Tetrads, configurational

Figure 2. Compositional tetrads (configurational triads) in chains of vinyl (Mj copolymers with a comonomer which places a single atom X in the main chain.

Two-dimensional techniques (HETCOR) also give information about triad and tetrad configuration [42]. The H multiplets at 1.89 and 1.5 ppm couple to three secondary carbon atoms at 34.94,34.23, and 33.42 ppm in the C spectrum. These methylene carbons do not show crosspeaks with any other region in the proton spectra. Thus, these results show that the major isomer isolated from the ACT reaction of 24 is all isotactic. Although this characterization does not allow determination of the absolute stereochemistry of 27, the tentative assignment shown is consistent with expectations based upon previous work done with chiral oxazolidine auxiliaries. 

Figure 1.22 Tetrad configurations in a vinyl polymer, illustrating the equivalence or nonequivalence of methylene protons.

T. Tanaka, Measurement of tetrad configurations in poly-(methyl acrylate) by 300 MHz PMR spectroscopy, Polym.,... 

Polymethylacrylate Measurement of tetrad configurations of triad, tetrad and pentad configurations [100]... 

Note Configurational sequences comprising two configurational units are termed diads, those with three such configurational units triads, and so on. In order of increasing sequence lengths they are called tetrads, pentads, hexads, heptads, octads, nonads, decads, undecads, etc. 

High magnetic fields and in particular C-NMR spectroscopy allow the analysis of even longer configurational sequences (tetrads up to nonads). This proved to be important in particular for the analysis of polyolefins like polypropylene or cycloolefin copolymers (COC). These polymers are available via transition-metal mediated (Ziegler-Natta, metallocene) insertion polymerizations, and the configurational analysis provides deep insight into the respective polymerization mechanisms as well as into the structure-property relationships. 

The proton NMR shifts of diad, triad, and tetrad protons in an isolated chain of PP are calculated, taking into account its configurations and conformations using Pople s approximation. The agreement between the observed and calculated results, however, is insufficient with respect to the large difference between syn and anti protons and the order of tetrad protons. Here, a priori probabilities of specified conformations necessary to the estimation of the chemical shifts are calculated using Flory s matrix method. 

Electronic structure Cation Electronic configuration Octahedron elongated along tetrad axis Octahedron compressed along tetrad axis Configuration of the most stable six-coordinated site... 

Figure 3.13 Crystal field states and electronic configurations of Fe2+ ions in regular octahedral and tetragonally distorted octahedral sites. The tetragonally distorted octahedron is elongated along the tetrad axis.

An overbar denotes coarse-grained quantities. The Newtonian constitutive Eq. (8.20c), in effect, defines the configuration-specific, anisotropic, kinematic viscosity-tetradic T (=vijkl) of the spatially periodic suspension. Subject to the attenuation conditions,... 

Deviations from regular smooth variation of properties of lanthanides occur at quarter-, half- and three-quarter filled 4/ configurations which have been attributed to tetrad effect. This effect has been attributed to small changes in Racah parameters when the ligands around the metal change during the reactions. The half-filled shell effect and the quarter-and three-quarter shell effects are caused by changes in El and 3 in the theoretical ionization potential expressions for /" ions [4],... 

If the stereochemistry of addition does depend upon the configuration found at the end of the chain, whether it is m or r, then we have a terminal model, or first-order Markovian statistics. At minimum we need tetrad data from NMR—i.e., data for (mmm), (mmr), etc.—to test for the terminal model. Remember, we can always calculate triad data from tetrad data using the relationships previously given in Equations 7-26. Equations 7-34 relate the relevant conditional probabilities to observable tetrad and triad sequences. 

To discuss the tetrad effect quantitatively, Nugent analyzed lanthanide and actinide elements using the approximate electronic repulsive energy equation proposed by Jprgensen [15]. He suggested that the electronic repulsive energy between the electrons of the f configuration is related to the electron number q. In fact, the macro tetrad effect is a representation of the relationship between and q. 

It should be pointed out that not all the ions discussed here are affected by the outer fields. In fact, lanthanide ions may be affected by solvents or coordination fields in chemical reactions. For example, E and E will change because of the coordination effect of water or organic molecules in an extraction. In addition, the amount of change would be different in different media. The tetrad effect would thus be different in different systems. The tetrad effect not only relates to the electronic configurations of lanthanide elements but is also affected by the surrounding conditions. Currently it is still not possible to predict the tetrad effect or to calculate it quantitatively. Tetrad effect theory still needs to be improved and further data need to be accumulated. 

That the double-double effect is an intrinsic feature of f-electron configurations is also confirmed by the fact that the pattern of the effect observed in the case of tetravalent actinides starts with Th(IV), i.e. with the 5f° configuration, and is consequently shifted by one place. For hexavalent actinides, the double-double effect most probably starts with uranium (51°) encompassing the U(VI)-Am(VI) tetrad for the first segment (see Sect. 4). 

Plots such as Fig. 2.15 can be made [18] for tetrad and other higher order probabilities (or fractions) as a function of Pm using the relationships given in Table 2.2. These are useful for peak assignments in NMR spectra (in which finer structures are observable), aided by certain necessary relationships (Table 2.3) among the frequencies of occurrences of sequences, which must hold regardless of the configurational statistics (Bemoullian or not). 

Fig. 20.2 shows the NMR spectra for the PVA in (A) the solution (B-D) gel and (E) solid states as measured by solution and solid-state NMR methods [16]. The NMR spectrum for the PVA aqueous solution obtained by the solution C NMR method (Fig. 20.2(A)) shows each of the signals for the CH and CH2 carbons split into multiple peaks due to its stereochemical configuration. The triply split peaks for the CH carbon are assigned to the mm, mr and rr triads from high frequency [17-19] and, furthermore, each of the triad peaks splits into pentad peaks. The split peaks for the CH2 carbon come from tetrad tacticity. In the solution NMR spectrum for the PVA gel shown in Fig. 20.2(B), the signal for the CH carbon splits into three peaks due to triad tacticity. The signals for the CH and CH2 carbon become broader as compared with those for the PVA solution. This is caused by the...

The a a methylene carbons normally display a tetrad sensitivity. In this case, the HHHH carbons are broadened by the presence of closely spaced resonances from sequences of different configurations (see Figure 5). Collectively, the entire region is simply proportional to the HH dyad concentration, that is,... 

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