Open chain structure, hydrogen bonds

The approximate equilibrium geometry of the open-chain structure of (HF is shown in Fig. 14. The energy of interaction relative to three HF molecules is —9.4 kcal/mol. The open-chain trimer structure is thus less stable than the cyclic trimer by approximately 2.2 kcal/mol. For the former structure we calculate an energy per hydrogen bond of —4.7 kcal/mol whereas for the latter we find only a value of —3.9 kcal/mol. Thus, the ring strain in the cyclic structure reduces the energy of interaction. The cyclic conformer, however, contains one more bond which ultimately leads to the higher stability. 

From what we have said so far, the formula C3H cannot represent an alkane. Not enough hydrogens are present to allow each carbon to form four bonds, unless there are multiple bonds. For example, the structural formula CH3—CH=CH2 fits the molecular formula but cannot represent an alkane because of the double bond. The C3H5 formula does become acceptable for an alkane if the carbon atoms form a ring, or cyclic, structure rather than the open-chain structure shown ... 

Oxalic acid crystallizes in three different forms as anhydrous a form, as anhydrous p form, and as a dihydrate. In the unit cell of the p-oxalic acid form, there are four cyclic dimers with two parallel hydrogen bonds similar to those found in carboxylic acids. Hydrogen bonds in the a-oxalic acid form are in open-chain structures separated from one another by the O—C=0 groups. The crystalline structure of the dihydrate is similar to the p form but with two molecules of water included, thus forming cyclic structure with four nonequivalent hydrogen bonds. 

Fig. 35.—(a) Stereo view of about a turn of the 3-fold double helix of potassium gellan (41). The two chains are drawn in open and filled bonds for distinction. Both intra- and inter-chain hydrogen bonds stabilize the helix. The vertical line is the helix axis. Octahedrally coordinated potassium ions (crossed circles) and triply hydrogen-bonded water molecules (open circles) located above the ions are integral components of the structure of 41. 

Dicarbonyl functions have been built into macrocyclic structures, and pKa values for the resulting macrocycles [60] have been determined (Alberts and Cram, 1979). When the open-chain model [62] is compared with the macrocycles [60], identical first pK values were found (pKa = 8.6). Thus for the diketones [60], no macrocyclic effect is noticeable. But for the dissociation of a second proton from the mono-aniorts of [60] much higher pKa values are found. To a certain extent. Coulomb repulsions (see Section 2) are probably the reason for this behaviour, but the large difference in the pKa values (ApKa = 2.9, see Table 26) argues for a special stabilization of the mono-anion. Again hydrogen bonds are not unreasonable. 

Open chain polyamine ligands have been widely studied. Often the coordination of zinc is compared with other first row transition metals and factors, such as behavior across a pH range, studied. The protonation patterns and stability constants are of particular interest. Octahedral zinc tris(ethylenediamine) structures have been characterized by X-ray diffraction with a number of different counter anions.94 The X-ray structure of zinc tris(ethylenediamine) with fluoride counter ions reveals extensive hydrogen bonding.95... 

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