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AIM and NBO analysis of the intramolecular hydrogen bond in guanidine derivatives

As a part of our recent interest in the effect of intramolecular hydrogen bonds on basicity, we recently measured gas-phase proton affinities (PAs) of several guanidines possessing up to three dimethylaminopropyl- or methoxypropyl- substituents. pKa measurements of the same compounds in acetonitrile were also carried out in order to elucidate effect of external solvation effects. In order to evaluate contribution of IMHB to the intrinsic PAs of the investigated guanidines, the strength of IMHB is analysed in terms of AIM and NBO approaches as they are widely accepted tools in identification and rationalization of the hydrogen bonded systems. For this purpose, a series of trisubstituted guanidine bases as well as their conjugate acids (Scheme 1), containing various proton accepting groups within the substituents, is considered. Energy of the IMHB (E_IMHB) is dissected into two main contributions: pure hydrogen bonding interaction (E_HB) and the “ ring forming” proces (E_RF). Pure hydrogen bonding contribution (E_HB) is estimated from the electron density at the bond critical points, as well as from the charge transfer between donating and accepting part of the hydrogen bond as obtained by NBO analysis. The energy of the “ ring forming” process (E_RF) is determined by comparison of the investigated structure with all-antiperiplanar (“ open” ) conformation of the corresponding compound.

intramolecular hydrogen bond; guanidine derivatives; AIM; NBO

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