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Two-plane orientation approach

Fig. 12- The geometry of interactions with sp nitrogen-containing heterocycles retreived from the Cambridge Structural Database shows that the metal prefers a head-on and in-plane approach to the lone electron pair of nitrogen, (a and b) Two perpendicular orientations are shown. Dashed lines are at 30° from the C-N-X (X = C, N) bisector. [Reprinted with permission from Vedani, A., Huhla, D. W. ]. Am. Chem. Soc. 112, 4759-4767. Copyright 1990 American Chemical Society.]... Fig. 12- The geometry of interactions with sp nitrogen-containing heterocycles retreived from the Cambridge Structural Database shows that the metal prefers a head-on and in-plane approach to the lone electron pair of nitrogen, (a and b) Two perpendicular orientations are shown. Dashed lines are at 30° from the C-N-X (X = C, N) bisector. [Reprinted with permission from Vedani, A., Huhla, D. W. ]. Am. Chem. Soc. 112, 4759-4767. Copyright 1990 American Chemical Society.]...
Herndon and co-workers4 developed a model for predicting regioselectivity which has been adapted to periselectivity problems by Paddon-Row.64 It makes two assumptions (a) the two reaction partners approach in parallel planes and (b) the distance between these planes is the same in all reactions. The second is a serious constraint, which explains a success rate of 14 correct predictions out of 17 cases (82%). The model is more reasonable when applied to regioselectivity, where two different orientations of the same cycloaddition are compared (122 correct predicttions in 133 cases, i.e. 91.7%).5 Nonetheless, to the best of our knowledge, FO theory provides the only simple way to study periselectivity available at present. [Pg.128]

The olefin retains its cri-configuration, i.e., the two cri-hydrogens of cyclic alkene must remain cis in the product. The stereochemistry at the remaining center comes from the way the two molecules approach each other. (1) As the alkene is unsymmetrical, the less sterically demanding carbonyl part of the ketene will be oriented above the larger alkene substituents, i.e., will be in the middle of the ring. (2) As the ketene is unsymmetrical, favored transition state must have the larger of the two substituents oriented away from the plane of the alkene. [Pg.160]

As the final topic of this chapter, the future-oriented approach of mechanically controlled DDS and sensing using molecular machines is presented. Molecular machines are certain kinds of state-of-the-art objects in current organic chemistry, supramolecular chemistry, and nanotechnology. A single molecule and/or a complex formed with a couple of molecules work as a machine in ultrasmall dimensions. However, most research efforts on molecular machines stay within the fine science level, and practical uses of molecular machines are still at the dream level. If we can control functions of molecular machines by conventional mechanical actions such as hand motions, it would open the way to common uses of molecular machines in daily life. In order to realize mechanical control of molecular machines, it would be required to couple two kinds of motions over very different length scales, that is, mechanical motions in meter or centimeter size and molecular motions in nanometer scale have to be combined. It can be rationally done if we use a two-dimensional medium where in-plane directions possess macroscopically visible dimensions and their thicknesses are maintained in the nanometer region [22]. Manual control of molecular machines can be accomplished at dynamic two-dimensional media. [Pg.36]

Cycloaddition of species with triple bonds, which should logically be addressed at this point, will be postponed to later chapters. The reluctance of acetylene to dimerize to cyclobutadiene (CBD) on the ground-state surface follows directly from Fig. 6.2. It is sufficient to note that when two acetylene molecules approach one another in the plane-rectangular (D2/1) orientation, the two additional tt orbitals in acetylene are retained as such in CBD, so they cannot alleviate the forbiddenness of the [ 2g + pathway [5, Fig. 4]. Discussion of the reaction between dioxygen and acetylene to form 1,2-dioxetene and the cycloreversion of tetraalkyl-l,2-dioxetanes to two ketonic fragments has to be postponed until the relation between space and spin symmetry has been introduced in Chapter 9. [Pg.171]

