Effect of quasi hydrostatic and non hydrostatic pressure on long S–S bonded sodium dithionite (Na2S2O4): A Raman Spectroscopic study

Abstract

Sodium dithionite, Na2S2O4, most commonly used reducing agent, is known to exist in equilibrium as [SO2−.] radical state in aqueous solution due to the easy cleavage of its very long S–S bond (2.389 ​Å). Overall, the structure of Na2S2O4 is akin to linkage system, where long S–S bond as a main bar is flanked by two short S–O bonds in eclipsed position. This prompted us to undertake solid state behaviour of Na2S2O4 under quasi-hydrostatic (up to 34.5 ​GPa) and non-hydrostatic (30.2 ​GPa) pressure environments using diamond anvil cell and Raman spectroscopy. Although increase in pressure, under both pressure environments, clearly showed blue shifting of the main vibrational modes, with some changes in dω/dP, relationship is observed between stretching of S–S bonds and stretching of S–O bonds. Interestingly, the pressure values of ∼0.3 ​GPa, ∼3.2 ​GPa, ∼7.8 ​GPa, and ∼18.7 ​GPa at which changes were observed in Raman spectra of Na2S2O4, have similarity in pressure values with those observed in high pressure studies of SO2 in literature. More importantly, development of a new S–S- bond around 261 ​cm−1 ​at high frequency side was observed under quasi-hydrostatic as well as non-hydrostatic pressure environment above 7.8 ​GPa, during compression which showed complete reversibility on decompression. These results can help as a simple model for understanding reactivity of disulphide bridged systems, especially proteins, under external stress.