Groups similarities between

In misiipcrviscd learning, the network tries to group the input data on the basis of similarities between theses data. Those data points which arc similar to each other arc allocated to the same neuron or to eloscly adjacent neurons. 

In clustering, data vectors are grouped together into clusters on the basis of intrinsic similarities between these vectors. In contrast to classification, no classes are defined beforehand. A commonly used method is the application of Kohonen networks (cf. Section 9.5.3). 

Despite these similarities between the G2 calculations of Dg for H2 and Hj, there is a profound difference that is only hinted at in the single-point energies in Table 10-1. In the first group of results we see that all the calculations, with the exception of the last one, give the same answer. In the lower block of results, for H2, this is not the case. Some results are duplicated and some are not. The results are more mixed. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

Equality of all groups in Equation 7-13 assures similarity between systems of different sizes. The types of similarity are geometrie, kinematie, and dynamie. The last tliree terms of Equation 7-13 represent the eonditions for geometric similarity, whieh require that all eor-responding dimensions in systems of different sizes have the same ratio to eaeh other. Eor geometrie similarity, Equation 7-13 beeomes... 

NaAs03 has an infinite polymeric chain anion similar to that in diopside (pp. 349, 529) but with a trimeric repeat unit LiAs03 is similar but with a dimeric repeat unit whereas /6-KASO3 appears to have a cyclic trimeric anion As309 which resembles the cyc/o-trimetaphosphates (p. 530). There is thus a certain structural similarity between arsenates and phosphates, though arsenic acid and the arsenates show less tendency to catenation (p. 526). The tetrahedral As 04) group also resembles PO4) in forming the central unit in several heteropoly acid anions (p. 1014). 

The traditional view of molecular bonds is that they are due to an increased probability of finding electrons between two nuclei, as compared to a sum of the contributions of the pure atomic orbitals. The canonical MOs are delocalized over the whole molecule and do not readily reflect this. There is, furthermore, little similarity between MOs for systems which by chemical measures should be similar, such as a series of alkanes. The canonical MOs therefore do not reflect the concept of functional groups. 

The chemical and particularly the physical similarity between thiophene and benzene has intrigued chemists for a long time, and comparison between phenyl and thienyl derivatives has often been made. Accordingly, it seems somewhat surprising that the effect of replacement of a vinyl group, —CH=CH—, in benzene by has... 

Itishould be noticed that the similarity between the osmophore theory and Witt s chromophore colour theory does not extand much beyond the initial conception and there seems to be no connection between the odour and the colour of a body, it is indeed quite the exception for a body to have both a strong odour and a strong colour. Two prolific sources of colour, viz. the diazo group and a large molecule have no counterpart as regards odour, and it is probably only by chance that quinone and chroman both have pronounced odours and are the sources of colour. 

The present review intends to be illustrative rather than comprehensive, and focuses on the results of this study leading to the hypothesis 9 — the three-dimensional shape similarity between interacting groups in reacting molecules is responsible for more specific and precise molecular recognition than would otherwise be achieved — and on the explanation of biological recognition on this basis. 

The idea of similarity between the isopentyl and 2-phenylethyl groups (Sect. 2.2.1.1) receives support from the results for the complex formation between 19 and 20 (Fig. 10). Comparison of the complex formation data for R8 = i-C5Hu with those for R8 = PhCH2, Ph(CH2)2, and Ph(CH2)3 indicates that, of the three phenylalkyl groups, the 2-phenylethyl group shows the closest similarity to the isopentyl group in the trend of the complex formation21 ... 

The ant Iridomyrmex pruinosus utilizes 2-heptanone as an alarm pheromone. Figure 17 illustrates the existence of a close correlation of the similarity between 2-heptanone and its analogs in molecular model silhouettes with their alarm activity45. It is noteworthy that the replacement of the methylene group in position 3 of 2-heptanone by the ether linkage yields n-butyl acetate which has the same activity as the natural pheromone. Considerable activity is still retained even when the carbonyl group is replaced by the hydroxyl group. 

Similarity effect on chemical events may be traced back to the old empirical rule that like dissolves like similia similibus solvuntur). This rule, however, implies the similarity between solute and solvent molecules in polarity or functional group, but not in molecular shape. 

Closely related to the carboxylic acids and nitriles discussed in the previous chapter are the carboxylic acid derivatives, compounds in which an acyl group is bonded to an electronegative atom or substituent that can net as a leaving group in a substitution reaction. Many kinds of acid derivatives are known, but we ll be concerned primarily with four of the more common ones acid halides, acid anhydrides, esters, and amides. Esters and amides are common in both laboratory and biological chemistry, while acid halides and acid anhydrides are used only in the laboratory. Thioesters and acyl phosphates are encountered primarily in biological chemistry. Note the structural similarity between acid anhydrides and acy) phosphates. 

The group was first to demonstrate NEMCA on metal oxides (IrC>2, R.UO2) and with their NEMCA studies on Ir02-Ti02/YSZ contributed significantly to establishing the similarity between electrochemical promotion and classical metal-support interactions. 

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