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The interplay between resonance-assisted hydrogen bonding and steric repulsions determines the structural stability of conjugated systems. To investigate the relation between electronic effects and π -delocalization in heteronuclear N-H···O interactions, the rotational spectra of three β -enaminone derivatives were investigated in the gas phase using a chirped-pulse Fourier transform microwave spectrometer. The three compounds were selected to probe the effects of donor acidity, steric repulsion, and extended π -conjugation. Contrary to previous models which predict a hydrogen bond strengthening in fully planar conjugated systems, the present results revealed that a twisted structure exhibits the strongest intramolecular hydrogen bond, while they are weaker for a planar analog. These experimental findings, corroborated by natural bond orbital analysis, demonstrate that for these compounds substituent-induced enhancement of donor acidity is more important for the strengthening of intramolecular interaction than the extent of the π -conjugation.
Batra et al. (2026) studied this question.