Imidazo quinolines, calculations

Ab initio calculations (STO-3G) on imidazo[4,5-/]quinolines have shown that in all cases (neutral, anionic, mono-, and dicationic) the 3H form 44a is favored over the IH form 44b [93THE(279)167]. 

Calculated Proton Affinities and pK Values for Imidazo [4,5-/ Quinolines... 

Calculating the energy values for anion, neutral, mono and dication forms it was also demonstrated that there is a nice correlation between computed gas phase proton affinities and pKa values determined in aqueous solution for imidazo[4,5-f]quinolines (Table 5 and Figure 5). 

TABLE 5. Calculated Proton Affinities and pKa Values for Imidazo[4,5-f]quinolines. 

Another conclusion can be driven out from the energy calculations is that the 3H-form is preferred over IH-form in imidazo[4,5-f]quinolines. As it can be seen from Figure 6 and Table 6 the 3H form is energetically favorable in each case. That point also needs to be enlightened experimentally by measuring the pKa values of the fixed derivatives by replacing the mobile hydrogens with methyl. This is to be done in future. 

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