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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 1 H double-quantum (DQ) NMR spectroscopy under fast magic-angle spinning (MAS). This NMR method combines the sensitivity of 1 H NMR chemical shifts to the strengths of hydrogen bonds with quantitative information about dipole-dipole couplings between pairs of protons. Thus, two-dimensional 1 H 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 hydrogenbonded moieties is elucidated in the bulk material. This process is associated with an Arrhenius activation energy of (145 AE 15) kJ mol À1 , which can be rationalised in terms of hydrogen-bond dissociation and the reorientation of the supramolecular polymer chain.

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