Crystal lattice energy factors

MOLECULAR CONFORMATION AND CRYSTAL LATTICE ENERGY FACTORS IN CONFORMATIONAL POLYMORPHS... 

For crystalline compounds, they noted that an important factor to consider is the crystal lattice energy. From theoretical considerations and subsequent empirical studies, they discovered that melting point (mp) serves as an excellent proxy for this factor. While this is a significant advance in our understanding of water solubility, it falls short as a means to predict solubility from the chemical structure alone a compound must be made and a mp determined experimentally. 

Crystal defects and imperfections inLuence the crystal lattice energy. These defects, including dislocations, give rise to increased surface energy and may be a major factor in improving dissolution performance of poorly water-soluble, crystalline substances. 

In summarizing reversed-phase SPE, both the and the log of the octanol-water partition coefficient of the compound are related to the aqueous solubility of the analyte, although the relationship is not always straightforward because there are other factors that affect solubility of an analyte but that do not affect sorption, such as crystal lattice energy for solids. In spite of this factor, one could theoretically estimate the from the solubility of the analyte and relate this solubility to the capacity of the solute for the reversed-phase sorbent. The ability to use solubility to predict capacity is also addressed... 

The crystal lattice energy is thus a factor of fundamental importance in all considerations of the solubility of amino acids and proteins. Un-... 

Ionic liquids have been known for nearly a century the first to be discovered was ethylammonium nitrate, CH3CH2NH3" N03, with a melting point of 12 °C. More generally, however, the ionic liquids in use today are salts in which the cation is unsymmetrical and in which one or both of the ions are bulky so that the charges are dispersed over a large volume. Both factors minimize the crystal lattice energy and disfavor formation of the solid. Typical cations are quaternary ammonium ions from heterocyclic amines, either 1,3-dialkylimidazolium ions, JV-alkylpyridinium ions, or ring-substituted AT-alkylpyridinium ions. 

The extent of the ionization produced by a Lewis acid is dependent on the nature of the more inert solvent component as well as on the Lewis acid. A trityl bromide-stannic bromide complex of one to one stoichiometry exists in the form of orange-red crystals, obviously ionic. But as is. always the case with crystalline substances, lattice energy is a very important factor in determining the stability and no quantitative predictions can be made about the behaviour of the same substance in solution. Thus the trityl bromide-stannic bromide system dilute in benzene solution seems to consist largely of free trityl bromide, free stannic bromide, and only a small amount of ion pairs.187 There is not even any very considerable fraction of covalent tfityl bromide-stannic bromide complex in solution. The extent of ion pair and ion formation roughly parallels the dielectric constant of the solvents used (Table V). The more polar solvent either provides a... 

Several issues remain to be addressed. The effect of the mutual penetration of the electron distributions should be analyzed, while the use of theoretical densities on isolated molecules does not take into account the induced polarization of the molecular charge distribution in a crystal. In the calculations by Coombes et al. (1996), the effect of electron correlation on the isolated molecule density is approximately accounted for by a scaling of the electrostatic contributions by a factor of 0.9. Some of these effects are in opposite directions and may roughly cancel. As pointed out by Price and coworkers, lattice energy calculations based on the average static structure ignore the dynamical aspects of the molecular crystal. However, the necessity to include electrostatic interactions in lattice energy calculations of molecular crystals is evident and has been established unequivocally. 

In typical organic crystals, molecular pairs are easily sorted out and ab initio methods that work for gas-phase dimers can be applied to the analysis of molecular dimers in the crystal coordination sphere. The entire lattice energy can then be approximated as a sum of pairwise molecule-molecule interactions examples are crystals of benzene [40], alloxan [41], and of more complex aziridine molecules [42]. This obviously neglects cooperative and, in general, many-body effects, which seem less important in hard closed-shell systems. The positive side of this approach is that molecular coordination spheres in crystals can be dissected and bonding factors can be better analyzed, as examples in the next few sections will show. 

---