Physical Character of Hydrogen Bonds

The most convenient classification scheme of solids is based on the physical character of the interatomic binding forces in various classes of crystalline materials. According to this classificahon, all solids fall into one of five general categories metallic, covalent, ionic, molecular, and hydrogen-bonded crystals. Some materials may belong to more than one category, thus, the distinction is in many cases not a sharp one. 

Laurence et al. [23, 24, 135] measured two solvatochromic scales for about 200 bases, each based on one reference hydrogen-bond donor, 4-nitrophenol or 4-nitroaniline, one standard state, the pure base, and one physical property, the enhanced bathochromic shift of the longest wavelength n n transition (with intramolecular charge-transfer character) upon hydrogen bonding. The solvatochromic comparison method was improved and inert solvents were carefully chosen [135] to fix the comparison lines of Equations 4.37 and 4.38 ... 

The physical properties of hydrofluorocarbons reflect their polar character, and possibly the importance of intermolecular hydrogen bonding (3). Hydrofluorocarbons often bod higher than either their PFC or hydrocarbon counterparts. For example, l-C H F bods at 91.5°C compared with 58°C for n-Q and 69°C for Within the series of fluorinated methanes, the boiling point reaches a maximum for CH2F2, which contains an equal... 

As a consequence of the mesomeric effect, the amide carbonyl group has less double bond character than that of a normal ketonic carbonyl group and it would be expected to absorb at lower frequency. This is found to be the case primary and secondary amides absorb strongly near 1690 cm-1 in dilute solution and at somewhat lower frequency in the solid phase. Tertiary amides are not affected by hydrogen bonding and show strong absorption at 1670-1630 cm-1 irrespective of the physical state of the sample. 

Ip values for Lil, LiBr, Nal, KI, Rbl and Csl are lower than for HF. In contrast, Qa values for all alkali metal halides are higher than for HF, and this appears more realistic with respect to the physical and chemical properties of the said compounds. The relatively high ionic character of HF is reflected in its strong tendency to association via hydrogen bonds. The ionic character of HCl, HBr and HI is much smaller than that of HF with Qa values generally higher than Ip. 

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