Water molecules, orientation

Are the water molecules oriented in the same way or differently around the positive and negative ions when sodium chloride dissolves Explain your conclusion. 

Chapter 9, on entropy and molecular rotation in crystals and liquids, is concerned mostly with statistical mechanics rather than quantum mechanics, but the two appear together in SP 74. Chapter 9 contains one of Pauling s most celebrated papers, SP 73, in which he explains the experimentally measured zero-point entropy of ice as due to water-molecule orientation disorder in the tetrahedrally H-bonded ice structure with asymmetric hydrogen bonds (in which the bonding proton is not at the center of the bond). This concept has proven fully valid, and the disorder phenomenon is now known to affect greatly the physical properties of ice via the... 

Fig. 15-1 Schematic representation of the change in water structure (water molecule orientation) due to the presence of a charged (hydrophilic) solute, (a) Pure water, (b) A solute forming strong bonds with water (dissolution favorable), (c) a solute forming weak bonds with water (dissolution unfavorable).

Figure 4.1 Copper sulfate pentaquo complex. In solution, CuS04 exists as a Cu2 + ion in octahedral co-ordination surrounded by the S042- ion and five water molecules orientated so that the oxygen atom points towards the copper ion. It is the effect of this hydration sphere on the electronic orbital structure of the copper which gives rise to d-d band transitions, and hence the blue color of the solution.

Any cation in liquid water will be surrounded by water molecules oriented with the oxygen atom facing the cation (Fig. 5.2). The number of water molecules in the coordination sphere will normally be close to the ideal coordination number expected for the cation (Appendix 4) but if the hydrated complex is to be stable, the bonding strength of the cation must match that of the water molecule. This is best illustrated by examples. 

However, it must not be imagined that the water molecules act by themselves and that they are unaffected by the presence of their neighbors. After all, dipoles interact with dipoles. Hence, the oriented water molecules also experience lateral interaction— a phenomenon that affects the net number of water molecules oriented in one direction and therefore the value of the dipole potential, gj-ipole (Section 6.7.6). Once the dipole potential is affected, the total potential difference across the interface gets affected, and consequently, the properties of the interface. 

Fig. 6.79. Dimer molecule. It is made up of two water molecules oriented in opposite directions. The net dipole moment of the dimer molecule is zero.

Figure 3.1 Schematic representations of a) a water molecule orientation near a nonpolar CHs-group, which is optimal if none of the hydrogen atoms or electron pairs is directed toward the nonpolar group ( = 0) b) contour line diagrams of three polar molecules with the first inner line of a solvation energy o/O kcal/mol, the second line of 1 kcal, the third line of 2 kcal/mol e/c and c) of the hydrophobic effect. Upon association of hydrophobic particles water or other solvent molecules are released. Entropy grows.

Solid sodium chloride dissolves as its ions are surrounded by solvent water molecules. Note how the polar water molecules orient themselves differently around the positive and negative ions. 

The dissolution of ionic species (Fig. 11.3) occurs through the ion-dipole forces described in Section 10.2. Each positive ion in solution is surrounded by water molecules oriented with the negative end of their dipole moments toward the positive ion. Each S04 anion in solution is surrounded by water molecules oriented with the positive end of their dipole moments toward the anion. When a halide such as KCl is dissolved, the anion forms a hydrogen bond with one of the H atoms in a water molecule that places the atoms O —H—Cl nearly in a straight line as described in Section 10.2. 

Figure 2. Interaction energy curve of two water molecules oriented as shown in Fig. 1. The curve has been computed at the MP2/aug-cc-pVTZ level. The interaction energy has been corrected by the BSSE.

Figure 1.2.1 Interaction energy (in kcal mol-1, computed at the MP2/aug-cc-pVTZ level) of two coplanar water molecules oriented as shown in the upper diagram (the dummy atom X used to mark the C2v axis of the water molecule taken as reference in the calculations). The lower diagrams present two views of the interaction energy surface for changes in the 0—0 distance (in A) and X-O—O angles...

Figure 11. Schematic of water molecule orientation near a nonpolar (—CH,) group.

One difference between an enzyme and, say, water in providing solvation shell for the substrate and transition state is that individual water molecules orientate themselves towards the substrate at the expense of their interaction with other water molecules. The dipoles within the one molecule of a rigid enzyme could be preorientated to create an electric field complementary to the charge distribution of the transition state. The electrostatic energy dominates the medium to long-range interaction between two molecules and it seems reasonable therefore to emphasise its importance. 

Electron spin resonance Solvation water molecule orientation... 

Nuclear magnetic resonance T], correlation time for rotation of the dipole moment Water molecule orientation (NMR line shape)... 

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