Bond sphere

The similarity of the retrieved protons to those of the query structure, and the distribution of chemical shifts among protons with the same HOSE codes, can be used as measures of prediction reliability. When common substructures cannot be found for a given proton (within a predefined number of bond spheres) interpolations are applied to obtain a prediction proprietary methods are often used in commercial programs. 

Figure 10.3-42. Deriving the reaction substructure, A diFferent number of bond spheres around the strategic bond can be Included in the reaction substructure, thereby influencing the specificity of a search in a reaction database.

The position of the ehosen strategic bond locates the reaction center. To derive the reaction siibstrncture, the user can select the number of bond, spheres around the strategic bond which should be included. The reaction substructure obtained is then n.scd as the query for a reaction substructure search in the database. Figure 10,3-42 illustrates the first and second bond spheres around a selected strategic bond of a retrosynthetic step. 

The number of bond spheres chosen influences the specificity of tlie reaction substructure query and the result of the search. In Figure 10.3-43 the reaction substructure queiy including the first bond sphere of the retrosynthetic step of Figure 10,3-42 is shown. 

The search in the Theilhcimer reaetion database [62] provides 161 reactions for this query. If the search is performed without any additional bond spheres (covering only atoms of the inner sphere with a dark gray bac kgroiind in Figure 10.3-42 as well as the added atom groups on the precursor side), 705 reactions arc obtained in the Theilhcimer database. The result of this search is less precise than that of the first search. Additionally, reactions forming any kind of C-0 bonds (c.g., making an ether bond instead of an ester bond) arc found. However, in both searches too many hits arc obtained in order to detect suitable reactions in a reasonable... 

Figure 10.3-54 illustrates how the reaction center is derived from the disconnection of the strategic bond and which additional bond spheres are considered in the definition of the reaction substructure search queiy. 

Other workers have added THF or H2O to the carrier gas stream in efforts to affect the requisite vapor pressure increase in Ba(/3-diketonate)2 compounds. " The objective of all these experiments was to coordinatively saturate the barium atom in the vapor phase by incorporation of the lone pair electrons on the oxygen atoms of the neutral ligands into the bonding sphere of the metal. At the temperatures demanded for vapor transport, the equilibrium for dissociation of these ligands is rather unfavorable therefore, any anticipated gain will be minimal. 

After the disconnection strategy is defined, the systems indicate the strategic bond together with their ranks. The user can now analyze the precursor or can verify the disconnection by performing a reaction substructure search in any of the interfaced reaction databases. To perform a search in the reaction database, the user can define the bond sphere to be considered as identity criterion. The first sphere, for instance, includes bonds attached to the atoms of the strategic bond. A hit is presented as a reaction with additional information from the reaction database, such as reaction condition, yield, and references. 

The following steps were proposed for the isomerization of n-butane to isobutane 1. The butane contacts the catalyst mass in such a manner that the hydrogen of the hydrogen chloride in combination with aluminum chloride (designated as Ho in the diagram) enters the bonding sphere of the carbon atoms (designated as Ci) of the butane. 

These last two cluster types are often referred to as M0X12 and M 0X0 to emphasize the differences in the cores, but it should be remembered that the six exo positions are evidently always occupied by halide or another Lewis base (Smith Corbett, 1985). To do otherwise, leaving a substantial fraction of the bonding sphere about a transition metal empty, would be unusual. 

In Figure 3 the estimation has been performed for 2,2-dichloro-l-phenylethanol using the HOSE code technique over a one- to four-bond sphere, showing the necessity of taking into account at least the three-bond sphere in order to get reliable results. From practical experience it can be concluded that for chemical shift estimation of an sp carbon a three-bond sphere is necessary in order to include y-effects, whereas for sp carbons a four-bond sphere usually gives acceptable results, because this includes also pnrn-substituents within aromatic systems. For some rare cases a correlation up to a six-bond sphere may be necessary the alternative is the visual inspection of the distribution of the chemical shift values of the reference spectra in order to select the correct entries... 

In another approach (see NMR Data Correlation with Chemical Structure),three-dimensional (3D) substructure descriptors are used as a basis for the prediction. In this case the distribution function is simplified and a subsequent analysis is not required. This extension of the HOSE code is based on the ability to count the total number of steric interactions over a three-, four-, and five-bond sphere and additional parameters to describe the axial and equatorial substitution pattern for each atom. In the simple case of cis/trans ksomerism the number of interactions is calculated by means of the display coordinates. More complex situations are described by up/down bonds in this case the calculation of the number of steric interactions for each atom is performed by comparison of the query structure with carefully selected references from a library of ring skeletons. Figure 6 exemplifies the method. The discussion of the stereochemical effects is focused on the two diastereotopic methyl groups at position 4. The experimental shift values are 22.0 and 33.6 ppm. The prediction without stereodescriptors results in 27.4 ppm for both atoms (Figure 6, upper part). If the stereochemical effects are considered, the predicted shifts (21.5 and 33.5 ppm) deviate insignificantly from the experimental values (Figure 6, bottom). 

Theoretical determinations of the counterion distribution around a PE were done by Solms by approximating them as tangentially bonded spheres which interact via the coulomb potential [64]. In his work, the Osmotic pressure and thermodynamic properties were determined via analytical integrals [64]. 

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