The composite vibrational structure near 3650–3200 and 3000–2400 cm −1 in the observed IR absorption spectrum of Chelidonic acid has been explained in terms of intra - and inter -molecular −O−H∙∙∙O H-bonding attributed to monomer and dimer species computed at B3LYP/6–311++G(d,p) level. Three of the six dimer species derived out of ten monomeric components have shown both intra - and inter -molecular H-bonding. Vibrational modes of the monomer and dimer species are satisfactorily identified with the observed IR and Raman bands including frequency shifts associated with the H-bondings. The H-bond interactions in the monomer and dimer species have been characterized in terms of electron density, ρ( r ), its Laplacian, ∇ 2 ρ( r ) and potential energy density at the O∙∙∙H bond critical points (BCPs) based on the Atoms in Molecules (AIM) theory. The attractive (van der Waals, H-bonds) and repulsive steric clash (SC) interactions are explained using computed reduced density gradient values from the noncovalent interaction (NCI) method. The AIM analysis confirms the presence of the intra - and inter -molecular H-bondings in the monomer/dimer species. The natural bond orbital (NBO) analysis of the natural charges and stabilization energy of the H-bonds for the dimer species further points to the stronger inter -than intra -molecular H-bonding.

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