Cooperativity and intermolecular hydrogen bonding in donor-acceptor complexes of phenol and polyhydroxybenzenes

Cooperativity between intermolecular and intramolecular hydrogen bonds is an important factor determining the strength of donor-acceptor complexes. Its impact in poly-1,2-diols, notably polyhydroxybenzenes, is subject to debate. Density functional theory calculations have been performed on complexes of phenol, catechol, pyrogallol and 1,2,3,4-tetrahydroxybenzene with pyridine, trimethylamine and trimethylphosphine oxide. Binding energies, proton NMR shifts, Quantum Theory of Atoms in Molecules analysis and Interacting Quantum Atoms (IQA) interaction energies reveal a positive cooperative effect of a topological intramolecular hydrogen bond (IMHB) in catechol relative to phenol, with insignificant or small negative effects of further HO groups. Except for catechol complexes, there are no topological IMHBs in any donors or their complexes, although all show Non-Covalent Interaction isosurfaces. The absence of a topological IMBH in catechol itself is explained by electron density contour plots. Complexes of Me3N and Me3PO with catechol, pyrogallol and 1,2,3,4-tetrahydroxybenzene display additional intermolecular hydrogen bonds, C-(HH)-H- horizontal ellipsis -C and C-(HO)-O- horizontal ellipsis =P, respectively, with the hydrogen atom ortho to the bound HO group. The predominantly covalent or electrostatic character of the various hydrogen bond types is discussed in terms of the IQA energy partition scheme, which is also useful for characterising IMHBs in the absence of a bond critical point. These results further our understanding of the nature and the limits of cooperative hydrogen bonding in donor-acceptor complexes.