Hydrogen bond-crystallization relationship

A linear correlation between. obtained from X-ray and neutron-diffraction data, and 0 has been observed (Jeffrey and Yeon, 1986). The survey was based on 20 compounds having OHO hydrogen bonds with / H. . . o varying from 123.4 pm (KH malonate) to 197.9 pm (a-form of oxalic acid dihydrate). Whether based solely on those crystals for which neutron data are available, or on X-ray and neutron data, a good linear relationship is revealed, showing that the lower o more deshielded is the... 

To a fair engineering approximation AH is not only a function of the hydrogen bonds in the crystal, but also a function of cavity occupation. Because the Clausius-Clapeyron equation determines the heat of hydrate formation by the slopes of plots of In P versus 1/T, one may easily determine relationships between heats of dissociation. 

Table 42 gives an overview of annular tautomerism data for azoles in the gas phase and in solution or crystals. In the gas phase the stability of alternative tautomers largely depends on their relative aromaticities. In Section 2 A.4.2.2 it was noted that 1,2-relationships between pyrrole- and pyridine-type nitrogen atoms favor aromaticity (Figure 21) and this is consistent with the relative stabilities of triazole and tetrazole tautomers in the gas phase (Table 42) <2010T2695>. In solution (and crystals) other factors such as solvent polarity, hydrogen bonding, and temperature become important and the relative stabilities can be reversed. Polar solvents tend to stabilize the tautomer with the largest dipole moment and this probably accounts for the observation of both 2H-1,2,3-triazole (p = 0.12D) and H-1,2,3-triazole (p = 4.55D) in...

Some values for the enthalpy of hydrogen-bond formation are given in Tkble 2.5. The sequence of energies is in qualitative agreement with the bond lengths observed in crystals, OH 0=C < OwH- -Ow < NH- 0=Cquantitative relationships could be proposed. 

Jeffrey GA (1989) Hydrogen bonding in crystal structures of nucleic acid components purines, pyrimidines, nucleosides and nucleotides. In Saenger W (ed), Landolt-BOrnstein. Numerical Data and Functional Relationships in Science and Technology. New Series, Group VII, Vol. Ib. Springer, Berlin, pp 277-348... 

Bellamy LJ, Owen AJ (1969) A simple relationship between infra-red stretching frequencies and hydrogen bond distances in crystals. Spectrochim Acta 25A 329-333... 

Thus there are plenty of reports on the resolutions of a wide variety of racemates with conventional resolving agents and on the crystal structures of the diastereomeric crystals of some combinations. These studies are rarely concerned with the correlation between the difference in stability of a pair of diastereomeric crystals and the efficiency of resolutions. However, the information about supramolecular hydrogen-bond systems obtained from these studies is very valuable for understanding such relationship in the resolutions of systematically selected racemates with a resolving agent. 

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