Fig. 5.10. Crystal splitting of the 730-720 cm" band, caused by the in-phase (CH2) rock vibration in crystalline long CH2 chain containing molecules. The unit cell shown has sections of two differently oriented CH2 chains, each performing the in-phase CH2 rock vibration. The bold-face CH2 groups all lie in one plane and the chain progression axis is more or less perpendicular to the page. The relative phase of the two differently oriented chains is different for the two unit cell modes. In the 730 cm" mode the two chains move their hydrogens toward and away from each other, and the close approach is shown. In the 720 cm" mode the two chains tend to stay out of each other s way. Fig. 5.10. Crystal splitting of the 730-720 cm" band, caused by the in-phase (CH2) rock vibration in crystalline long CH2 chain containing molecules. The unit cell shown has sections of two differently oriented CH2 chains, each performing the in-phase CH2 rock vibration. The bold-face CH2 groups all lie in one plane and the chain progression axis is more or less perpendicular to the page. The relative phase of the two differently oriented chains is different for the two unit cell modes. In the 730 cm" mode the two chains move their hydrogens toward and away from each other, and the close approach is shown. In the 720 cm" mode the two chains tend to stay out of each other s way.
Further, the presence of anisotropic distortion of the basal plane of a-plane wurtzite layers, [s 7 Sy ), will lift-off the degeneracy of the x and e Sy. Therefore, the IR dielectric functions Sx and Sy provide access to the frequencies and broadening parameters of the TO and LO phonons with Ei symmetry polarized along the x = [1120] and y = [1100] directions. In other words, the splitting of the TO and LO with Ei symmetry that is predicted theoretically by Equation 9 [14,16] can be obtained from the IR eUipsometry data analysis. Note, that polar c-plane GaN heteroepitaxial layers that experience anisotropic distortion of the basal plane, for instance when grown on a-plane sapphire [29] will also allow assessment of the Ei phonon splitting [17, 18]. In this case, the optical measurement will depend on the orientation of the plane of incidence and incident polarization with respect to the two in-plane directions X = [1120] and y = [1100]. The standard eUipsometry measurement for non-c-plane-oriented and anisotropically strained wurtzite crystals is inapplicable and the generalized eUipsometry approach is needed. [Pg.234]

Formation of trans isomers in overwhelming predominance in the ISOC reaction leading to five-membered rings (Entries a-d) has been ascribed to the orientation in which H% H , and R are on the exo face of TS 182b (this avoids a possible strain between R and NO or between H and [48b] that is presumably present in TS 182 a). Since elimination of silanol involving H in no way interferes with the orientation of H and R, a trans relationship between H and is abundantly clear. This fully accords with the widely accepted view that approach of the dipole and dipolarophile takes place in two parallel planes [49] and that the endo TS is preferred in the absence of obvious steric effects [50]. Formation of approximately 5% cis isomer when the dipolarophile terminus is disubstituted is accountable in terms of the cycloaddition taking place via TS 182a. [Pg.27]

The parameters K1/ K2/ and K3 are defined by the refractive indices of the crystal and sample and by the incidence angle [32]. If the sample has uniaxial symmetry, only two polarized spectra are necessary to characterize the orientation. If the optical axis is along the plane of the sample, such as for stretched polymer films, only the two s-polarized spectra are needed to determine kz and kx. These are then used to calculate a dichroic ratio or a P2) value with Equation (25) (replacing absorbance with absorption index). In contrast, a uniaxial sample with its optical axis perpendicular to the crystal surface requires the acquisition of spectra with both p- and s-polarizations, but the Z- and X-axes are now equivalent. This approach was used, through dichroic ratio measurements, to monitor the orientation of polymer chains at various depths during the drying of latex [33]. This type of symmetry is often encountered in non-polymeric samples, for instance, in ultrathin films of lipids or self-assembled monolayers. [Pg.310]


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Two approaches

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