title: Proton transfer aiding phase transitions in oxalic acid dihydrate under pressure.
abstract: Oxalic acid dihydrate, an important molecular solid in crystal chemistry, ecology and physiology, has been studied for nearly 100 years now. The most debated issues regarding its proton dynamics have arisen due to an unusually short hydrogen bond between the acid and water molecules. Using combined in situ spectroscopic studies and first-principles simulations at high pressures, we show that the structural modification associated with this hydrogen bond is much more significant than ever assumed. Initially, under pressure, proton migration takes place along this strong hydrogen bond at a very low pressure of 2 GPa. This results in the protonation of water with systematic formation of dianionic oxalate and hydronium ion motifs, thus reversing the hydrogen bond hierarchy in the high pressure phase II. The resulting hydrogen bond between a hydronium ion and a carboxylic group shows remarkable strengthening under pressure, even in the pure ionic phase III. The loss of cooperativity of hydrogen bonds leads to another phase transition at ∼ 9 GPa through reorientation of other hydrogen bonds. The high pressure phase IV is stabilized by a strong hydrogen bond between the dominant CO2 and H2O groups of oxalate and hydronium ions, respectively. These findings suggest that oxalate systems may provide useful insights into proton transfer reactions and assembly of simple molecules under extreme conditions.
authors.affiliation: High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India. himalphy@gmail.com hbhatt@barc.gov.in.
authors.forename: Himal
authors.initials: H
authors.lastname: Bhatt
authors.affiliation: High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India. himalphy@gmail.com hbhatt@barc.gov.in.
authors.forename: A K
authors.initials: AK
authors.lastname: Mishra
authors.affiliation: High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India. himalphy@gmail.com hbhatt@barc.gov.in.
authors.forename: Chitra
authors.initials: C
authors.lastname: Murli
authors.affiliation: High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India. himalphy@gmail.com hbhatt@barc.gov.in.
authors.forename: Ashok K
authors.initials: AK
authors.lastname: Verma
authors.affiliation: High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India. himalphy@gmail.com hbhatt@barc.gov.in.
authors.forename: Nandini
authors.initials: N
authors.lastname: Garg
authors.affiliation: High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India. himalphy@gmail.com hbhatt@barc.gov.in.
authors.forename: M N
authors.initials: MN
authors.lastname: Deo
authors.affiliation: High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India. himalphy@gmail.com hbhatt@barc.gov.in.
authors.forename: Surinder M
authors.initials: SM
authors.lastname: Sharma