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.
Using solid-state 15N
NMR spectroscopy, the cis/trans isomerization
in a two-dimensional
(2-D) array of surface-mounted azobenzene-based switches was detected
for the first time. In order to achieve this, a new class of rod-shaped
molecular switches, suitable for formation of 2-D regular arrays on
large facets of tris(o-phenylenedioxy)cyclotriphosphazene
(TPP) nanocrystals, was synthesized. A mechanochemical approach was
used to prepare corresponding host–guest surface inclusions
in a TPP matrix. Comparison of thermal steps in solution and supramolecular
surface inclusions revealed that switching of individual molecules
is not compromised by the close proximity of neighbors.
A straightforward method is proposed for the determination of free energies of nucleobase pairing by monitoring conformational changes upon intermolecular binding.
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