Conformations results

The rotational barrier in methylsilane (Table 3.4, entry 5) is significantly smaller than that in ethane (1.7 versus 2.88 kcal/mol). This reflects the decreased electron-electron rqjulsions in the eclipsed conformation resulting from the longer carbon-silicon bond length (1.87 A) compared to the carbon-carbon bond length (1.54 A) in ethane. 

Shinkai [15] concluded that p-zert-butyl calix[n]ar-ene tetra esters form stable monolayers at the air-water interface and the metal responds, therein, quite differently from that in solution. They reported that examination of the metal template effect on the conformer distribution established that when the metal cation present in the base used serves as a template, the cone conformer results are predominant [16]. Hence, Na in... 

There are various ways to check the quality of the resulting structures with respect to experiment. A typical check would be to compare the mean square end-to-end distance with results from scattering experiments. However, since the experimental samples are highly polydisperse, the results from scattering experiments are somewhat questionable [195]. Furthermore, a crucial check is the direct comparison of conformations of systems. In order to be able to compare the conformations resulting from simulations unanimously to experiment we reintroduce the chemical details into the coarse-grained chain. This is one of the reasons why it is so important to device a mapping procedure which stays close to the chemical structure of the objects. We have a one-... 

SCHEME 2.10 Structures and relative energies (kcal/mol) of several tautomers/conformers resulting from the bielectronic oxidation of 2-4, computed at the PBE0/6-31 + G(d,p) level in vacuum and in aqueous solution (in parenthesis, by PCM solvation model, data from Ref. [19]). 

The explanations for the relative rates of reaction have been based on three factors (1) The rate of reaction increases as the electron density in the diene system increases thus isoprene reacts faster than butadiene and a complex electron-rich 2-silylmethylbutadiene reacts even faster. (2) The rate of reaction increases as the steric hindrance due to the diene substituents decreases thus frans-piperylene reacts more slowly than dimethylbu-tadiene or isoprene. (3) A decrease in the equilibrium concentration of the cisoid conformer results in a slower reaction rate thus cw-piperylene or cis/trans-2,4-hexadiene react more slowly than /rans-piperylene or transltrans-2,4-hexadiene, respectively.175177... 

Crystalline trithia dication 151 contains a 3-ccntcrA-clcctron bond with a central hypervalent sulfur atom. It was shown to be sufficiently stable to be isolated and studied spectroscopically. One important feature is its boat-boat conformation resulting from the formation of the transannular bond between the three sulfur atoms <1988JA1280>. 

Ab initio calculations of 2,3-disubstituted dienes have predicted that the tram conformation is more stable than the cis conformation. Results of these calculations are shown in Table 21. 

Much controversy is found in the literature regarding the conformational preference of the six-membered ring piperidine (5)4 5. However, most experimental evidence is consistent with a predominance of the H-equatorial conformer by 0.25-0.74 kealmol-1. As noted above, the C—C—N—H and C—C—N—lp torsional parameters were adjusted to reproduce an intermediate value of 0.30 kcal mol-1. MM2 calculations of this system have revealed, perhaps contrary to chemical intuition, that most of the energy difference between the H-axial and H-equatorial conformers results from torsional energy while the 1,3-diaxial interactions have only a negligible contribution5. 

Model 25, invoking a preferential rotameric disposition of TFAE dictated by peri interactions, and solvate conformations resulting from CHB between the solute carbinyl proton and the anthryl ring, conectly predicts the senses of nonequivalence observed for 2-butanol (Fig. 4), and for 2-octanol as well. In these conformations, the substituent over the larger portion of the anthryl ring is the more shielded. 

Fig. 4 Lowest energy conformations resulting from a Monte Carlo conformational search on an unsubstituted phenylene ethynylene dodecamer. The ideal helical conformation is shown in the lower right-hand corner. All of the other conformations are destabilized by the addition of bulky side chains...

The many different conformers resulting from rotation around the carbon-carbon bonds in simple molecules like ethane and w-butane may be shown by Newman projections (Figure 2.7). The most stable is the anti or trans projection where the steric hindrance is minimized. There are a number of eclipsed and gauche arrangements of which only one of... 

Ipc)2BH adopts a conformation which minimizes steric interactions. This conformation results in transition states H and I, where the S, M, and L substituents are, respectively, the 3-H, 4-CH2, and 2-CHCH3 groups of the carbocyclic structure. The steric environment at boron in this conformation is such that Z-alkenes encounter less steric encumbrance in transition state I than in H. 

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