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

Abstract

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.

Noncovalent bonding assessment by pair distribution function

“…48 The crystal structure of BIPY:SQA is known to be sensitive to the external environment, exhibiting proton transfer events and structural transitions as a function of applied temperature and pressure 20 and electric elds 21 which are coupled to colour changes along the yellow-orange-red spectrum and have been determined using single crystal X-ray diffraction (SCXRD), powder X-ray and neutron diffraction, and optical and infrared spectroscopy. 49 Fig. 1 Schematic showing the preparation of the BIPY:SQA 1 : 1 adduct and its temperature dependent behaviour, undergoing a solid-state reaction at 453 K.…”

Double proton transfer in hydrated formic acid dimer: Interplay of spatial symmetry and solvent-generated force on reactivity

“…Simulations at higher temperature allow to qualitatively assess conditions at higher pressure which has been used to induce proton transfer in other systems. 89,90…”

“…Simulations at higher temperature allow to qualitatively assess conditions at higher pressure which been used to induce proton transfer in other systems. 89,90 Results from previous simulation studies are somewhat ambiguous. Recent ab initio MD simulations using the BLYP functional including dispersion correction point towards pronounced hydrophobicity of cyclic FAD 62 as was also found from earlier simulations using a fully empirical force field.…”

Double Proton Transfer in Hydrated Formic Acid Dimer: Interplay of Spatial Symmetry and Solvent-Generated Force on Reactivity

“…The consequences of this become more prevalent at higher temperatures. Simulations at temperatures T ≥ 500 K are not directly relevant to experiment but the effects found here also apply a) to systems under pressure which have been investigated experimentally 73,74 and b) to systems with lower barriers for (D)PT than FAD considered here. Effective electrostatic force on the hydrogen atom for a PT towards O acc in the respective H-bond is "supportive", and "inhibiting" otherwise.…”