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.
Highlights
As one of the basic independent thermodynamic parameters, the pressure is important for characterizing the state in condensed matter physics[32,33,34]
It is obvious that the most prominent Raman bands of SQBP are attributed to BP, with some weak bands originating from squaric acid (SQ)
Due to the cooperative and competitive effects of hydrogen bonds, the Raman bands of SQBP below 900 cm−1 tend to shift to lower wavenumbers in comparison with the reactants, and the corresponding normal modes are attributed to the twisting, bending and ring-ring stretching vibrations of the whole molecular skeleton
Summary
As one of the basic independent thermodynamic parameters, the pressure is important for characterizing the state in condensed matter physics[32,33,34]. From an experimental point of view on high pressure research, to elucidate the mechanism of phase transition and the changes of crystal or molecular geometry, corresponding Raman spectroscopy is commonly adopted for structural characterizations[38,39,40]. SQBP comprises a protonated squaric acid cation(SQ+) and a 4,4′-bipyridine anion(BP−), with possible changes of external parameters such as pressure or temperature, double proton-transfer complex (SQ2+BP2−)could be obtained[46, 47]. A possible higher-order phase transition is proposed in the pressure range of 3~6 GPa
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