Autoassociation

Kramer, M. A., Nonlinear principal component analysis using autoassociative neural networks. AIChE J. 37, 233-243 (1991). 

All of the studies above have used back propagation multilayer perceptrons and many other varieties of neural network exist that have been applied to PyMS data. These include minimal neural networks,117119 radial basis functions,114120 self-organizing feature maps,110121 and autoassociative neural networks.122123... 

Kramer, M. A. Autoassociative neural networks. Comput. Chem. Eng. 1992,16, 313-328. 

Other interesting data in these reactions concern the H/D isotopic effect of the nucleophile/catalyst, for example when [2-hydroxypyridine] = [2 — 02H] = 0.08, fcobsH/ obsD = 1-5. Since a very poor H/D effect is usual in SjvAr reactions with neutral nucleophiles (amines) in apolar solvents10, the authors conclude that the unusually high H/D effect should be due to a difference in the Xh/Xd = 1.75 of the molecular complex. Nevertheless, the same effect could be explained on the basis of an autoassociation of... 

This interpretation was also supported by the spectra of the corresponding N-methyl-leucine derivative in which the H-donor of the selectand was substituted by a methyl group and therefore not available for hydrogen bonding. Both complexes showed a similar spectral behavior as the weak 5-complex of DNB-Leu The C=0 stretch was always shifted from 1725 (uncomplexed autoassociated selector) to 1739 cm (indicative for disrupted H-bonds) in the 5-complex and R-complex as well. These FT-IR data may be regarded as an unequivocal proof for the existence of a stereoselective H-bond between the NH of DNB-Leu and the selector s carbonyl group (Figures 1.10 and 1.11). 

What happens for a nonracemic mixture of enantiomers Is it possible to calculate the values of the chiral properties of the solution from knowledge of the properties of the enantiopure compound In principle, yes, on the condition that there is no autoassociation or aggregation in solution. Then, the observed properties will be simply the weighted combination of the properties of two enantiomers. A nice example of where this normal law may be broken was discovered by Horeau in 1967 it is the nonequivalence between enantiomeric excess (ee) and optical purity (op, with op = [a]exi/[ ]max) for 2,2-methylethyl-succinic acid. In chloroform op is inferior to ee, while in methanol op = ee. This was explained by the formation of diastereomeric aggregates in chloroform, while the solvation by methanol suppresses the autoassociation. 

If several molecules of the chiral auxiliary are involved in autoassociation or in any type of molecular species, then the passage from enantiopure auxiliary to nonenantiopure auxiliary may produce new diastereomeric entities. Consequently, the enantioimpure system may have new stereochemical properties. 

Influence of temperature on the process of polymerization of acrylic acid in dioxane and toluene was examined. It was found that in dioxane an increase in temperature destroys the oligomeric auto-associations of acrylic acid and gives rise to monomer-solvent association, making matrix effect less pronounced. In toluene, an increase in temperature converts the cyclodimeric autoassociations of the monomer into linear oligomers and the matrix effect appears. 

Let us consider this problem in more detail. In fact, the autoassociates of the hydroxyl-containing compounds and amines can be composed of molecules, having different reactivities in the interaction with amine. Therefore, the actually observed reaction rate constant is complex in its composition. Thus, even the simplest noncatalytic (in the absence of proton donors) reactions of the epoxy compound with amine, considering all the donor-acceptor interactions, is generally described by the following kinetic scheme ... 

At present, the problem of the structure of the effective rate constant or, in other words, the nature of the transition state in the epoxy ring opening still remains to be solved. As a first approximation, one can neglect the process of amine autoassociation. In this case, the structure of the effective noncatalytic reaction rate constant, assuming that AjE E, is clear [cf. Scheme (3)]... 

Finally, a series of works 5 14,16 17,29,30) attempted to estimate the effective kinetic parameters of both the noncatalytic and catalytic reactions by taking into account all the donor-acceptor interactions [Scheme (8)] except for autoassociation reactions. 

M(OR)5 species do not readily form coordination complexes but prefer to autoassociate into dimers, trimers, etc. The coordination chemistry of M(OMe)5 has been investigated by low... 

As is seen from the previous section, autoassociation of pyrrole involves NH-77- bonding. Hydrogen bonding of this type can also occur between pyrrole and other aromatic and 77-electron systems. The strength of such bonds is less than that of the pyrrole dimer and considerably smaller than observed for NH-X bonds (vide infra), as shown by the frequency shift of the NH stretching vibration (Table IV). 

Their u9Sn NMR spectroscopy has been used to study autoassociation in methyl- and butyl-tin trialkoxides (64, 68, 72). Figure 7 illustrates the variation of chemical shift with temperature for the neat liquid butyltin trialkoxides, BuSn(OR)3. (72)... 

The affinity of tannins to bind proteins is favored by their ability to work as mul-tidentate ligands in which one tannin is able to bind to more than one point in the same protein structure or bind to more than one protein at a time [22], Proteins can also wrap around tannins [16], and the tannins have the ability to autoassociate with other tannins, forming stacks, even when they are bound to proteins [21],... 

Practically all OH groups are involved in H bonding in a bulk polymer19), since there are many proton acceptors available in epoxy-aromatic amine networks. At room temperature the concentration of free OH groups in networks is lower than 1-2%. The formation enthalpies of different H bonds in the networks measured by the shift of v(OH) vibrations in IR spectra are shown in Table 4. It is seen that the largest part of all H bonds ( 90% in a stoichiometric mixture) comes from the autoassociation of OH groups. 

Experimental dipole moments of azoles are collected in values for three compounds are as follows 1,2,3-triazole 1.79 (benzene, 25 °C), benzotriazole 4.10 (dioxane, 25 °C), l-methylbenzotriazole4.1 D (benzene, 25 °C). More recent measurements combined with CNDO/2 calculations have been carried out by Mauret et al. (75bsfi675> the results are shown in Table 4. The measurement of molecular polarizations represents not only a valuable method for investigating autoassociation effects of azoles, but can also be widely used for the determination of tautomerism in the azole field (75BSF1675, 76Ahc(S1)32, 76AHC(S1)281). 

C NMR spectra of organotin derivatives 36-38 and their autoassociated complexes (see Section 4.05.3.2.3) have been recorded and chemical shift shown to be diagnostic of coordination between the tin atom and oxygen of a carbonyl group of adjacent molecules <1995JPR242>. 

The coordination number may also be increased by autoassociation. For example, 2,2-di-t-butyl-l,3,2-oxathiastannolane (2-1) in CDCI3 solution at 0.02 M concentration, when it is present principally as the monomer, shows 8 +52. The signal progressively moves upfield as the concentration increases, reaching a value of -25 at 0.45 M, when a substantial amount of the dimer is present. In the solid state, where X-ray crystallography confirms that the compound exists as the dimer, the value of 8 is -100.61... 

Another characteristic structural feature of cinchona alkaloids is their multifunctional character and, thus, autoassociation phenomena are possible that could result in the strong dependency of their efficiency on the concentration and temperature [22, 23]. 

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