The resonance-assisted hydrogen bond (RAHB) is a model of synergistic interplay between pi -delocalization and hydrogen-bond (H-bond) strengthening originally introduced (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Sec. 1989, 111, 1023; Bertolasi, V.; Gilli, P.; Ferretti, V.1 Gilli, G. J. Aln. Chem. Sec. 1991, 113, 4917) for explaining the abnormally strong intramolecular O-H...O bonds formed by the ...O=C-C=C-OH... beta -enolone fragment I which are typical of B-diketone enols. The applicability of this model to the intramolecular N-H...O hydrogen bonds formed by a number of heteroconjugated systems (...O=C-C=C-NH..., beta -enaminones II; O=C-C=N-NH..., ketohydrazones III; and ...O=N-C=C-NH..., nitrosoenamines IV) is investigated. The X-ray crystal structures of five molecules which close a six-membered ring by an intramolecular N-H...O bond through the resonant ...O=X-C=X-NH... (X = C, N) fragments II-IV are compared to those of two other molecules closing the same ring through the nonresonant ...O=C-C-C-NH... beta -aminone moiety V. Experimental findings are complemented by a CSD (Cambridge Structural Database) search of all compounds forming intramolecular N-H...O bonds through the molecular fragments II-V and by a comprehensive analysis of the IR v(NH) stretching frequencies and H-1 NMR delta (NH) chemical shifts available for compounds of these classes of known crystal structure. It is shown that all the descriptors of H-bond strength [d(N...O) shorthening, decrease of v(NH), increase of delta (NH), and increase of pi -delocalization within the heteroconjugated fragment] are mutually intercorrelated according to RAHB rules, which can then account for the strength of heteronuclear N-H...O bonds in II-IV as well as for that of the homonuclear O-H...O bonds in I. Heteronuclear N-H...O bonds appear, however, to have distinctive features. In particular, their strength turns out to be partially hampered by the proton affinity difference (BPA) between the N and O atoms, so that very strong H-bonds (2.65 greater than or equal to d(N...O) greater than or equal to 2.48 Angstrom, 3200 greater than or equal to v(NH) greater than or equal to 2340 cm(-1), 13 less than or equal to delta (NH) less than or equal to 18 ppm) can occur only when the pi -delocalization of the heterodienic moiety is associated with proper electron-attracting substituents which are able to decrease this Delta PA by increasing the NH acidity. Moreover, at variance with strong O-H...O RAHBs, whose protons are mostly found in nearly symmetrical positions, even the strongest N-H...O RAHBs are highly dissymmetric, despite the very similar changes undergone by both IR and H-1 NMR spectra in O-H...O and N-H...O H-bonded systems. Specificities of heteronuclear H-bonds are shown to be interpretable by the electrostatic-covalent H-bond model (ECHBM) which was previously developed for the homonuclear case (Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, C. J. Am. Chem. Sec. 1994, 116, 909). The conclusions drawn are corroborated by extended DFT quantum-mechanical calculations at the B3LYP/6-31+G(d,p)B3LYP/6-31+G(d,p) level of theory and by full geometry optimization carried out on 27 variously substituted heterodienes II-IV and nonresonant beta -aminones v. calculations allow the estimation of H-bond energies that are found to be approximately 2.75 kcal mol(-1) for nonresonant V and 5.22, 6.12, and 7.03 kcal mol(-1) for unsubstituted resonant II, III, and IV, respectively. Proper substitutions of beta -enaminone II nearly double H-bond energies, making them comparable to those calculated for homonuclear O-H...O RAHB in beta -diketone enols (9.51 and 13.08 kcal mol(-1) for malondialdehyde and acetylacetone, respectively).