Functional unprotonated basic

Optical Spectra. The main (a) band in a variety of visual pigments exhibits absorption maxima in the range between 430 and 580 nm. It is this variability, as well as the basic bathochromic shift relative to a free PRSB in solution, which have provided the basis for most of the spectroscopic theories relevant to the structure of the chromophore and its environment in the binding site. Attempts to rationalize the shift in terms of charge-transfer complex formation between the (unprotonated) Schiff base and a protein functional group (200,210,212,228) have never... 

Figure 1 7-2 Composition of EDTA solutions as a function of pH. Note that the fully protonated form, H4Y is only a major component in very acidic solutions (pH < 3). Throughout the pH range of 3 to 10, the species H2Y and HY - are predominant. The fully unprotonated form Y is a significant component only in very basic solutions (pH > 10).

Of course, the intrinsic presence of neutral molecules in pure ionic liquids becomes more and more relevant if the equilibrium constant is less and less in favor of the ionic compound. For the N-protonated l-alkyUmidazolium salts the amount of neutral, unprotonated alkylimidazole is obviously a function of the anion s basicity. Here, the wide range of potential anion basicities has provoked the question as to whether these systems should at all be considered as ionic liquids. However, some convincing applications of these substances in catalysis [28] and organic synthesis [29] (see BASIL process in Chapter 9) and physical measurements [30] have led to the now widely accepted view that at least combinations of 1-alkylimidazoles with strong acids can be regarded as ionic liquids. 

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