Unfavorable orientation

Siace these masses of polycrystaUine diamond possess extensive diamond-to-diamond bonding, they have, ia contrast to siagle-crystal diamond, excellent crack resistance, siace any crack that begias ia oae crystal oa an easy cracking plane (parallel to an octahedral face) is halted by neighboring crystals that are unfavorably oriented for their propagation. 

The potential energy is often described in terms of an oscillating function like the one shown in Figure 10.9(a) where the minima correspond to the relative orientations in which the interactions are most favorable, and the maxima correspond to unfavorable orientations. In ethane, the minima would occur at the staggered conformation and the maxima at the eclipsed conformation. In symmetrical molecules like ethane, the potential function reflects the symmetry and has a number of equivalent maxima and minima. In less symmetric molecules, the function may be more complex and show a number of minima of various depths and maxima of various heights. For our purposes, we will consider only molecules with symmetric potential functions and designate the number of minima in a complete rotation as r. For molecules like ethane and H3C-CCI3, r = 3. 

The strength of the Fajb interaction and its variations with distance and orientation can be conveniently visualized in terms of the overlap of 7ra and 7tb NBOs, on the basis of a Mulliken-type approximation (cf. Eq. (1.34)). As an example, the top two panels of Fig. 3.38 compare the overlapping 7ta-7tb orbital contours for trails 1 and cis 2 isomers of butadiene. As shown in Fig. 3.38, the overlap in the cis isomer 2 (S = 0.2054) is slightly weaker than that in the trans isomer 1 (S = 0.2209), due to the unfavorable orientation of the 7ta across the nodal plane of the 7tb in the latter case. Consistently with the weaker 7ta-7tb overlap, the JtA F nh ) interaction is less, namely 0.0608 in 1 versus 0.0564 in 2. The delocalization tail of the 7fa NEMO is correspondingly less than its value in the trans isomer... 

The synthesis of the bis-P-cyclodextrin 35 is outlined in Fig. 14. Interestingly, however, the X-ray structure of 36, the de-tosylated 35, revealed that P-carotene would not be incorporated into both CD units due to the unfavorable orientation of the diamide linker which blocks the entrance to the second P-CD. In agreement with this result are experiments with the target Ru complex 37 which displayed central cleavage on P-carotene 1 5delding retinal 2 as the major product (Fig. 15) (19). 

The different threshold behaviors for heads and tails orientations shown in Figures 7 and 8 indicate that, at very low energies, reaction is restricted to only one end of the molecule. We directly observe that there is no reaction for attack in the unfavored orientation. The different thresholds for attack at different ends of these molecules requires the final state of the system, at the respective thresholds, to be somehow different for attack at the opposite ends of the molecule. For CFjBr, we believe that different products may be formed, depending on the end attacked, but the same species in different internal states could also be a possibility. In these experiments there are two likely low-energy reaction channels. 

The rule cannot be extended to include the cleavage of 2,3-anhydro-pentosides with sodium methylmercaptide, and attempts to convert 2,3-anhydropentosides to 2-desoxypentopyranosides by this method have met with very limited success. In those cases where substitution was expected to occur at carbon atom 2, the nucleophilic portion of the reagent molecule entered predominantly at position 3, giving derivatives from which 3-desoxypentopyranosides were obtained the reasons for this unfavorable orientation are not known. 

Where the dimer yields are not very high (<50%), it is reasonable to conclude that the presence of the unfavorably oriented second molecule (B) does not affect the reaction taking place in other words, the reaction cavity free volume of the favourably oriented molecules (A) is not interfered by that of B. 

Betaines 208a and 208c are incapable of formation of furan derivatives, because of an unfavorable orientation of the carbonyl fragment and the nearest hydroxyl group. These betaines may, however, be sources of anhydro derivatives, as well as 208b, if it is assumed that elimination of triphenylphosphine oxide is accompanied by intramolecular attack of a distant hydroxyl group on the /3-carbon atom, as shown. 

In contrast to the photoreaction of 39a and 39b, the solid state photocycliza-tions of 39c and 39h-j to give the oxetanes did not occur. The x-ray analyses of these imides indicate that all molecules are apparently in unfavorable orientation for the intramolecular cyclization, what was termed as (E,Z) conformation at the imide chromophores. 

Thermodynamic calculations, based on integrated intensity measurements, show that the crown form has the lower enthalpy, and also the lower entropy (.dS =8.5 ) or 6 2 eu ). The low entropy of the crown finds an explanation in the high symmetry of this conformation, compared to the low symmetry of the boat-chair. The best boat-chair for tetroxocane appears to be the BC-1,3,5,7 conformation. However, the ether functions at positions 3 and 7 have their dipoles close together in an unfavorable orientation but... 

It is clearly to be seen from electron micrographs that the conversion of carbon spiroids into nano-onions really proceeds from the core to the periphery. They show entirely spiral objects at first. These are transformed into completely concentric onions via the hybrid form of an onion core with a spiral shell (Figure 4.30). It is presumably even just a part of the spiroid structures actually present in the sample that are detected in these examinations as their projections appear onion-like at unfavorable orientation. 

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