Geometric orientations

A number of other syntheses were discussed by Takeuchi and Furusaki and the most common involved reaction of hydroxylamine with selected a,/3-unsaturated ketones to give isoxazolidine-3- or -5-ols, which exist in equilibrium with an open-chain counterpart (77AHC(21)207). A similar equilibrium was observed in the reaction of a,/3-unsaturated ketones with N-hydroxyurea. The geometric orientation of the ring substituents was studied as a dynamic process (Scheme 158) (75TL2337). 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

For complex organic molecules, geometric considerations alone lead one to the conclusion that only a small fraction of bimolecular collisions can lead to reaction. One can represent the fraction of the collisions that have the proper geometric orientation for reaction by a steric factor (Ps). Except for the very simplest reactions, this factor will be considerably less than unity. On the basis of simple collision theory, it is not possible to make numerical estimates of Ps, although it may occasionally be possible to make use of one s experience with similar reactions to determine whether Ps for a given... 

The effect of crown ethers on the geometrical orientation in base-promoted E2 eliminations has been studied by several groups. Bartsch et al. (1973) have investigated the effect of several parameters on the potassium alkoxide-promoted eliminations of HBr from 2-bromobutane (Table 45). In the absence of crown ethers the relative amount of 1-butene formed increases, while the trans/cis ratio of the 2-butenes decreases, with decreasing solvent polarity. Furthermore, the proportion of 1-butene increases and the trans/cis ratio decreases on increasing the base concentration. These effects were explained in terms of steric interactions between the base and a- and /7-alkyl groups in the... 

Two methods for including explicit electrostatic interactions are proposed. In the first, and more difficult approach, one would need to conduct extensive quantum mechanical calculations of the potential energy variation between a model surface and one adjacent water molecule using thousands of different geometrical orientations. This approach has been used in a limited fashion to study the interaction potential between water and surface Si-OH groups on aluminosilicates, silicates and zeolites (37-39). 

Sousa LR, Larson JM (1977) Crown ether model systems for the study of photoexcited state response to geometrically oriented perturbers. The effect of alkali metal ions on emission from naphthalene derivatives. J Am Chem Soc 99 307-310... 

The energy difference An of n+ and n is attributed to the interaction of n and 2- An is dependent on the separation of the two nitrogen atoms by the number of cr bonds and also on their mutual geometrical orientation. 

Rectangular or Square Parallel Keys and Their Corresponding Keyways Cylindrical and I/IO Conical Shaft Ends Geometrical Orientation and Directions of Movements Mechanical Vibration—Balance Quality Requirements of Rigid Rotors—Part I ... 

Chain Pair Modeling. In the following analysis, we assume that the chains are regular helices, i.e. that they have screw symmetry, with a repeat distance, t. In a perfect crystal, such chains must either be parallel or antiparallel. Four interhelical parameters are required to define the geometric orientation of chain A relative to chain B (see Figure 2). The parameters and their ranges are ... 

Figure 2. Interhelical parameters required to define the geometric orientation of chain A relative to chain B.

One approach to a solution of this problem was put forward by Hansen (14), who derived general equations which express the overall experimental film absorbance in terms of the external reflectance of the substrate. These relations contain within them expressions for the individual anisotropic extinction coefficients in each geometric orientation. Solution of these general equations for the anisotropic extinction coefficients allows for an unambiguous description of the dipole orientation distribution when combined with a defined orientation model. 

The importance of e.s.r. in this field is mainly in achieving a detailed characterization of radical structures. Because the resonance line due to the unpaired electron is split by magnetic nuclei in the radical in a very characteristic way, the technique actually counts the various types of nuclei and allows in many cases an accurate determination of their respective position and geometrical orientation. Patterns of relationships between structure and e.s.r. parameters, based on many experimental results, have been developed and applied to new systems successfully. Details of e.s.r. methods are given in many books and reviews on the subject (see especially Fessenden and Schuler, 1970 Wertz and Bolton, 1972). 

Logically it would be expected that ion pairing phenomena would also affect the product distribution in base-promoted -elimination from 2-alkyl halides. In such systems one must distinguish between positional orientation, which refers to the relative proportions of 1-and 2-alkenes formed, and geometrical orientation, relating to the cis and trans 2-alkenes produced. 

Thus it is found that the relative amounts of olefinic products which result from reactions of 2-bromobutane with MeOK/MeOH, EtOK/EtOH and t-BuOK/DMSO are insensitive to changes in the base concentration (Bartsch et al., 1973a). In contrast, for eliminations induced by t-BuOK/t-BuOH, both positional and geometrical orientations are base concentration dependent. As the concentration of t-BuOK increases, a relatively higher proportion of 1-butene is... 

Figures 15 and 16 illustrate the effect of addition of varying amounts of DMSO upon positional and geometrical orientation in reactions of 2-bromobutane with 0 5 M potassium t-butoxide in...

In some cases the geometrical orientation is not identified in the original document. In most of such cases the sample is a mixture of isomers. If data is found only for unspecified geometrical isomers the recommendations are based on such data. However, if specific geometrical isomers are identified as well, both kinds of data are listed in separate tables. Then the recommendations are based on data for the specific isomers. 

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