Many efforts have been recently devoted
to the design and investigation
of multicomponent pharmaceutical solids, such as salts and cocrystals.
The experimental distinction between these solid forms is often challenging.
Here, we show that the transformation of a salt into a cocrystal with
a short hydrogen bond does not occur as a sharp phase transition but
rather a smooth shift of the positional probability of the hydrogen
atoms. A combination of solid-state NMR spectroscopy, X-ray diffraction,
and diffuse reflectance measurements with density functional theory
calculations that include nuclear quantum effects (NQEs) provides
evidence of temperature-induced hydrogen atom shift in cocrystals
with short hydrogen bonds. We demonstrate that for the predictions
of the salt/cocrystal solid forms with short H-bonds, the computations
have to include NQEs (particularly hydrogen nuclei delocalization)
and temperature effects.