Triad effect

Reactions of [M(CO)5SR]- (M = Cr, Mo, W R = H, Ph) in THF with P-donors lead to facile replacement of cis CO ligands at ambient temperatures, thus illustrating the very strong m-labilization by the thiolate ligands. In the absence of P-donors the complexes dimerize rapidly and reversibly to form [M(CO)4SR] which also undergo reactions with P-donors to form substituted mononuclear products. Rates of substitution into [M(CO)5SR] increase along the series M = Mo > Cr > W in accordance with the triad effect. No kinetic studies were carried out to investigate the effects, if any, of [P-donor] on the rates. 

In poly(methyl thiirane) the tertiary carbon of the polymer chain is stereosensitive in carbon tetrachloride and shows dyad effects [38,41]. The same carbon shows triad effects in the case of poly( -butyl thiirane) [42] and poly(l-methoxy methyl thiirane) [25]. The position of isotactic peaks was again confirmed using optically active products. 

O atom quadruple coupling constants. The strength of the Jt-bonding in [MofCO) ] is ca 10% less than in [Cr(CO)g] or [W(CO)6] and the results are compared with those of earlier investigations using other techniques, particularly with theoretical work that emphasizes jr-bonding as the major contributor to M—CO bond strengths. Such results are clearly related to the triad effect. ... 

The foregoing discussion has been conducted in terms of Mi sequences. Additional relationships of the sort we have been considering also exist for dyads, triads, and so forth, of different types of specific composition. Thus an ability to investigate microstructure experimentally allows some rather subtle mechanistic effects to be studied. In the next section we shall see how such information is obtained. 

For the case of copolymers, suppose we consider the various triads of repeat units. There are six possibilities MjMjMj, M1M1M2, M2M1M2, M2 M2 M2, M2 M2 Ml, and Mi M2 Mi. These can be divided into two groups of three, depending on the identity of the central unit. Thus the center of a triad can be bracketed by two monomers identical to itself, different from itself, or by one of each. In each of these cases the central repeat unit is in a different environment, and a characteristic proton in that repeat unit will resonate at a different location, depending on the effect of that environment. 

By changing Ser 221 in subtilisin to Ala the reaction rate (both kcat and kcat/Km) is reduced by a factor of about 10 compared with the wild-type enzyme. The Km value and, by inference, the initial binding of substrate are essentially unchanged. This mutation prevents formation of the covalent bond with the substrate and therefore abolishes the reaction mechanism outlined in Figure 11.5. When the Ser 221 to Ala mutant is further mutated by changes of His 64 to Ala or Asp 32 to Ala or both, as expected there is no effect on the catalytic reaction rate, since the reaction mechanism that involves the catalytic triad is no longer in operation. However, the enzyme still has an appreciable catalytic effect peptide hydrolysis is still about 10 -10 times the nonenzymatic rate. Whatever the reaction mechanism... 

Millan and coworkers (99-101) also studied the effect of tacticity on the nucleophilic substitution reactions of PVC. Sodium thiophenate and phenol were used for these reactions. The central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads was found to be most reactive. It was concluded that initiation of degradation may occur by normal structures, and polyene build-up may be favored by syndiotic sequence. This... 

Suffice it to say that Dobereiner s research established the notion of triads as a powerful concept, which several other chemists were soon to take up with much effect. Indeed, Do-bereiner s triads, which would appear on the periodic table grouped in vertical columns, represented the first step in fitting the elements into a system that would account for their chemical properties and would reveal their physical relationships. 

Overberger and associate110) also prepared copolymers of 6(6)-vinylbenzimid azole with acrylic add 62 (PBzJm - AA). In 40%-propanol water, the activities of the copolymers indicated a strong dependence on the carboxylate-benzimidazole-carboxylate triad. These effects were ascribed to strong electrostatic interactions between these sequences and substrates. 

Triad information is more powerful, but typically is subject to more experimental error and signal assignments are often ambiguous (Section 7.3.3.12). Triad data for the MMA-S system are consistent with the terminal model and support the view that any penultimate unit effects on specificity are small.Mv lS... 

Analysis of the data collected by Hill et al. illustrates how a four parameter problem can be reduced to one involving only two parameters by appropriately formulating the problem. In addition an approach to fitting models directly to triad fraction data is shown. Our analysis confirms the conclusion made by Hill et al. that a penultimate effect does exist for both styrene and acrlyonitrile. 

The characteristics of C NMR spectra for all copolymers were similar. The triad distributions for all copolymo" from NMR monomer insertion are shown in Table 2. Based on the triad distribution of ethylene/l-hex aae copolymers in Table 2, we found that microstructurc of copolymer obtainrai fiom aluminoxane system was slightly different in monomer incorporation, but found significantly when borated system was applied. We suspected that this difference was arising fiom the diffaences in bimetallic complex active species between [aluminoxane] [catalyst] and [Borate] [catalyst] which had the electronic and gMmetric effects fiom the sterric effect of larger molecule of borate compare to the aluminoxane on the behaviors of comonomer insertion in our systems. 

Such configurational as well as conformational effects have been also reported by MILLAN et al. in the case of nucleophilic substitution of poly(vinyl chloride) with sodium thiophenate (14) and with sodium isooctylthioglycolate or isooctylthiosalicylate (15). The authors have shown that these reactions proceed selectively on the isotactic TT diads which can only exist either in the GTTG isotactic or in the TTTG heterotactic triads, the former ones being much more reactive than the latter ones. 

The physical properties (7-10) of our E-V copolymers are sensitive to their microstructures. Both solution (Kerr effect or electrical birefringence) and solid-state (crystallinity, glass-transitions, blend compatibility, etc.) properties depend on the detailed microstructures of E-V copolymers, such as comonomer and stereosequence distribution. I3C NMR analysis (2) of E-V copolymers yields microstructural information up to and including the comonomer triad level. However, properties such as crystallinity depend on E-V microstructure on a scale larger than comonomer triads. 

The relative proportions of triads is determined by the synthetic conditions chosen as described above for acrylic acid copolymers of acrylamide derived by either direct copolymerization or by hydrolysis. Also, the polymerization pH has a considerable effect on the reactivity in acrylamide/acrylic acid copolymerization. 

In contrast to the case of Cp2ZrX2/MAO giving atactic poly(alkene)s, Cp MCl2/MAO, M = Zr (139) and Hf (140), are the catalyst precursors of the syndiotactic polymerization of 1-butene and propylene [176]. Triad distribution indicated that this is chain-end controlled syndiospecific polymerization. The syndiospecificity is attributed to the increase of steric encumbrance around the metal center. Thus, Cp HfX2 is the most effective syndiospecific catalyst component in this system. 

The explanation for the solvent and salt effect in Scheme 22 lies in the dynamics of the photogenerated ion-radical triad in equation (81). Thus, the ion-pair annihilation is favored in nonpolar solvents such as dichloromethane to afford the alkylation product237 (equation 82). 

In contrast, resonance delocalization and bond alternation in the C—C=C—C backbone are only slightly affected by H-bond formation (namely, the C4—C5 bond order varies by only 0.008 between H-bonded and open conformers), because such resonance shifts do not intrinsically alter the charge distribution in the H-bonded O- H—O triad. This example illustrates the principle that H-bonding is not generally coupled to resonance per se, but only to such resonance as leads to effective CAHB enhancement (Section 5.2.2). 

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