Hydrogen bond, between two water molecules

Hydrogen bonds are particularly strong dipole-dipole interactions that form between the H-atom of one molecule and an F, O, or N atom of an adjacent molecule. The partial positive charge on the hydrogen atom is attracted to the partial negative charge on the electron pair of the other atom. The hydrogen bond between two water molecules is shown as the dashed line below ... 

FIGURE 2.6 A hydrogen bond between two water molecules accounts for the liquid phase of water at standard temperatures and pressures. 

The solubility behavior of alcohols also reflects their ability to form hydrogen bonds. In sharp contrast to hydrocarbons, the lower alcohols are miscible with water. Since alcohol molecules are held together by the same sort of intermolecular forces as water molecules, there can be mixing of the two kinds of molecules the energy required to break a hydrogen bond between two water molecules or two alcohol molecules is provided by formation of a hydrogen bond between a water molecule and an alcohol molecule. ... 

Figure 1.3. Hydrogen bonding between two water molecules. The small circles symbolize lone-pair electrons of the O atoms.

Although it is called a hydrogen bond, it is not a bond in the same sense that a covalent bond is a bond. The following figure shows a hydrogen bond between two water molecules... 

Using grammatically correct English sentences, describe the difference between the hydrogen bond between two water molecules and the O-H bond in a particular water molecule. 

Our first example pertains to a chemical bond. Now let us take in the same way a quite different situation, where we have relatively weak intermolecular interactions namely, the hydrogen bond between two water molecules. The binding energy in such a case is of the order of D = 6 kcal mol = 0.00956 a.u. = 2097 cm i.e., about twenty times smaller... 

The above example pertains to a chemical bond. Let us take, in the same way, a quite different situation where we have relatively weak intermolecular interactions, namely the hydrogen bond between two water molecules. The binding energy in such a case is of the order of D = 6 kcal/mol = 0.00956 a.u. = 2097 cm", i.e. about twenty times smaller as before. Tb stay within a single oscillator model, let us treat each water molecule as a point-like mass. Then, fji = 16560 a.u. Let us stay with the same value of o = 1. We obtain (p. 171) a = 17.794 and hence bo = 17.294, b = 16.294,..., bn = 0.294, bn> i < 0. Thus, (accidentally) we also have 18 vibrational levels. 

Morse type. However, whai we would get, would certainly resemble a Morse potential. Indeed, the resulting curve approaches zero at 7 oo (as do all the Morse curves), and at the equilibrium point, Rg will be shifted up by /(/ -I-which we may ensure by taking a Morse curve with a little lower dissociation energy D ly = D — J(J + l)h /(2fiR ). As seen from Fig. 4.14, we should not worry too much about what to take as the parameter a, which controls the well width. Let us keep it constant. We could take any example we want, but since in Chapter 4 we used the Morse potential for the hydrogen bond between two water molecules, we have alreatty a feeling for what happens there, so therefore let us stick to this example. 

What is the average length (in angstroms) of a hydrogen bond between two water molecules ... 

This scheme allows us to determine the average number of water molecules to which a water molecule located at a distance r from a central molecule (either solvent or solute) is hydrogen bonded. This quantity might be smaller than the average number of hydrogen bonds in which a water molecule located at a distance r from a central molecule (either solvent or solute) is participating because of the possibility of multiple hydrogen bonds between two water molecules permitted by the above definition. 

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