Quadruple hydrogen bonds of ureido-pyrimidinone moieties investigated in the solid state by H-1 double-quantum MAS NMR spectroscopy

The structure of the quadruple hydrogen bond formed by ureido-pyrimidinone moieties is investigated in dimerised model compounds, as well as in a supramolecular polymer, by solid-state H-1 double-quantum (DQ) NMR spectroscopy under fast magic-angle spinning (MAS). This NMR method combines the sensitivity of H-1 NMR chemical shifts to the strengths of hydrogen bonds with quantitative information about dipole dipole couplings between pairs of protons. Thus, two-dimensional H-1 DQ MAS spectra provide particularly detailed insight into the arrangement of hydrogen bonds and allow proton proton distances to be measured. For the supramolecular polymer, a thermally induced irreversible tautomeric rearrangement of the hydrogen-bonded moieties is elucidated in the bulk material. This process is associated with an Arrhenius activation energy of (145 +/- 15) kJ mol(-1), which can be rationalised in terms of hydrogen-bond dissociation and the reorientation of the supramolecular polymer chain.