Giant deuteron migration during the isosymmetric phase transition in deuterated 3,5-pyridinedicarboxylic acid

A positive to negative uniaxial thermal expansion crossover in an organic benzothienobenzothiophene structure

Compound 6,6'-([1]benzothieno[3,2-b][1]benzothiophene-2,7-diyl)bis(butan-1-ol) (BTBT-C4OH) displays a continuous type 0 first-order isosymmetric phase transition at 200 K which is accompanied by a continuous change of the thermal expansion along the b axis from positive to negative. The equivalent isotropic atomic displacement parameters for all non-hydrogen atoms as well as all the eigenvalues of the anisotropic atomic displacement tensor show discontinuous behavior at the phase transition. The eigenvalues of the translational tensor in a rigid-body description of the molecule are all discontinuous at the phase transition, but the librational eigenvalues are discontinuous only in their temperature derivative. BTBT-C4OH displays a similar type of quasi-supercritical phase transition as bis(hydroxyhexyl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-C6OH), despite the difference in molecular packing and the very large difference in thermal expansion magnitudes.

Pressure-induced isosymmetric phase transition in biurea

An isosymmetric phase transition of biurea has been found in which the molecules appear to “snap” from one conformer to another upon compression.

Geometric H/D isotope effect in a series of organic salts involving short O–H⋯O hydrogen bonds between carboxyl and carboxylate groups

Noticeable elongations of donor–acceptor distances upon deuteration are confirmed in short O–H⋯O hydrogen bonds between carboxyl and carboxylate groups.

Structure and physicochemical characterization of a naproxen-picolinamide cocrystal

Naproxen (NPX) is a nonsteroidal anti-inflammatory drug with pain- and fever-relieving properties, currently marketed in the sodium salt form to overcome solubility problems; however, alternative solutions for improving its solubility across all pH values are desirable. NPX is suitable for cocrystal formation, with hydrogen-bonding possibilities via the COOH group. The crystal structure is presented of a 1:1 cocrystal of NPX with picolinamide as a coformer [systematic name: (S)-2-(6-methoxynaphthalen-2-yl)propanoic acid-pyridine-2-carboxamide (1/1), C14H14O3·C6H6N2O]. The pharmaceutically relevant physical properties were investigated and the intrinsic dissolution rate was found to be essentially the same as that of commercial naproxen. An NMR crystallography approach was used to investigate the H-atom positions in the two crystallographically unique COOH-CONH hydrogen-bonded dimers. 1H solid-state NMR distinguished the two carboxyl protons, despite the very similar crystallographic environments. The nature of the hydrogen bonding was confirmed by solid-state NMR and density functional theory calculations.

Measurement of proton tunneling in short hydrogen bonds in single crystals of 3,5 pyridinedicarboxylic acid using nuclear magnetic resonance spectroscopy

In this Letter, we present NMR spin-lattice and relaxometry data for proton transfer in one of the shortest known N-H⋯O hydrogen bonds in a single crystal of 3,5 pyridinedicarboxylic acid (35PDCA). It is widely believed that proton transfer by quantum tunneling does not occur in short hydrogen bonds since the ground state energy level lies above the potential barrier, yet these data show a temperature independent, proton tunneling rate below 77 K and a clear deviation from classical dynamics below 91 K. This study therefore suggests that proton tunneling occurs in all hydrogen bonds at low temperature and the crossover temperature to classical hopping must be determined when evaluating whether proton tunneling persists at higher temperature, for example in enzyme catalysis under physiological conditions.

An unusual temperature induced isostructural phase transition in a scheelite, Li(0.5)Ce(0.5)MoO4

High resolution synchrotron X-ray diffraction, dielectric and Raman scattering study of a scheelite compound Li0.5Ce0.5MoO4 (LCM) revealed that it transforms to a self similar structure above 400 °C. The thermally induced isostructural phase transition (IPT), a phenomenon which has rarely been reported in the literature, is preceded by partial softening of the zone centre phonons followed by their hardening above the IPT transition temperature. The high temperature isostructural phase, which exhibits expanded lattice parameters and cell volume, nucleates and grows in the low temperature matrix over a very wide temperature range. Both the phases show nearly identical thermal expansion suggesting similarities in symmetry, unaltered coordination environments around the atoms across the transition.

Energy Analysis of Competing Non-Covalent Interaction in 1:1 and 1:2 Adducts of Collidine with Benzoic Acids by Means of X-Ray Diffraction

The hydrogen bond pattern and the types of non-covalent interactions in the crystals of the 1:1 and 1:2 adducts of 2,4,6-trimethylpyridine and benzoic acids are studied using high-resolution X-ray diffraction. The geometries of the hydrogen bonds are estimated using a combined XRD/DFT approach that provides the geometrical parameters within the margin of error of neutron diffraction studies. The energies of the non-covalent interactions are estimated on the base of the experimental electron density distribution function. It is shown that the structures of the adducts are governed by the NOH and OHO hydrogen bonds. In turn, C-H...O contacts and stacking interactions define the packing of the adducts in the crystal. On the other hand, it is important to note that the latter interactions affect the competition of the former hydrogen bonds in some 1:2 adducts.