The strength of hydrogen bonds

The strongest hydrogen bonds resemble covalent bonds, the weakest ones are like van der Waals interactions, and the majority have energies lying between these two extremes. The strength of ahydrogen bond corresponds to the enthalpy of dissociation of the reaction  

Strong and weak hydrogen bonds obviously have very different properties. Table 11.2.2 lists the properties observed for different types of hydrogen bonds. 

In ice-Ih, the O-H- O bond energy is 25 kJ mol-1, which results from the following interactions  

X-H- Y interaction mostly covalent mostly electrostatic electrostatic 

Examples Acid salts, acids, proton sponges, HF complexes Acids, alcohols, hydrates, phenols, biological molecules Weak base, basic salts C-H- O/N O/N-H- ji 

---

Alcohols with low molar masses are liquids, and alcohols have much lower vapor pressures than do hydrocarbons with approximately the same molar mass. For example, ethanol is a liquid at room temperature, but butane, which has a higher molar mass than ethanol, is a gas. The relatively low volatility of alcohols is a sign of the strength of hydrogen bonds. The ability of alcohols to form hydrogen bonds also accounts for the solubility in water of alcohols with low molar mass. 

Clearly, the strength of hydrogen bonds depends on the reaction medium. In practice, the nonpolar solvent toluene is routinely used. It can be considered to mimic a hydrophobic binding pocket of an enzyme and clearly supports the formation of moderate (1.5-2.2A) and even strong (1.2-1.5 A) hydrogen bonds [42]. 

Since the strength of hydrogen bonds is significantly smaller than that of covalent bonds, molecular systems formed via hydrogen bonds show strucmral flexibility and flnctnations. In other words, hydrogen bonds are highly dynamic. 

On a purely statistical basis, we may expect the ratio of products from 3 to correlate with the number of available hydrogens at the various positions of substitution. That is, 4, 5, 6, and 7 would be formed in the ratio 6 3 2 l (50% 25% 17% 8%). However, as can be seen from Table 4-6, the strengths of hydrogen bonds to primary, secondary, and tertiary carbons are not the same and, from the argument given in Section 4-4E we would expect the weaker C-H bonds to be preferentially attacked by CI-. The proportion of 7 formed is about three times that expected on a statistical basis which is in accord with our expectation that the tertiary C-H bond of 2-methylbutane should be the weakest of the C-H bonds. (See Table 4-6.)... 

Cellulose fibers are insoluble in water because the strength of hydrogen bonding of a cellulose molecule in the fiber with surface water molecules is not sufficient to overcome the intermolecular forces that hold it in the fiber. 

Although only a1 and e are used as additional descriptors so far, some others might be useful in the future. One of the most apparent is the local electronic polarizability of the molecule in the vicinity of a surface segment i, which we may represent by a local refractive index n,. Such local polarizability or local refractive index would allow for a refinement of the electrostatic misfit term, which presently only takes into account an average electronic polarizability of organic molecules. It is also likely that the local electronic polarizability is of importance to the strength of hydrogen bonds, which so far in COSMO-RS are only a function of polarity. Finally, from a physical perspective, the vdW interactions should be a function of the local polarizability as well. 

---