The basicity of the hydrogen bonds of formamidines 1–19 was measured by means of the formation constant KHB of their complexes with p-fluorophenol and the frequency shift Δν(OH) of methanol hydrogen-bonded to 1–19. The study of the ν(C=N) band shows that hydrogen bonding takes place with the imino nitrogen atom. On the hydrogen-bonding basicity scale, the formamidines appear to be more basic than the corresponding amides and pyridines, and as basic as the imidazoles. The field effect of electron-withdrawing substituents and the steric effect of bulky alkyl groups on the imino nitrogen atom markedly decrease the hydrogen-bonding basicity.
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Raczyńska et al. (1992) studied this question.