Hydrogen bond separation

Strong hydrogen bonds can be symmetrical or unsymmetrical. In some crystal structures, the midpoint of the O 0 hydrogen-bonded separation is a crystal-... 

It is well known that molecules of PBG in a numbo of solvents (called coiling solvents) exist in the form of a-helixes stabilized by intramolecular hydrogen bonds. Separation of the solution into isotropic and anisotropic phases is observed in these solvents (such as dioxane, chloroform, methylene chloride, etc.) for a certain critical concentration of the polypeptide (c ) in accordance with the theoretical concepts of Flory [3] (see also Chapter 1). The anisotropic phase in a solution of PBG is separated as liquid spherulites which become larger and form a continuous anisotropic phase of the cholesteric type in cooling or concentration of the solution. 

At Its most basic level separating the total strain of a structure into its components is a qualita tive exercise For example a computer drawn model of the eclipsed conformation of butane using ideal bond angles and bond distances (Figure 3 8) reveals that two pairs of hydrogens are separated by a distance of only 175 pm a value considerably smaller than the sum of their van der Waals radii (2 X 120 pm = 240 pm) Thus this conformation is destabilized not only by the torsional strain associ ated with its eclipsed bonds but also by van der Waals strain... 

C which causes the strands to separate by breaking the hydrogen bonds between them [Figure 28 14(Zi)]... 

The solution is then cooled to 60°C allowing new hydrogen bonds to form [Fig ure 28 14(c)] However the reaction mixture contains much larger concentrations of two primer molecules than DNA and the new hydrogen bonds are between the separated DNA strands and the primers rather than between the two strands... 

The attraction for two neutral atoms separated by more than four Angstroms is approximately zero. The depth of the potential wells is minimal. For the AMBER force field, hydrogen bonds have well depths of about 0.5 kcal/mol the magnitude of individual van der Waals well depths is usually less. 

The most common hydrophobic adsorbents are activated carbon and siUcahte. The latter is of particular interest since the affinity for water is very low indeed the heat of adsorption is even smaller than the latent heat of vaporization (3). It seems clear that the channel stmcture of siUcahte must inhibit the hydrogen bonding between occluded water molecules, thus enhancing the hydrophobic nature of the adsorbent. As a result, siUcahte has some potential as a selective adsorbent for the separation of alcohols and other organics from dilute aqueous solutions (4). 

The entropy value of gaseous HCl is a sum of contributions from the various transitions summarized in Table 4. Independent calculations based on the spectroscopic data of H Cl and H Cl separately, show the entropy of HCl at 298 K to be 186.686 and 187.372 J/(mol K) (44.619 and 44.783 cal/(mol K), respectively. The low temperature (rhombic) phase is ferroelectric (6). SoHd hydrogen chloride consists of hydrogen-bonded molecular crystals consisting of zigzag chains having an angle of 93.5° (6). Proton nmr studies at low temperatures have also shown the existence of a dimer (HC1)2 (7). 

Similar models for the crystal stmcture of Fortisan Cellulose II came from two separate studies despite quite different measured values of the diffraction intensities (66,70). Both studies concluded that the two chains in the unit cell were packed antiparallel. Hydrogen bonding between chains at the corners and the centers of the unit cells, not found in Cellulose I, was proposed to account for the increased stabiUty of Cellulose II. The same model, with... 

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