Structure and hydrogen bonding

At atmospheric pressure, solid H2O can adopt one of two polymorphs, depending upon the conditions of crystallization. At higher pressures, five pol5Tnorphs exist which differ in their arrangement of the oxygen atoms in the crystal lattice. We shall be concerned here only with the normal form of ice. 

Enthalpy of vaporization, AvapT7°(373K)/kJmoU 40.65 Entropy of vaporization, A,ap S°(373 K)/ J moU K 109 Relative permittivity (at 298 K) 78.39 

When water acts as a solvent, hydrogen bonds between water molecules are destroyed as water-solute interactions form the latter may be ion-dipole interactions (e.g. when NaCl dissolves) or new hydrogen bonds (e.g. when H2O and MeOH mix). 

In dealing with acid-base equilibria in aqueous solution, three equilibrium constants are of special significance  

Essential equations relating to acid-base equilibria are listed below. Expressions involving concentrations are approximations, since we should strictly be using activities (see main text). Moreover, for a weak acid, HA, we assume that the concentration in aqueous solution of the dissociated acid at equilibrium is negligible with respect to the concentration of acid present initially similarly for a weak base. 

Review example 1 Calculate the pH of aqueous 0.020 m acetic acid (Kg = 1.7 x 10 ) 

The iwitia/concentration of MeC02H is 0.020mol dm-, and since the degree of dissociation is very small, the equilibrium concentration of MeC02H 0.020 mol dm-.  

At a concentration of 5.00 x 10- moldm-, Ca(OH)2 is fully ionized, with two moles of [OH]- provided by each moleofCa(OH)2. 

To check that the assumption that (MeC02H]equiim [MeCO,Hli iri,i is valid  

This confirms that the degree of dissociation is very small and the assumption made was valid. 

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Gragson D E and Richmond G I 1998 Investigations of the structure and hydrogen bonding of water molecules at liquid surfaces by vibrational sum frequency spectroscopy J. Phys. Chem. 102 3847... 

Emsley E (1984) The Composition, Structure and Hydrogen Bonding of the /)-Diketones. 57 147-191... 

Etter MC, Urbanczyk-Lipkowska Z, Baer S et al (1986) The crystal structures and hydrogen-bond properties of three 3-hydroxyflavone derivatives. J Mol Struct 144 155-167... 

For a recent overview of structure and hydrogen bonding in the organic solid state, see Chem. Materials, Special Issue 1994,6, No. 8. 

Structure and Hydrogen Bonding. In order to provide information about the... 

Figure 7 Crystal structures and hydrogen-bonding networks of BHCA (a) bilayer structure, (b) crossing structure, and epibile acids with 2-pentanol, (c) 3EDCA, and (d) 3ECA.

The following are our current research results indicating that not only the molecular structures and hydrogen-bonding abilities of resolving agents and racemic substrates but also those of solvent are very important in realizing successful resolution. 

J. Emsley, The composition, structure and hydrogen bonding of the (3-diketones. Struct. Bonding 57, 147-191 (1984)... 

O.S. Sukhanov et al., Molecular structure and hydrogen bonding in polyhydrated complexes of adenine A DFT study. J. Phys. Chem. B 107, 2846-2852 (2003)... 

Figures 11.3.1(a) and 11.3.1(b) show the structures and hydrogen bonding distances of 2-butyne-HCl and 2-butyne-2HCl, respectively. In 2-butyne-HCl,...

Jeffrey, G.A. and Huang, D.-B., The tetrasaccharide nystose trihydrate crystal structure and hydrogen bonding,... 

Table 14. Comparison of computed equilibrium structures and hydrogen-bond energies of HF, (HF)2 and (HF),...

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