= Solution
Use \b[two-dimensional <NOESY>], or <ROESY> where molecular tumbling makes <NOESY> less favorable, to look for through-space correlations between proton sites on the two ends that are brought together in the closed <conformation>. Assign <resonances> first and compare predicted ring contacts with an open reference or model.
The <nuclear Overhauser effect> involves <NMR cross-relaxation> through magnetic dipole–dipole coupling. Its strong approximate $r^{-6}$ dependence makes it sensitive to short interproton distances. A <NOESY> cross-peak therefore reports spatial proximity, including between sites far apart along the covalent skeleton; it is not a <NMR scalar coupling> transmitted along that skeleton.
Run a dilution series: end-to-end contacts that persist at <concentrations> where intermolecular association is negligible support intramolecular ring closure. Hydrogen-bonded NH <chemical shifts> and <diffusion> measurements can support the assignment, but neither alone proves ring topology. A flexible open chain can transiently bring ends together, and intermolecular contacts can also give NOEs. Several geometry-consistent cross-peaks, <concentration> behavior and controlled mixing times make the case much stronger than one peak.
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