation ratios for addition to vinyl fluoride and trifluoroethylene vary only slightly, but in Table 12 there is a marked increase as the radical becomes more branched. Perhaps the most striking feature of the table is that the increase in orientation ratio is almost entirely due to a reduction in the rate of addition to the most substituted end of the olefin. The relative rates of attack at the unsubstituted ends of vinyl fluoride and vinylidene fluoride vary only slightly across the table. In other words, the high orientation ratio is almost entirely due to the difficulty of addition at the substituted end of the olefin, a conclusion supported by the great variation in the relative rate of addition to tetrafluoroethylene (i.e. where both ends are fully substituted). 

Fig. 1 Correlation of log (orientation ratio) of perfluoroalkyl radicals with the pA"a of the corresponding perfluoroalkyl hydrides (El-Soueni et al., 1977)...

If steric hindrance is important we should see evidence for this in the effect substitutents in the olefin have on the orientation ratio (see Table 14). The first feature to notice about Table 14 is that the methyl and trifluoromethyl substituent groups have similar directive effects towards the three electrophilic radicals. This is in sharp contrast to electrophilic ionic addition where, for... 

The orientation ratios (vinyl fluoride and trifluoroethylene... 

For the DRl-MMA 35/65, N = 1.4 10 molecules/cm and P - 0.8 10 It implies a mean orientation ratio equal to 0.3, which is comparable to what can be achieved using the dc electric field poling technique such as corona poling. 

Various data sets were obtained by using the different sensors however, in this paper we concentrate on results from measurements with the laser slope gauge as well as with the X-band scatterometer operating at co-(VV) and cross-polarisation (HV). In order to gain insight into the specific damping behaviour of OLA in the presence of rain, we calculated the normalised orientation ratio, NOR, which we define as... 

Fig. 1. Frequency dependence of the normalised orientation ratio, NOR (see (1)), measured with the two-dimensional laser slope gauge at different wind speeds (columns) without rain (upper row) and with rain at a rain rate of R=160 mm h 1 (lower row). The water surface was not covered with any slick.

We have studied the dependence of the wave orientation ratio (measured with the two-dimensional laser slope gauge) on slick coverage and on wind speed. It can be inferred from our results that, on a slick-covered water surface, the rain-induced effects have an influence on the orientation of the wave field, particularly on the high-frequency part (up to 25 Hz), which is expected to be due to high frequency ring waves generated by the rain. 

In the free radical polymerization of vinyl acetate, CH2=CH(OOCCH3), on the other hand, there are weak dipole-dipole interactions between the ester groups in the transition state, which facilitates an occasional attack on the a-carbon atom despite steric hindrance by these groups. Poly(vinyl acetate) therefore contains l%-2% head-to-head structures, that is, the ajP orientation ratio is 0.01-0.02. The orientation ratio depends on the attacking species, as well as on the nature of the attacked monomer (Table 15-3). The attack is even almost exclusively at the a position with certain initiators, as, for example, in the copolymerization of butadiene and propylene with certain modified Ziegler catalysts. 

Table 15-3. Orientation Ratio, a IP, for the Placement of Free Radicals...

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