Patterns of hydrogen bonding involving thiourea in the series of thiourea⋅trans-1,2-bispyridyl ethylene cocrystals – A comparative study

Salts of trans-3-Styryl Pyridine: The Effect of N-Quaternization on Solid-State Photoreactivity, and Photochemical Crystal Melting

Investigating solid-state photoreactivity, driven by crystal packing, has been a major enduring research theme in Crystal Engineering. Trans-3-styryl pyridine (3-StPy), an unsymmetric olefin, is photo-stable. However, when converted to a series of salts, they exhibited solid-state photoreactivity under UV irradiation. Crystal structures of 3-StPy, its protonated salts, namely, (3-StPyH)(HSO4) ⋅ H2O (1), (3-StPyH)(CF3CO2) (2), and (3-StPyH)(p-Tol-SO3) (3), and methylated salt (3-StPyMe)I (4) were determined by X-ray diffraction. 3-StPy molecules were found not to align in the parallel arrangement required to undergo solid-state [2+2] photocycloaddition reaction; however, upon protonation and methylation, the unsymmetric 3-StPyH+ and [3-StPyMe]+ cations aligned in a head-to-tail parallel arrangement, predominantly governed by the cation⋅⋅⋅π interactions. Various structural features, e. g., the patterns of hydrogen bonding, etc. have been addressed, and established by Hirshfeld surface analysis. The salt with p-tolyl sulfonate anion (3) with Z'>1 represents a rare crystal class. It was also noted that the crystals of 1 and 2 melted into a liquid state upon photodimerization reaction. UV-vis absorption and fluorescence properties have been explored. The electronic structures of 3-StPy, 3-StPyH+, [3-StPyMe]+, and the dimer [3,3'-MPyPhCB]2+ cations have been elucidated by DFT calculations, and the effect of N-quaternization on crystal structures and photophysical properties has been discussed.

A comprehensive computational and principal component analysis on various choline chloride-based deep eutectic solvents to reveal their structural and spectroscopic properties

In this study, the quantum chemical properties, nonbonding interactions, and spectroscopic insights of a wide variety of choline chloride (ChCl)-based deep eutectic solvents were investigated employing molecular dynamics (MD), density functional theory, and spectroscopic analyses. Nine experimentally reported ChCl-based deep eutectic solvents (DESs) were selected for this study where ChCl was common in all the DESs and the hydrogen bond donors (HBDs) were varied. The most energetically favorable cluster was selected using MD simulation followed by density functional theory calculation. The most stable cluster structures were fully optimized, and their quantum chemical properties and IR spectra were computed at the ωB97XD/6-31G++(d,p) level of theory. Principal component analysis was performed to distinguish their behavioral differences and to find out if any correlation exists among the 1:1 and 1:2 clusters. The atom-atom radial distribution functions based on MD simulations revealed that several hydrogen bonds were formed among the donor and acceptor molecules. However, the most prominent hydrogen bonds were found to be N-H_HBD⋯Cl^- for ChCl:U, ChCl:TU, and ChCl:Ace and O-H_HBD⋯Cl^- for ChCl:Glu, ChCl:Ma, ChCl:Ox, ChCl:Gly, and ChCl:Phe. Both N-H_HBD⋯Cl^- and O-H_HBD⋯Cl^- were major interactions for ChCl:Pro, where Cl^- worked as a bridge between Ch⁺ and the respective donors. In addition, the -OH of Ch⁺ showed strong intermolecular interactions with the acceptor groups of the donor molecules, such as C=O and O-H. This study has tried to extract a pattern of the contributions of HBDs by comparing the structural, spectroscopic, and thermodynamic properties of ChCl-based DESs, which have also been successfully correlated with the intermolecular interactions.