Hydrogen bonding is a kind of molecular interactions very important in biological systems due to high ability of structures formation and simultaneously a large flexibility which results in high efficiency in molecular recognition and replication processes. Intra molecular hydrogen bonds reveal enhanced thermodynamic and structural stability, which makes them very useful model systems in study of the nature of hydrogen bonding. Some specific features of these bent hydrogen bonds appear, not only interactions decrease, but also substantial lowering of the intensity of stretching n(X-H) vibrations is observed. It is a result of vector character of dipole moment derivatives of basic and acidic parts of a bridge. The bond length increments resulting from chelate ring formation show a pattern of ortho quinoid resonance structure, even in molecules with weak p electron coupling between acidic and basic components of the system (Mannich bases). A procedure of estimation of the energy of intramolecular hydrogen bond is not solved problem, yet. A model is proposed on the basis of thermodynamic cycle. The energetic consequences of steric effects can be estimated within this model, which increase of estimated energy of hydrogen bond on about 2.5 kcal/mol. These effects are compared with related in the systems with direct electron coupling between acid and base (Schiff bases, also important from the practical point of view). It is stated, that the steric effects in these rigid molecules, are higher about three times than in Mannich bases. The amount of keto resonance form in description of the structure of hydrogen bonded, open and zwitterionic forms, in Schiff bases, is estimated. It is shown, that only after the opening the chelate rings of zwitterionic form gives the predominance of keto structures.

References:

[1] A. Koll, S. M. Melikova, A. Karpfen, P. Wolschann, J. Mol. Struct. 2001, 559, 127.

[2] A. Koll, Int. J. Mol. Struct. 2003, 4, 434.

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