Hydrogen-bond basicity

Where FCl is the solute gas-liquid partition coefficient, r is the tendency of the solvent to interact through k- and n-electron pairs (Lewis basicity), s the contribution from dipole-dipole and dipole-induced dipole interactions (in molecular solvents), a is the hydrogen bond basicity of the solvent, b is its hydrogen bond acidity and I is how well the solvent will separate members of a homologous series, with contributions from solvent cavity formation and dispersion interactions. 

Some exceptions to this general observation were found halide and nitrite salts have unusually high hydrogen-bond basicities (as would be expected), while penta-cyanopropionide, picrate, triflate, and perfluorobezenesulfonate salts not only had unusually low hydrogen-bond basicities (also as would be expected) but also lower s values, perhaps due to the weakening of the Coulombic interactions by delocalization of the charge on the anions. 

Attempts have also been made to separate non-specific effects of the local electrical field from hydrogen-bonding effects for a small group of ionic liquids through the use of the k scale of dipolarity/polarizability, the a scale of hydrogen bond donor acidity, and the (i scale of hydrogen bond basicity (see Table 3.5-1) [13, 16]. 

At first glance the hydrogen bond basicity (3 is controlled solely by the anions, with basicity decreasing in the order Cl > [RS03] >[BF4] > [PF ] > [N03] > [SCN] . However, while the general trend is clear, this is not the order that one would have expected, and the cations are obviously playing a role. Again, this may be a consequence of competition for the basic site (anion) between the test solute and the acidic site (cation) of the ionic liquid. It is unfortunate that no study to date has used a common anion across all possible cations. 

A hydrogen-bond basicity scale has been developed that can be used to determine the relative basicity of molecules. Table 8.2 gives the p hb values for several common heteroatom containing molecules. This is obtained from the protonated form (conjugated acid) of the base in question. The larger the number, the more basic is that compound. 

The correlation of Snyder s solvent strength e° with molecular dipolarity and polarizability (7t ) and the hydrogen-bond acidity (a) and the hydrogen-bond basicity ((3) solvatochromic parameters for adsorption chromatography can be achieved, although most papers on solvatochromic parameters deal with reversed-phase systems [18]. 

Abraham, M. H., P. L. Grellier, D. V. Prior, J. J. Morris, P. J. Taylor, C. Laurence, and M. Berthelot. 1989b. A Thermodynamically-Based Scale of Solute Hydrogen Bond Basicity. Tetrahedron Letts. 30, 2571. 

Polymer (dry) Hydrogen bond acidity Hydrogen bond basicity Polarizability Et (30)... 

In an excellent paper, Zhao et al. [29] assembled a carefully reviewed literature set of human absorption data on 241 drugs. They showed that a linear regression model built with 5 Abraham descriptors could fit percent human absorption data reasonably well (r2 = 0.83, RMSE = 14%). The descriptors are excess molar refraction (E), polarizability (S), hydrogen bond acidity (A), hydrogen bond basicity (B), and McGowan volume (V), all related to lipophilicity, hydrophilicity, and size. In a follow-on paper, data on rat absorption for 151 drugs was collected from the literature and modeled using the Abraham descriptors [30]. A model with only descriptors A and B had r2 = 0.66, RMSE = 15%. 

A scale was developed of relative hydrogen-bond basicity for a wide variety of compounds by means of their retention in gas chromatography21. Results indicate that the... 

By keeping either BH+ or HBA constant, hydrogen-bond basicity or acidity scales can be built41-43. 

The scale of hydrogen-bond basicity p hb, initially developed by Taft and coworkers14, has been extended by Abraham and coworkers81, and is based on the formation of hydrogen-bond complexes of a base B with a hydrogen-bond reference donor as 4-fluorophenol in CCI4 at 25 °C ... 

On the basis of IR evidence it has been proposed84 that alkylamidates are the strongest known hydrogen-bonding carbonyl bases and that the order of hydrogen-bond basicity may be alkylamidates w FLMPA > vinylogous amides benzamidates > amides (Table 8). 

TABLE 8. Hydrogen-bond basicity of amidates RCON N+Me3 and, for comparison, some carboxyamides in CCI4... 

Attempts to evaluate the effective solute hydrogen bond acidity and basicity40 produce quantitative scales of solute hydrogen-bonding, which may be of interest for application also to biochemical processes41. The indicator pair 4-nitroanilinc/4-nitro-/V,/V-dimethylaminoaniline are used to state the equivalence42 of solute and solvent scales of hydrogen bond basicity. 

Recently195, the hydrogen bond basicity scale (p.K hb as logarithm of the formation constant of 4-fluorophenol/base complexes in carbon tetrachloride, equilibrium 21) has been measured for several nitro derivatives (nitromethane, nitrobenzene, IV-nitrocamphorimine, 2-nitropropane, 4-nitro-o-xylene, 4-nitroanisole, lV,./V-diethyl-4-nitroaniline, l-dimethylamino-2-nitroethylene, l-piperidino-2-nitroethylene) ... 

Complex 1 1 is considered the only complex present, but the hydrogen bond may be either two (76) or three center (77). Nitroenamines are more prone to complex with 4-fluorophenol than the nitroanilines and they form the strongest hydrogen bond presently known for nitro-bases. In particular, l-piperidino-2-nitroethylene (78) and 1-dimethylamino-2-nitroethylene (79) (both in E form) present a hydrogen bond basicity comparable to that of tributylamine. 

Abraham s solute descriptors to yield a predictive regression equation. Further, the solute dipolarity/polarizability, hydrogen bond acidity, and hydrogen bond basicity were found to favor blood and solute size favor brain. 

The hydrogen-bond donor and acceptor properties of a packing are another dimension of column selectivity. They are independent of the silanol ion-exchange activity described above, and affect the selectivity of a packing in a separate way. We have measured these properties by the relative retention between butylparaben, a hydrogen-bond donor, and dipropylphthalate. The same property, or at least a very similar property, has been called hydrogen bond basicity by Snyder.In our case, it has been defined as follows ... 

The hydrogen-bond basicities of a very extensive range of aldehydes and ketones have been measured, and are reported in terms of Taft s p/fuB scale. ... 

The ionic liquids incorporating various anions associated with the same cation [BMIM] exhibited different basicity and dipolarity values. The effect of the cation on hydrogen bond basicity and dipolarity was quite small. Therefore, it was inferred that the anion has a greater influence than the cation on the overall hydrogen bond basicities of the ionic liquids. 

It has often been found that enzymes retain their activity when suspended in ionic liquids of low hydrogen bond basicity, but they are inactive in those with high hydrogen bond basicity (e.g., Cl , NOi", and acetate-containing ionic liquids) 274). Some recent biocatalytic reactions are discussed below, with an emphasis on the function of the ionic liquids in enzyme catalysis. 

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