  Water is the most important solvent in nature. It is a crucial issue to study
interactions among water molecules. Nuclear magnetic resonance (NMR)
spectroscopy is one of the most powerful tools to detect magnetic interactions
for the structure analysis of a molecule with broad applications. But
conventional NMR spectroscopy requires macroscopic sample quantities with
hampers in investigating nanoscale structures. Through quantum control of a
single spin quantum sensor, magnetic resonance spectroscopy of nanoscale
organic molecules and single molecules has been achieved. However, the
measurement of the dipolar interaction of nuclear spins within a molecule at
nanoscale and the analysis of its structure remain a big challenge. Here we
succeed in detecting the NMR spectrum from an ice crystal with
(6-nanometer)$^3$ detection volume. More importantly, the magnetic dipolar
coupling between two proton nuclear spins of a water molecule was recorded. The
resolved intra-molecule magnetic dipolar interactions are about 15 kHz and 33
kHz with spectral resolution at a few kHz. Analysis of the interaction-resolved
NMR spectroscopy provides a spatial view of nanoscale ice crystal, from which
the orientation of a water-molecule bond is derived and further the length of
the bond can be got. This work enables NMR spectroscopy applications in single
molecule structure analysis, provides a further tool for nanocrystalline and
confined water research.
