Steric hindrance estimation

The percent conversion of the monomer to the polypeptide can be estimated by the quantity 100 (Aq - At) / Aq, where Aq and At are the integrated intensities of the ester band at the start and time t, respectively. At 40 °C, it tended to saturate at about 30 % for the Y-type films of L-NaphAla-C-i s, and L-PyrAla-G 8, although the conversion for the LB film of long-chain ester of alanine (L-Ala- Ci s) reached to 90 % [52]. This difference is considered to be due to a larger steric hindrance of the aromatic rings. 

The amplitude and correlation time tc of the libration in the stacked state were estimated by using the diffusion in a cone model. The semiangle of the cone 6c at 20° C were obtained to be 22° and 26° for the TiL component and PBG- d2 and PBLG- d2, respectively. The tc values were obtained to be about 10 12 s at 19°C in both states. The 6C of r1L component is 4° smaller than that of PBLG- d2, showing that the amplitude of the libration in the stacked state is smaller than that in the free state due to the steric hindrance between the adjacent side chains forming the stacking... 

The Grant-Cheney steric hindrance model74> to estimate the C-13 splittings were applied. The evidence was said to be ambiguous but they felt that there definitely are two forms of the monomer present. 

The lower reactivity of benzaldehyde with respect to acetaldehyde was found also in the vapour phase aldolisation over lithium phosphate [390]. Over the same catalyst, the reactivity order in the self-condensations of aldehydes could be estimated as CH3CHO > CH3CH2CHO (CH3)2-CHCHO. The reactivity of isobutyraldehyde in the self-condensation was almost undetectable, probably due to steric hindrance on the a-carbon, but this substance was able to react as a hydrogen acceptor with cyclohexanone. With propionaldehyde over a calcium hydroxide catalyst, a Cannizzaro-type reaction occurred to some extent simultaneously with the aldolisation [390]. This unexpected result was also recorded by other authors [391], who established that the tendency to aldolisation decreased, and the tendency to the Cannizzaro reaction increased, with... 

It has been pointed out by Adrian81 that there is steric hindrance in the triphenylmethyl radical, the phenyl groups being twisted about the bond to the central atom through about 32°. This twist decreases the calculated resonance energy from 35 kcal/mole (for planarity) to 21 kcal/mole. The steric repulsion energy of the two halves of hexa-phenylethane is estimated to be about 36 kcal/mole, and the enthalpy of dissociation is about 16.5 kcal/mole. 

Thin-film dialysis can thus be considered as a way to estimate Stokes radius with a precision approximating 3% (24) when suitable models of known dimensions are available for comparison. In addition to this it offers an excellent approach to the study of molecular interactions, selfassociation, and binding (29, 30). It therefore is excellent for supplementing the type of information which can be derived from high-resolu-tion NMR study which in many cases together with model building and the restrictions of steric hindrance can reveal the precise positions certain atoms in a molecule occupy in space relative to each other. Many times however with NMR, a single unique position cannot be extracted from the data but instead only several possibilities. Here supplementary knowl-... 

Exercise 12-27 The equatorial form of methylcyclohexane is 1.5 kcal mole-1 more stable than the axial form because the axial form has steric hindrance between the methyl and two hydrogens, one in the 3- and the other in the 5-position. Knowing that c/s-decalin is about 2 kcal mole-1 less stable than frans-decalin, what would you estimate for the relative stabilities of cis- and frans-9-methyldecalin (numbering as in Figure 12-22) ... 

Interest in thermotropic liquid crystals has focussed mainly on macroscopic properties studies relating these properties to the microscopic molecular order are new. Lyotropic liquid crystals, e.g. lipid-water systems, however, are better known from a microscopic point of view. We detail the descriptions of chain flexibility that were obtained from recent DMR experiments on deuterated soap molecules. Models were developed, and most chain deformations appear to result from intramolecular isomeric rotations that are compatible with intermodular steric hindrance. The characteristic times of chain motions can be estimated from earlier proton resonance experiments. There is a possibility of collective motions in the bilayer. The biological relevance of these findings is considered briefly. Recent similar DMR studies of thermotropic liquid crystals also suggest some molecular flexibility. 

However, using rj as the only parameter in the optimization can often be misleading, since the maximum degree of conversion is not only determined by the number of reacting hydroxyl groups, but also by the mean cross-sectional area (A.J of the reacted group (sterical hindrance effects). The maximum number of bonded groups on the surface can be estimated as... 

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