Characterization of O–H⋯O interactions in linear and cyclic clusters of boric acid: An ab initio, DFT, QTAIM and NBO study

We investigated geometries, O–H stretching frequencies, binding energies, and n O → σ O – H ∗ charge transfer properties of linear and cyclic clusters of boric acid (BA) by means of density functional theory and MP2 method. Our calculations indicate that the structure and energetics of linear (BA)2–8 clusters are qualitatively different from the conventional H-bonded systems, which usually exhibit distinct cooperative effects, as cluster size increases. M06/6-311++G∗∗ calculations reveal that for the linear (BA)2–8, the average H-bonding energy per monomer increases from −12.54 kcal/mol in dimer to −12.97 kcal/mol in octamer; i.e. 3.4% cooperativity enhancement. Because the n O → σ O – H ∗ interaction hardly exhibits cooperative effects, the capability of the linear BA clusters to localize electrons at the O⋯H bond critical point is almost independent of cluster size and thereby leads to the non-cooperative changes in the O⋯H lengths and strengths. In contrast, a ring-like cluster consisting of six monomers of the BA indicates significant cooperative effects in lengths of O⋯H HBs, ν O – H stretching frequency, charge transfers, and electron localization at O⋯H bond critical points. The calculated non-additive energies also show that the cooperative effects due to n O → σ O – H ∗ interactions in cyclic (BA)6 indeed provide additional stabilities for the cluster.