Molecular size effect

Water is a special liquid that forms unique bonds involving protons between the oxygen atoms of neighboring molecules, the so-called hydrogen bond. The solvation forces are then due not simply to molecular size effects, but also and most importantly to the directional nature of the bond. They can be attractive or hydrophobic (hydration forces between two hydrophobic surfaces) and repulsive or hydrophilic (between two hydrophilic surfaces). These forces arise from the disruption or modification of the hydrogen-bonding network of water by the surfaces. These forces are also found to decay exponentially with distance [6]. 

Moore JS, Bharathi P, Devadoss C (1996) Energy transfer in dendritic macromolecules molecular size effects and the role of an energy gradient. J Am Chem Soc 118 9635-9644... 

Next, they applied the stepwise MLR to the training set providing TPSA, LA, and MW as the descriptors so as to take into account hydroaffinity, lipoaffinity, and molecular size effects. However, the calculation failed to generate a statistically sound linear equation using all three descriptors. Instead the following linear model was generated (Eq. 42) which was comparable to Clark s Eq. 31 ... 

These conclusions can only be tentative with the limited data here. However the potential scope for future studies of the kinetics of adsorption and partitioning of mixed sorbates is evident. This is of particular interest with microporous sorbents, such as carbons, especially where molecular size effects may be predominant. Such an aspect is indeed particularly relevant in gas separation processes which may involve membranes and pressure swing adsorption. 

Molecular size effect and a radical diffusion model... 

Solid-fluid phase diagrams of binary hard sphere mixtures have been studied quite extensively using MC simulations. Kranendonk and Frenkel [202-205] and Kofke [206] have studied the solid-fluid equilibrium for binary hard sphere mixtures for the case of substitutionally disordered solid solutions. Several interesting features emerge from these studies. Azeotropy and solid-solid immiscibility appear very quickly in the phase diagram as the size ratio is changed from unity. This is primarily a consequence of the nonideality in the solid phase. Another aspect of these results concerns the empirical Hume-Rothery rule, developed in the context of metal alloy phase equilibrium, that mixtures of spherical molecules with diameter ratios below about 0.85 should exhibit only limited solubility in the solid phase [207]. The simulation results for hard sphere tend to be consistent with this rule. However, it should be noted that the Hume-Rothery rule was formulated in terms of the ratio of nearest neighbor distances in the pure metals rather than hard sphere diameters. Thus, this observation should be interpreted as an indication that molecular size effects are important in metal alloy equilibria rather than as a quantitative confirmation of the Hume-Rothery rule. 

The rate of diffusion of mercaptans into human hair is undoubtedly influenced by steric considerations. For example, molecular size (effective minimum molecular diameter) of the mercaptan molecule should affect the rate of diffusion into hair. Therefore, the rate of reduction of human hair by ethyl mercaptan in neutral to alkaline media, where diffusion is ratedetermining, should be faster than that of higher homologs. (The possible effects of variation in the structure of cystinyl residues in hair on the rate of reduction was considered in the previous section on cystinyl residues of differing reactivities.)... 

Heavier isotopes have smaller atomic volume deuterated compounds have smaller molar volumes than the corresponding unlabeled compounds. The molar volume differences between, e.g., deuterated and ordinary benzene are due to a molecular size effect caused by differences in zero point intermo-lecular motion of the molecules. 

The molecular size effect is used in size-exclusion liquid chromatography. The steric effect, a difference in molecular shape, is mainly applied to normal-phase liquid chromatography. Enantiomeric separation is usually carried out with normal-phase liquid chromatography, using specially... 

To test the molecular size effect on the H-transfer reaction, the following reaction has been studied (Xiao et al. 1997) ... 

To investigate the possible molecular size effect on the calculated energetics, the P scission reactions of a primary pentyl, hexyl, and heptyl radical have also been studied (Xiao et al. 1997). The calculated activation energy Ea and heat of reaction AH at the B3LYP/6-31G level are 30.43 and 23.41 kcal/mol for butyl O ethylene + ethyl 30.52 and 23.58 kcal/mol for pentyl o ethylene + propyl 30.38 and 23.35 kcal/mol for hexyl ethylene + butyl and 30.39 and 23.37 kcd/mol for heptyl O ethylene + pentyl. The results again suggest that the kinetics is largely controlled by the type of reaction, rather than the size of the system. 

Thus, we concluded that the observed asymmetric induction in the gas-solid reaction should be due to molecular size effect of reagents. 

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