Experimental, DFT dimeric modeling and AIM study of H-bond-mediated composite vibrational structure of Chelidonic acid.

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, ∇2ρ(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.