As for the latter two experiments it should be noted that in NMR the difference between “small molecule” and
“big molecule” is not just a matter of semantics intended to convey
a difference in molecular size: small molecules (typically below
about 800 Da) exhibit entirely different relaxational characteristics
from large molecules (typically over 1000 Da). This difference
manifests itself e.g. in the resonance linewidths (larger molecules
give broader resonances) and the sign of the NOE, which is positive
for small molecules and negative for large molecules. For molecules
whose size is close to ca. 1000 Da the NOE disappears, and the
ROESY experiment was designed to overcome this problem. Nevertheless,
since the ROESY experiment can give less exact results,
usually the NOESY is the experiment of first choice unless the
molecule falls into the no-NOE region. As a further complication,
the molecular weight where the NOE changes sign depends on field
strength: larger fields tend to push a given small molecule more
towards the big-molecule zone. For a more detailed discussion of
these subjects the reader is referred to references