Compression studies of face-to-face π-stacking interaction in sodium squarate salts: Na2C4O4 and Na2C4O4●3H2O

Pressure-induced hydrogen bonding modulating Fermi resonance between fundamental modes in xylitol molecule

Xylitol, as a typical polyol, has a broad range of application prospects. However, the molecular states of xylitol under different environments are rarely reported until now. In this work, the state changes of xylitol molecules under high pressure were analyzed by Raman spectra. A Fermi resonance phenomenon in the fundamental mode of xylitol at 2945 (±0.06) cm-1 and 2955 (±0.41) cm-1 was observed at 0.99 GPa. The Fermi doublets possess the same symmetry and close energy levels, which had not been changed by pressures. However, the high pressure shortened the atomic distances and applied the extra disturbance, providing the necessary conditions for energy transfer. Besides, the Fermi doublets decoupling happened at 4 GPa due to the breaking of hydrogen bonding. This work provides an important reference for studying molecular states and weak interactions of polyols under high pressures.

Hydrogen-Bonded Cyclic Dimers at Large Compression: The Case of 1H-pyrrolo[3,2-h]quinoline and 2-(2'-pyridyl)pyrrole

1H-pyrrolo[3,2-h]qinoline (PQ) and 2-(2′-pyridyl)pyrrole (PP) are important systems in the study of proton-transfer reactions. These molecules possess hydrogen bond donor (pyrrole) and acceptor (pyridine) groups, which leads to the formation of cyclic dimers in their crystals. Herein, we present a joint experimental (Raman scattering) and computational (DFT modelling) study on the high-pressure behaviour of PQ and PP molecular crystals. Our results indicate that compression up to 10 GPa (100 kbar) leads to considerable strengthening of the intermolecular hydrogen bond within the cyclic dimers. However, the intramolecular N–H∙∙∙N interaction is either weakly affected by pressure, as witnessed in PQ, or weakened due to compression-induced distortions of the molecule, as was found for PP. Therefore, we propose that the compression of these systems should facilitate double proton transfer within the cyclic dimers of PQ and PP, while intramolecular transfer should either remain unaffected (for PQ) or weakened (for PP).

Thermochromism and piezochromism of an atomically precise high-nuclearity silver sulfide nanocluster

A novel high-nuclearity silver sulfide nanocluster [Ag₅₀S₇(SC₆H₄F)₃₆(dppp)₆]·4DMI, (hereafter abbreviated as 1⋅4DMI) was synthesised. Solvent-free crystals of 1 displayed a completely reversible narrowing and broadening of the optical band gap that was accompanied by visual thermochromism and piezochromism changeovers, when stimulated by varying temperatures between 113 and 413 K or by changing the pressure from 1 atm to 7.5 GPa.

High Pressure Spectroscopic Investigation on Proton Transfer in Squaric Acid and 4,4'-Bipyridine Co-crystal

In attempt to the obtain detailed geometric information of proton transfer compound (subsequently denote as SQBP) formed between squaric acid (SQ)and 4,4′-bipyridine(BP), and to investigate the mechanisms of pressure-induced double proton transfer and related structural phase transition, we carried out in-situ high pressure Raman spectroscopy of SQBP up to 20 GPa. A solid-solid phase transition together with double proton transfer phenomenon was confirmed by Raman spectroscopy at about 1.5 GPa, and the activation of C = O stretching mode in Raman spectra indicates a square-ring structure of SQ with four symmetric C = O bond formation. These results are further supported by first-principals calculations and in-situ high pressure infrared absorption spectroscopy. Additionally, Raman intensity analysis suggests that a higher-order phase transition with planar BP molecular structure occurred in the pressure range of 3~6 GPa. As a result, the π electron delocalization effect in BP dominated the intensity enhancement of C = O stretching mode in SQ. To the best of our knowledge, this is the first time observation of the intensity enhancement of proton donor’s normal modes induced by proton acceptor’s π electron delocalization.