Quadruple hydrogen bonds of ureido-pyrimidinone moieties investigated in the solid state by 1H 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 1H double-quantum (DQ) NMR spectroscopy under fast magic-angle spinning (MAS). This NMR method combines the sensitivity of 1H NMR chemical shifts to the strengths of hydrogen bonds with quantitative information about dipole–dipole couplings between pairs of protons. Thus, two-dimensional 1H 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