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We investigate the anisotropic thermal expansion behavior of a co-crystalline system composed of 4,4′-azopyridine and trimesic acid (TMA-azo). Using variable-temperature single-crystal X-ray diffraction (SC-XRD), low-frequency Raman spectroscopy, and terahertz time-domain spectroscopy (THz-TDS), we observe significant temperature-induced shifting and broadening of the vibrational absorption features, indicating changes in the intermolecular potential. Our findings reveal that thermal expansion is driven by anharmonic interactions and the potential energy topography, rather than increased molecular dynamics. Density functional theory (DFT) simulations support these results, highlighting significant softening of the potential energy surface (PES) with temperature. This comprehensive approach offers valuable insights into the relationship between structural dynamics and thermal properties, providing a robust framework for designing materials with tailored thermal expansion characteristics. [ABSTRACT FROM AUTHOR]

The aqueous solubility of solid-state pharmaceuticals can often be enhanced by cocrystallization with a coformer to create a binary cocrystal with preferred physical properties. Greater understanding of the internal and external forces that dictate molecular structure and intermolecular packing arrangements enables more efficient design of new cocrystals. Low-frequency (sub-200 cm−1) Raman spectroscopy experiments and solid-state density functional theory simulations have been utilized together to investigate the crystal lattice vibrations of mycophenolic acid, an immunosuppressive drug, in its pure form and as a cocrystal with 2,2′-dipyridylamine. The lattice vibrations primarily consist of large-amplitude translations and rotations of the crystal components, thereby providing insights into the critical intermolecular forces governing cohesion of the molecular solids. The simulations reveal that despite mycophenolic acid having a significantly unfavorable conformation in the cocrystal as compared to the pure solid, the cocrystal exhibits greater thermodynamic stability over a wide temperature range. The energetic penalty due to the conformational strain is more than compensated for by the strong intermolecular forces between the drug and 2,2′-dipyridylamine. Quantifying the balance of internal and external energy factors in cocrystal formation indicates a path forward in the development of future mycophenolic acid cocrystals. [ABSTRACT FROM AUTHOR]

Solid-state density functional theory calculations were used for analyzing the experimental solid-state terahertz (THz) spectrum of the beta-crystal form of the high explosive octahydro-1,3,5,7-tetrazocine (HMX). The modification of the molecular structure in the solid state through crystal packing effects and the formation of weak C-H...O hydrogen bonds cause the deficiency of isolated-molecule calculations.

The barrier to internal rotation of a methyl group attached to an unsaturated linkage in an organic molecule is changed by complex formation with a weakly bound argon atom. This is a result of a dispersion interaction between the argon and pie-electrons of the molecule in the vicinity of the methyl group.

Solvent reorganization at the molecular level was examined with particular focus on the changes in the placements and/or orientations of a nearby solvent molecule that might take place when a chromophore absorbs light. Using high-resolution electronic spectroscopy in molecular beams, the study demonstrates such changes in the solute-solvent complex of indole and water. Analysis of the high-resolution spectrum provides insights into properties of the intermolecular potential-energy surfaces governing changes in electronic states.